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Article

Yield and Heat Unit Requirements for Several Citrus Cultivars over Several Seasons in Egypt

by
Mahmoud Abdel-Sattar
1,*,
Rashid S. Al-Obeed
1 and
Abdulwahed M. Aboukarima
2
1
Department of Plant Production, College of Food and Agriculture Sciences, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia
2
Department of Agricultural Engineering, College of Food and Agriculture Sciences, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(7), 712; https://doi.org/10.3390/horticulturae10070712
Submission received: 9 May 2024 / Revised: 1 July 2024 / Accepted: 4 July 2024 / Published: 5 July 2024
(This article belongs to the Section Fruit Production Systems)
Browse Figures
Versions Notes

Abstract

:
Citrus cultivars have various temperature needs for development and output at different times from flowering to harvesting, making climate one of the numerous factors that affect citrus’ productivity and quality. In this study, the yield and heat unit requirements for several commercial citrus species over several seasons from 2010/2011 to 2021/2022 in Egypt were investigated. For this investigation, the time of flowering and the time of fruit harvesting were recorded. However, the required heat units from flowering to fruit harvesting were calculated based on daily records of air temperature, taking into account that all results below zero (negative results) are not used and all maximum air temperatures above 35.0 °C (≥35.1) are changed to 35.0 °C. In addition, the base air temperature of 13.0 °C was utilized for determining the required heat units. The results showed that in the experimental area, the overall mean of minimum air temperature, maximum air temperature, air relative humidity, and precipitation during the experimental periods had values of 15.2 °C, 28.70 °C, 59.3%, and 0.3 mm/day, respectively. Moreover, the lowest number of days required from flowering to fruit harvesting was observed to be 290.0 days for Fremont mandarin. Furthermore, the highest number of days required from flowering to fruit harvesting was observed to be 482 and 440 days, respectively, for Bearss Seedless lime and Valencia orange. Our study has highlighted a notable diversity among the investigated citrus cultivars, particularly highlighting specific cultivars that possess high yield. The cultivar that shows the greatest yield over the investigated seasons of the study was Valencia orange with 147.6 kg/tree. Moreover, the average values of the heat unit requirements for Washington Navel orange, Valencia orange, Murcott mandarin, Fremont mandarin, and Bearss Seedless lime were 3112.7, 3628.3, 3221.9, 3027.6, and 4398.4 °C day, respectively. This is the first report regarding the heat unit’s determination for several citrus cultivars grown in Egypt, and we expect this research will provide a new awareness in accepting and discovering novel locations where citrus cultivars can be positively developed in Egypt. It will also prove to be a source of basic information for the development of the citrus crop model.

1. Introduction

The greatest fruit crop worldwide, accounting for around 18% of the total fruit crop area, is citrus [1]. With a total production of 158.5 million tons in the year 2022, citrus is produced in around 10.1 million hectares [2]. With 162 species, the genus citrus is one of the most significant taxonomic components of the Rutaceae family. It is abundant in edible fruits like oranges, lemons, tangerines, mandarins, and limes [3,4]. In terms of economic, social, and cultural value, citrus are among the main fruit crops traded internationally. They are highly sought after for their use as food additives in foods and beverages, as well as for their fresh consumption and juice production [5,6,7]. Citrus show a significant character in our regular food because of their attractive colors, unique scents, and delectable tastes, as well as their high nutritional content and health-improving properties [8,9]. Citrus are long-season, evergreen perennial plants that are primarily planted in the tropical and subtropical regions of the world between latitudes of 40 degrees either north or south of the Equator [10,11].
Citrus can adapt to a wide range of weather conditions, but they also need favorable ecological conditions all year round [12]. However, climate conditions and production methods are just two of the many variables that affect citrus yield and quality [13]. The regulation of citrus development and growth are difficult processes that depend on numerous external and internal stimuli that can act both simultaneously and sequentially [5]. Climate change, which is defined by heat stress and aridity probabilities, as in the Mediterranean region, has already had an impact on citrus output. The climate in the areas where citrus varieties are now being created is changing due to temperature rise and other unfavorable climatic phenomena [14]. In the citrus sector, production losses constitute a significant financial restriction.
Citrus is produced in 135 countries and regions all over the world, but the productivity of citrus is highly variable from country to country. The changeable production seasons represent the main obstacles to citrus production [15]. The amount of heat that plants receive determines how quickly they grow, and each species has a preferred air temperature range. For each class, there is an ideal range of air temperature [16]. These air temperature ranges’ heat containment hours are referred to as heat units or growth degree days. Heat units can, however, be used to assess if a place is suitable for growing citrus, to appraise the span of phenological (growth) phases, and to forecast fruit ripeness times [16]. Additionally, heat unit information can be utilized as a decision support tool for selecting and evaluating citrus cultivars [17]. Furthermore, in various places around the world, air temperature—both high and low—is one of the environmental factors that most restricts citrus’ development and output. To obtain mature fruits, certain total heat requirements must be met for temperate-zone fruit species [18,19]. However, different citrus cultivars have different air temperature needs at different stages of their development and growth [12]. The quantity of heat that builds up during the growing season has a direct impact on how well the plants grow and develop [12]. Additionally, heat units, as well as the precise start dates for each phonological stage of citrus growth (root activation, spring flush, flowering, and color break), varied [20]. Although heat waves and excessively high air temperatures have a negative impact on productivity, the best air temperature range for citrus development and fruiting is between 13.0 and 35.0 °C [17].
Information on the heat requirements of citrus trees seems to be useful for managing citrus cultivar yield production. The heat units for the fruit growth and maturation of ‘Folha Murcha’ orange ranged from 4462 °C day to 5090 °C day [21]. In the state of São Paulo, Brazil, orange cultivars need between 2500 and 3600 °C day for fruit to attain maturation [22]. Therefore, the goal of this study was to highlight the fluctuation of a set of climate factors, particularly the air temperature range, over a period of eleven years for five citrus cultivars planted under the ecological conditions of a region located in Egypt, and its impact on heat unit requirements and yield. This study will advance our understanding of how air temperature fluctuations affect citrus production.

2. Materials and Methods

2.1. The Experimental Site and Plant Description

In this study, field studies were conducted in private commercial orchards located close to Nobaryia city, El-Behera Governorate, Egypt (latitude 30°44′47.8″ N; longitude 30°09′15.2″ E). Generally, the climate of Egypt is relatively wet and cool in winter (October to March) and is dry and hot in summer (April–September) [23]. Five commercial citrus cultivars that budded on sour orange rootstock (Citrus aurantium L.) were used to collect the required data. The trees were six years old prior to the eleven consecutive growing seasons (2010/2011 to 2021/2022). The five citrus cultivars were Washington Navel orange (Citrus sinensis (L.) Osbeck), Valencia orange (Citrus sinensis (L.) Osbeck), Murcott mandarin (Citrus reticulata Blanco) × (Citrus sinensis L.), Fremont mandarin (Citrus clementina Hort. ex Tanaka) × (Ponkan mandarin; Citrus reticulata Blanco), and Bearss Seedless lime (Citrus latifolia Tanaka). The citrus trees were planted under a drip irrigation system. The soil texture was classified as a sandy soil with an average pH of 7.4–7.6. The drip irrigation system had two lines and four drippers per tree (8 L/h). Every cultivar had 400 trees/ha, planted 5 × 5 m apart.
All citrus trees were kept in good condition by applying conventional agricultural techniques, such as hoeing, irrigation, pruning, diseases, and pest management, as advised by the Egyptian Ministry of Agriculture. According to Abdel-Sattar et al. [24], mineral and organic fertilizers were applied to provide fertilized elements to each citrus tree. Except for ‘Bearss’ lime, which is green in hue, citrus was historically plucked when it turned a brilliant yellow color. Any defects such as splitting, bruises, or husk cuts were removed.

2.2. Measurements and Calculations of the Related Variables

2.2.1. Calculating Daily Heat Units (DHUs)

Table 1 depicts the time cycle from the plant growth to the harvesting periods. There are different models to calculate the daily heat units (DHUs) that are available in the literature [25,26]. These models are based on the crop threshold temperature (base temperature, Tb) for citrus, which is Tb = 13.0 °C [17]. Other models use TB (upper base temperature) instead of Tb; however, there is no published justification for the choice of Tb or TB values when calculating DHUs [25]. According to the methods used in [17], in this study, we calculated DHUs using the recommended method, which is indicated by the calculations in Table 2. However, the authors of [17] reported that when calculating DHUs, all results below zero (negative results) are not used, and all maximum air temperatures above 35.0 °C (≥35.1) are changed to 35.0 °C, as shown in Table 2.

2.2.2. Climatic Data

The climatic data were obtained from a weather station that is located in the same area of the orchards that the daily data were obtained from. Daily precipitation, daily air relative humidity at 2 m, daily maximum air temperature at 2 m, daily average temperature at 2 m, and daily minimum air temperature at 2 m were the measured meteorological data. For presentation purposes, the weather factors data were averaged to be yearly -expect precipitation values, which were presented as mm/day.

2.2.3. Yield Determination

All fruits were harvested from each experimental tree (4 experimental trees × 4 trees per replicate = 16 trees per cultivar) at the harvesting time in each season. Then, the yield was evaluated in terms of saleable products minus damaged and oversized fruits. Fruits harvested from each cultivar were weighed using a digital weighing scale (ME1002E, Mettler Toledo, Greifensee, Switzerland) with a 0.01 g precision to calculate the yield per tree (kg/tree), which is then converted to tons/ha.

2.3. Statistical Analysis

Citrus cultivars were arranged in a randomized complete block design, according to Snedecor and Cochran [27]. Every cultivar had four replicates, with four trees representing each replicate (five cultivars × four replicates × four trees per replicate = 80 trees). To indicate the significant differences between citrus cultivars and growing seasons, ANOVA was applied using SAS software version 9.13 [28]. However, the least significant difference (LSD) test was employed to evaluate the mean differences (p = 0.05). We also calculated the standard deviation and coefficient of variation using an Excel spreadsheet, to measure the dispersion or variation among the data. Furthermore, correlation analysis was employed to assess the relationships between the investigated variables [29]; however, the correlation analysis was performed using SPSS software Version 26. Additionally, the multiple linear regression method [30] was utilized in this study to examine the influence of air temperature, air relative humidity, and precipitation on the yield. However, multiple linear regression analysis was completed using an Excel spreadsheet to obtain the regression coefficients ( a 0 ,   a 1 ,   a 2 , a 3 , a n d   a 4 ) of the following relationship:
Y i e l d   t o n s h a = a 0 + a 1 × X 1 + a 2 × X 2 + a 3 × X 3 + a 4 × X 4
where X1 is the average daily precipitation (mm/day), X2 is the yearly average air relative humidity (%), X3 is the yearly average maximum air temperature (°C), and X4 is the yearly average minimum air temperature (°C).

3. Results and Discussion

3.1. The Time Cycle from Plant Growth to Harvesting Periods

Based on the data in Table 1, the citrus growing activity starts at different periods. According to Khurshid and Sanderson [31], the duration of the growth stages of citrus can vary from year to year due to different climatic conditions, particularly temperature. Low temperatures in the winter induce buds to become dormant, while allowing them to blossom [32]. Spring bud sprouting shifts when the number of hours at a low temperature increases. However, normal growth and development have been impacted, and flowering behavior has changed due to altering climatic factors [33]. In citrus, flowering time depends on the species, the tree age, and the climatic conditions. Citrus flowering is divided into three distinct stages, as follows: anthesis, bud differentiation, and flower bud induction [34]. Citrus flowers generally bloom in the spring after the inductive winter season in subtropical climates [35]. Depending on the species, cultural management, and habitat, citrus trees can be harvested five to six months after they begin to flower and they begin to give fruit in three to five years from the time they are planted as propagated seedlings.
The findings (Table 1) demonstrated that the citrus cultivars’ commencement dates for flowering and harvesting varied. However, Table 3 indicates how the number of days needed from flowering to fruit harvesting varied for citrus cultivars depending on the production season. While the Bearss Seedless lime and Valencia orange cultivars showed signs of late maturation, the Fremont mandarin cultivar and Washington Navel orange showed signs of early maturity (Table 1). The Murcott mandarin cultivar gained medium maturity among the other cultivars (an average of 384.2 days), as shown in Table 3. The average number of days required from flowering to fruit harvesting ranged from 290.00 days to 482.17 days. This characteristic of numerous cultivars offers a good chance for fruit consumers, fruit processors, and exporters to expand the availability of citrus in the local, as well as international, markets. The least number of days required by the Fremont mandarin—290 days—was not close to that of other cultivars, as this cultivar shows the characteristic of early maturity, whereas the maximum number of days for maturity was required by the Bearss Seedless lime and Valencia orange, i.e., 482 and 440 days, respectively. The findings showed that the tested citrus had a range of harvest dates, with certain cultivars showing early maturity (such as the Fremont mandarin), while others, including the Washington Navel orange and Murcott mandarin, were discovered to be mid-maturing. However, it was discovered that the Bearss Seedless lime and Valencia orange cultivars were harvesting slowly.

3.2. Analysis of the Collected Meteorological Data

The information in Figure 1 depicts the mean of the recorded maximum air temperature, average air temperature, and average minimum air temperature from the year 2010 to the year 2022. Meanwhile, the information in Figure 2 depicts the mean of the recorded air relative humidity, from the year 2010 to the year 2022. Moreover, the information in Figure 3 depicts the mean of the recorded precipitation from the year 2010 to the year 2022.
Figure 1, Figure 2 and Figure 3 make it evident that there was a low variation in the meteorological characteristics. Throughout the trial periods, the overall average minimum air temperature was 15.2 °C, with a standard deviation of ±0.4 °C and a variation coefficient of 2.7% (Figure 1). Additionally, the overall average maximum air temperature was 28.7 °C, with a standard deviation of ±0.5 °C and a variation coefficient of 1.9% (Figure 1). Moreover, the overall average mean air temperature was 21.2 °C, with a standard deviation of ±0.4 °C and a variation coefficient of 2.0% (Figure 1). Furthermore, the overall average air relative humidity rate was 59.3%, with a standard deviation of ±2.1% and a variation coefficient of 3.5% (Figure 2). Moreover, the overall average precipitation rate was 0.30 mm/day, with a standard deviation of ±0.2% and a variation coefficient of 72% (Figure 3). Nevertheless, the air temperature range was suitable for citrus production [17]. Both low and high air temperatures have several adverse effects on citrus trees, including the loss of flowers and fruitlets. Moreover, hot weather also delays flowering and has an impact on crop yield and fruit quality [36]. Therefore, the climate should be viewed as a resource that needs to be managed or as a factor that has to be handled because it is associated with other production factors [37]. The climate in Egypt is excellent for growing oranges [38].

3.3. Citrus Cultivars’ Yield Responses to the Investigated Seasons

The variation of citrus cultivars’ yield with production years is shown in Table 4 for Washington navel orange, Valencia orange, Murcott mandarin, Fremont mandarin, and Bearss Seedless lime, respectively. The Washington navel orange is the most widely cultivated cultivar of navel orange. Between mid-October and March, on average, Washington navel oranges reach maturity [38]. Oranges were picked on 30 January in this study (Table 1), whereas fruits were picked on 15 December and 14 December in the 2013 and 2014 growing seasons, respectively, in the study by Zayan et al. [39]. The average production varied across the research periods and ranged from 42.7 to 59.7 tons/ha, with the year 2010/2021 producing a lower yield than the year 2017/2018, which generated 59.8 tons/h, as indicated in Table 4. According to El-Khalifa et al.’s study [40] from the 2019/2020 season, Egypt produced 25.4 tons/ha of Washington navel oranges. According to Zayan et al. [39], the average value for Washington navel oranges grown in Egypt in the years 2013 and 2014 was 32.0 tons/ha. El-Khalifa et al. [40] recorded a yield of 25.4 tons/ha for Egypt-planted Washington navel oranges in the 2019/2020 growing season. Agro-technical practices, which are crucial to the development, blooming, and fruit set of various crops, may be to blame for the variations in yield among previous studies and our results [41].
The Valencia orange ranks in the second position after the Navel orange. In this research, the Valencia orange was harvested on 15 May (Table 1). In the study of El-Aidy et al. [42], the harvest time of Valencia orange was in the first week of May. The average yield was different during the investigated seasons of this research and was in the range of 47.6 to 66.7 tons/ha, as the year 2020/2021 produced a lower yield of 47.6 tons/ha compared to the year 2010/2011, which yielded 66.7 tons/ha, as shown in Table 3. El-Shirbeny et al. [43] showed that the yield of the Valencia orange was in the range from 16.4 (tons/ha) to 69.5 (tons/ha) in the 2013/2014 and 2014/2015 seasons in Egypt. However, the yield is dependent on the amount of irrigation water [44], the type of fertilizer [45], and on the rootstock [46], as rootstock choice is one of the most important aspects in orchard management because scion cultivars respond differently to growth, fruit quality, and nutrients accumulation when grown on diverse rootstocks [47]. Therefore, agricultural approaches should typically include all yield-reducing factors that would aid in citriculture sustainability evaluation [48] and this is a significant concern due to the behavior of Valencia orange cultivation varies according to farms [49].
The Murcott mandarin used in this study was picked on 30 March (Table 1). The commercial harvest period for the Murcott mandarin is from January to March [50]. This fruit reaches full maturity between January and the end of March, making them the latest mandarin-type fruit to reach maturity, having little to no competition from other fruit kinds and a high monetary value [51]. However, according to the horticultural maturity stage (i.e., full color), which was observed around 297–300 days after full bloom [52], the fruit harvest for the Murcott mandarin was carried out on 14 March during the 2018 season and on 11 March during the 2019 season [53]. Furthermore, Fikry et al. [54] harvested Murcott mandarins in the first week of February because the experiment was carried out on 5-year-old ‘Murcott’ mandarin trees (Citrus reticulata, Blanco) that were grafted on the rootstock of ‘Volkamer’ lemon trees (Citrus volkameriana) over two consecutive growing seasons in 2018 and 2019 in Egypt. The average yield was different during the investigation seasons of this research work and was in the range of 39.8 tons/ha to 47.7 tons/ha, as the year 2019/2020 produced a lower yield of 39.8 tons/ha compared to the year 2020/2021, which yielded 47.7 tons/ha, as shown in Table 4. The average yield of Murcott mandarin trees that were budded on two citrus rootstocks (Volkamer lemon and Sour orange) during three successive seasons—2016, 2017, and 2018—under Egyptian environmental conditions was 15.5 tons/ha and 14.5 tons/ha, respectively [55]. Moreover, the yield was 23.7 tons/ha in the 2018 season, and for the season of 2019, the yield was 24.2 tons/ha for control treatment; when they used 4.5% nano-kaolin, the yield was 33.8 tons/ha for the 2018 year and was 33.0 tons/ha for the 2019 year [56]. However, during the two fruitful seasons of 2018 and 2019, research was conducted on six-year-old Murcott mandarin trees (a naturally occurring mandarin hybrid from tangerine to sweet orange) that were budded on ‘Volkamer’ lemon rootstock and grown in sandy soil with drip irrigation in a private orchard in El-Nubaria, Beheira governorate, Egypt. Generally speaking, there are regional differences in the typical agro-technical procedures of irrigation, pruning, and pest control for commercial citrus farms across the nation. Hamdy [57] reported that pruning caused a significant effect on the yield of Murcott mandarin fruit, and chemical thinning [58] also had an impact on the yield of Murcott mandarins. In addition, the climate has a significant impact on the features of mandarins, and fruits grow larger as the air temperature and relative humidity rise [59].
In this research, the Fremont mandarin was harvested on 1 January (Table 1); however, under Egyptian environmental conditions, Hamdy et al. [55] harvested the Fremont mandarin on 15 December during the two successive seasons of 2013 and 2014 on 5-year-old trees. However, under Turkish conditions, Ülker and Kamiloğlu [60] harvested Fremont mandarin in December. Furthermore, the average yield varied according to the investigated seasons of this research (season 2011 to season 2021), and was in the range of 47.6 tons/ha to 54.8 tons/ha, as the years 2017/2018 and the 2020/2021 season produced a lower yield of 47.6 tons/ha compared to the year 2013/2014, which yielded 54.8 tons/ha, as shown in Table 4. According to Hamdy et al. [55], the average yield of Fremont mandarin trees that were budded on two citrus rootstocks—Volkamer lemon and Sour orange—during three consecutive seasons in Egypt in—2016, 2017, and 2018—was 12.2 tons/ha and 11.2 tons/ha, respectively.
In this research, the Bearss Seedless lime was harvested on 15 July (Table 1) under Egyptian environmental conditions. Additionally, the average yield varied according to the investigated seasons of this research and was in the range of 42.9 to 50.0 tons/ha, as the years 2011/2012 and 2012/2013 produced a lower yield of 42.9 tons/ha compared to the year 2017/2018, which yielded 50.0 tons/ha, as shown in Table 4. The Bearss Seedless lime is a heavy producer, often producing fruit year-round in warm climates. This makes it a great option for those who want a reliable source of fresh limes for cooking and cocktails. Additionally, it is a compact tree that can be grown in a container, making it a good choice for those with limited space [61]. Finally, it is clear that the total average citrus yield (kg/tree) was in the range of 112.0 kg/tree to 147.6 kg/tree. However, the highest yield was observed for Valencia oranges, and the lowest yield belonged to the Murcott mandarin.

3.4. Statistical Analysis of Citrus Yield Responses Based on the Investigated Seasons

Table 5 presents the influence of cultivars, growing season, and their interaction on yield, with units of tons/ha. The yield differed significantly (p-value < 0.05) in relation to the cultivars and the growing season. At the same time, we noted that the interaction between cultivars and growing has a significant impact on the change in the yield. Citrus fruit growth and development regulation is a complex process that depends on numerous internal and external variables that can act both sequentially and simultaneously [5].
Concerning the comparisons between the average level of yield on the five cultivars, the level of yield was high for the Valencia orange compared to the other cultivars (Table 6). This is mainly due to the behavior of the cultivar [62,63]. However, according to Cojocaru et al. [64], the yield is influenced by the cultivar, and cultivars differ in their suitability to a specific climate [31]. Thus, the high yield of Valencia orange in this study may be related to the best climatic conditions in the area, in addition to the adequate agronomic management of the crop [49]. Moreover, the floral load depends on the cultivar [65]. The size and weight of the fruit have a major impact on the output; according to certain researchers [66], the larger the fruit’s diameter and weight, the higher the yield in orange cv. strength. However, all yield observations in this study were recorded during a specific air temperature range of the yearly average of 15.1–28.6 °C (Figure 1), air relative humidity between 54.7 and 65.5% (Figure 2), and precipitation between 0.1 and 0.7 mm/day (Figure 3). In the study of Nawaz et al. [67], it was observed that Kinnow mandarin behaves differently under uneven agro-climatic conditions due to the oscillation in abiotic and biotic stress, and changes in fruit growth, development, and ripening. In agriculture, the main interest is to have a higher yield.
It is clear from Table 7 that there was a variation in the yield of the investigated citrus, as its range was from 45.9 tons/ha to 53.6 tons/ha for the investigated periods. The 2016/2017 growing season stands out as having the highest yield (Table 7). This trend may be due to the fact that the limiting factors in citrus production were suitable in this season, as the average air relative humidity was 61.9%, with mild temperature conditions of a minimum air temperature of 14.7 °C and a maximum air temperature of 27.9 °C, and a reasonable precipitation of 0.6 mm/day. However, the climate has a significant impact on both productivity and product quality. Every kind of plant needs a certain spectrum of climate elements during every stage of growth [68]. Humidity and rainfall have an impact on citrus productivity. Climate has an impact on citrus plants’ flowering phase, which calls for consistent humidity and water stress [68]. Citrus is ideal for growing in areas that have rainfall between 1000 and 3000 mm/year, a temperature of 13 to 35 °C, and a humidity of 70 to 95% [68]. Additionally, in the bibliography, it is reported that different cultivars present a significant difference in production efficiency [69,70,71]. Fruit yield was affected in different seasons due to an irregular fruiting pattern [66]. The fluctuating trend in weather parameters has not only influenced plant phenophases, but also fruiting habits, whereby the fruit drops at different stages, and consequently, the yield and quality characteristics are influenced [65]. Furthermore, genotype, intrinsic features, and climatic adaptability in a given place may all influence the variation in cultivar fruit productivity [72]. To produce citrus with excellent fruit yield and quality, cultivars should be selected that respond to various agro-climatic zones.

3.5. Citrus Cultivars’ Yield Responses to Climate Parameters

Ahmad et al. [73] attested to the fact that specific temperature conditions are necessary for citrus fruit growth, and that the temperature has a significant impact on yield. Every year, citrus fruit grows and ripens from 25 February to 30 January, for the Washington Navel orange as an example; other periods for different cultivars are shown in Table 1, and the climate at these times of each year has a significant impact on the yield. Consequently, the correlation between climate factors and the yield for each citrus cultivar was examined, and the correlation coefficient was obtained. As seen from Table 8, the correlation between yield and climate factors fluctuated between weak, very weak, and moderate for all cultivars, and the correlation coefficient ranged between r = −0.022 and r = 0.650. However, precipitation had a positive correlation with yield for the Washington Navel orange, the Murcott mandarin, and the Bearss Seedless lime (r = 0.053; r = 0.528; r = 0.122; p ≤ 0.01, respectively); for the Valencia orange and the Fremont mandarin, precipitation had a moderate negative correlation with the yield (r = −0.503; r = −0.484; p ≤ 0.01, respectively). Air relative humidity had a negative correlation with the yield for Washington Navel oranges, Valencia oranges, Fremont mandarins, and Bearss Seedless limes (r = −0.046; r = −0.541; r = −0.279; r = −0.022; p ≤ 0.01, respectively), and for Murcott mandarins, air relative humidity had a moderate positive correlation with the yield (r = 0.650). The maximum air temperature had a negative correlation with the yield for Washington Navel oranges and Murcott mandarins (r = −0.427; r = −0.226; p ≤ 0.01); for Valencia oranges, Fremont mandarins, and Bearss Seedless limes, the maximum air temperature had a positive correlation with yield (r = 0.370; r = 0.161; r = −0.279; r = 0.282; p ≤ 0.01, respectively). The minimum air temperature had a negative correlation with the yield for Washington Navel oranges (r = −0.400; p ≤ 0.01); for Valencia oranges, Murcott mandarins, Fremont mandarins, and Bearss Seedless limes, the minimum air temperature had a positive correlation with the yield (r = 0.118; r = 0.180; r = −0.279; r = 0.195; r = 0.399; p ≤ 0.01, respectively). However, Wang et al. [29] reported that mean air temperature, air relative humidity, minimum air temperature, and maximum air temperature correlated with the yield to different degrees.
Agriculture and climate change are related phenomena that have multiple effects. Climate change affects citrus fruits because annual abiotic stress—water stress and air temperature being the main environmental factors—caused a drastic yield impairment [74]. These elements result in abnormal plant growth, development, and reproduction, as well as morphological, genetic, biochemical, and physiological abnormalities, that ultimately lower the crops’ commercial yield [74]. Reductions in fruit set, a drop in fruit size and growth, low tree output, and other phenological effects of high temperatures and water stress during hazardous citrus phenological phases occur. The air temperature was the only environmental factor that affected citrus yield in our research, because regular irrigation was achieved. Good management practices in citrus orchards are necessary to lessen the negative plant physiological stresses. It is necessary to enhance agronomic management techniques in order to address heat and water deficit stress. Pruning, irrigation, pests, nutrition, diseases, and other factors are all part of agronomic management methods and they play a significant role in the quantity and quality of citrus fruits [75].
Using the regression tool from the Analysis Tools list in an Excel spreadsheet, through the Data Analysis command button on the Data tab, a multiple linear regression model was created to estimate the yield of five citrus cultivars based on the yearly minimum air temperature, maximum air temperature, air relative humidity, and average daily precipitation. However, the variations of citrus yield with weather parameters were evaluated using regression Equation (1). The regression statistics and significance, F, for the regression models (Equation (1)) and the regression coefficients of different cultivars are depicted in Table 9 and Table 10, respectively. The lower values of R2 (Table 9 and Table 10) for all regression models indicate that the relationship between the yield and weather parameters may be nonlinear. However, nonlinear forecasting models were found to be superior for the yield forecasting of wheat crops [76].
For Washington Navel oranges, the R square is 0.766 (Table 9), meaning that the investigated factors account for more than 76.6% of the output. Precipitation and minimum air temperature had considerable impacts on Washington Navel orange production (Table 10). For Valencia oranges, the R square is 0.299 (Table 9), meaning that the investigated factors account for more than 29.9% of the output. The minimum air temperature considerably impacted Valencia orange production (Table 10). For Murcott Mandarins, the R square is 0.602 (Table 9), meaning that the investigated factors account for more than 60.2% of the output. The maximum air temperature considerably impacted Murcott Mandarin production (Table 10). For Fremont Mandarins, the R square is 0.474 (Table 9), meaning that the investigated factors account for more than 47.4% of the output. The minimum air temperature considerably impacted Fremont Mandarin production (Table 10). For Bearss Seedless limes, the R square is 0.196 (Table 9), meaning that the investigated factors account for more than 19.6% of the output. Precipitation considerably impacted Bearss Seedless lime production (Table 10). The outcomes of the multiple linear regression show that the yield of citrus cultivars significantly depends on climatic variables. In the study of Bhattacharyya et al. [77], with citrus, the R square was 0.91, meaning that the investigated factors account for more than 91% of the output. The area and maximum yield have a considerable impact on citrus production. Citrus yield is significantly influenced by temperature, lowest temperature, and the lowest percentage of relative humidity. The outcome shows that the lowest temperature and area had a positive relationship, whereas minimum relative humidity and maximum temperature had a negative relationship. When aggregated to all cultivars, the predictions explain 19.6–76.6% of the observed variance between seasons, depending on the citrus cultivar. The study recommends that future research on citrus should focus on the importance of climate change related to yield. The negative values of the coefficients for weather variables showed that an increase in the weather variables results in a decrease in the citrus yield. On the other hand, the weather variables yielded positive coefficients, which means that an increase in the weather variables will increase the yield of citrus.

3.6. DHUs Requirements

The climate is just one of several elements that affect citrus productivity and quality, as different citrus cultivars require varying temperatures for development and output at different times from blooming to harvesting. Calculating heat units is a crucial step in determining which cultivars are best suited for a given region. Researchers and producers can utilize a system of heat unit calculations of cultivars of commercial value that directly affects the growth and development of the trees. Additionally, knowledge of the need for heating units can aid citrus farmers in making decisions, in particular, in relation to yield. However, to successfully recommend the unusual and local citrus germplasm for commercial cultivation in specific areas in the future, systematic efforts will be required to test all the citrus cultivars that are currently available in Egypt for their heat unit requirements, as well as to calculate heat units in other areas.
The amount of heat a plant receives determines how quickly they develop. If no other circumstances (like water) are restricting, there is a temperature range where each species will flourish at its best. It is generally agreed upon that the ideal temperature range for citrus plant growth is between 13.0 and 35.0 °C. Heat units or increasing degree days are the hours of heat that fall within this range. Heat units can be applied to citrus to determine a region’s appropriateness for citrus cultivation, to calculate the length of phenological (growth) stages, and to forecast when fruits will ripen [78]. From flowering to fruit harvest, the heat units’ requirements for various citrus cultivars varied. Table 11 provides values of the heat unit requirements for several citrus cultivars in Egypt from blooming to fruit harvesting over the production season of 2010/2011 to 2021/2022. The average heat unit requirements for Washington Navel oranges, Valencia oranges, Murcott mandarins, Fremont mandarins, and Bearss Seedless limes were 3112.7, 3628.3, 3221.9, 3027.6, and 4398.4 °C day, respectively (Table 11). In a previous study, the variances in the date of maturation between ‘early’ and ‘late’ cultivars (e.g., Valencia and Washington Navel oranges) are supposed to reveal variances in heat unit necessities, whereby late cultivars necessitate a larger amount of heat units [79].
The broad range in heat unit requirements was brought about by the various maturation times of several cultivars (Table 1). The findings of Singh et al. [80], which show that each genotype requires a specific quantity of heat unit accumulation to complete several phenophases that results in the variation in maturity duration, are fully supported by the observations. However, Ananthanaryanan and Pillai [81] noted that other factors influencing fruit ripeness were heat, relative humidity, and rainfall. The amount of heat units fluctuates because of changing air temperatures and the number of days. However, the heat units for several citrus cultivars in Pakistan, as determined by Khan et al. [12], were in agreement with our findings. The citrus cultivars have different temperature requirements for their proper growth and development at various phenological stages. It is considered that citrus plants show no signs of growth below average air temperatures (base temperature) less than 13.0 °C [82], which is the degree required for growth and development [83], and the hours of heat above this range are referred to as heat unit or growth degree days. The heat unit calculations of cultivars of commercial importance can be used by various scientists/growers to identify the best microclimate that is suited for growing new cultivars within the wide range of climatic regions, along with exploring new potential areas where the various specific citrus cultivars can be grown [12]. The plant development rate is governed by the quantity of heat it receives. The low productivity of citrus is most often attributed to factors related to many changes arising in their fluctuating growth conditions under the influence of environmental factors and thus results in a reduced citrus yield [84].
In comparison to the global averages, Egypt’s national average citrus output is quite low. However, Egypt produced about 4632.7 thousand tons in the year of 2019, while the world produced 143,755.6 thousand tons in the same year [2]. Inappropriate cultivation methods, aged trees, a lack of routine trimming, off-type cultivars, poor irrigation techniques, and transmissible diseases are the main causes of the low output of citrus, as was reported in [81]. Additionally, the citrus productivity in Egypt is influenced by a variety of environmental factors, However, temperature changes generally harm citrus orchard development and productivity [85]. On the other hand, the increase in citrus productivity is attributed to optimal weather conditions and temperatures during the flowering of the trees, which increased the fruit set [74].

4. Conclusions

In this study, we investigated the variation in yield and heat units of five citrus cultivars grown in Egypt. When aggregated to all the citrus cultivars, the predictions using multiple linear regression explain 19.6–76.6% of the observed variance between seasons, depending on citrus cultivars. However, minimum air temperature, precipitation, maximum air temperature, and air relative humidity acted as independent variables, and citrus yield acted as the dependent variable. The findings of our study have revealed a significant biodiversity among the citrus cultivars examined. It is possible to say that among the cultivars studied, the one that showed the best production performance in the area where the test was carried out was the Valencia orange. The limitations of this study include the fact that it relied on limited data and specific citrus cultivars. However, this study was conceptualized to quantify the change in high and low fluctuating heat units during a crop’s growing season in the growing environment of Egypt, to assess the impact of such changes in terms of heat units on tree productivity.

Author Contributions

Conceptualization, M.A.-S., A.M.A. and R.S.A.-O.; Data curation, M.A.-S. and A.M.A.; Formal analysis, M.A.-S., A.M.A. and R.S.A.-O.; Methodology, M.A.-S., A.M.A. and R.S.A.-O.; Investigation, M.A.-S. and R.S.A.-O.; Resources, M.A.-S., R.S.A.-O. and A.M.A.; Software, M.A.-S., R.S.A.-O. and A.M.A.; Validation, M.A.-S. and R.S.A.-O.; Visualization, M.A.-S. and A.M.A.; Writing—original draft preparation, M.A.-S., R.S.A.-O. and A.M.A.; Writing—review and editing, M.A.-S., R.S.A.-O. and A.M.A.; Supervision, M.A.-S.; Funding acquisition, M.A.-S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Researchers Supporting Project (number: RSPD2024R707), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors extend their appreciation to the Researchers Supporting Project (number: RSPD2024R707), King Saud University, Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Al-Saif, A.M.; Abdel-Sattar, M.; Eshra, D.H.; Sas-Paszt, L.; Mattar, M.A. Predicting the chemical attributes of fresh citrus fruits using artificial neural network and linear regression models. Horticulturae 2022, 8, 1016. [Google Scholar] [CrossRef]
  2. FAO. Food and Agriculture Organization of the United Nations. FAO Statistical Databases 2020. Available online: https://www.fao.org/faostat/ (accessed on 22 August 2022).
  3. Tanaka, T. Fundamental discussion of Citrus classification. Stud. Citrog. 1977, 14, 1–6. [Google Scholar]
  4. Taia, W.K.; Ibrahim, M.M.; Abdel-Sattar, M. Pollen morphological variations among some cultivated citrus species and its related genera in Egypt. Jordan J. Biol. Sci. 2020, 13, 499–508. [Google Scholar]
  5. Iglesias, D.J.; Cercós, M.; Colmenero-Flores, J.M.; Naranjo, M.A.; Ríos, G.; Carrera, E.; Ruiz-Rivero, O.; Lliso, I.; Morillon, R.; Tadeo, F.R.; et al. Physiology of citrus fruiting. Braz. J. Plant Physiol. 2007, 19, 333–362. [Google Scholar] [CrossRef]
  6. Rafiq, S.; Kaul, R.; Sofi, S.A.; Bashir, N.; Nazir, F.; Nayik, G.A. Citrus peel as a source of functional ingredient: A review. J. Saudi Soc. Agric. Sci. 2018, 17, 351–358. [Google Scholar] [CrossRef]
  7. Talón, M.; Caruso, M.; Gmitter, F.G., Jr. The Genus Citrus, 1st ed.; Elsevier: London, UK, 2020; p. 720. [Google Scholar]
  8. Karn, A.; Zhao, C.; Yang, F.; Cui, J.; Gao, Z.; Wang, M.; Wang, F.; Xiao, H.; Zheng, J. In-vivo biotransformation of citrus functional components and their effects on health. Crit. Rev. Food Sci. Nutr. 2020, 61, 756–776. [Google Scholar] [CrossRef] [PubMed]
  9. Ma, G.; Zhang, L.; Sugiura, M.; Kato, M. Citrus and health. In The Genus Citrus; Woodhead Publishing: Cambridge, UK, 2020; pp. 495–511. [Google Scholar]
  10. Srivastava, A.K.; Hu, C. (Eds.) Fruit Crops: Diagnosis and Management of Nutrient Constraints; Elsevier: Amsterdam, The Netherlands, 2020; pp. 723–737. ISBN 978-0-12-818732-6. [Google Scholar] [CrossRef]
  11. Klimek-Szczykutowicz, M.; Szopa, A.; Ekiert, H. Citrus limon (Lemon) phenomenon—A review of the chemistry, pharmacological properties, applications in the modern pharmaceutical, food, and cosmetics industries, and biotechnological studies. Plants 2020, 9, 119. [Google Scholar] [CrossRef]
  12. Khan, A.-U.-R.; Nawaz, M.A.; Khan, M.N.; Ahmed, W.; Rehman, A.; Munir, N.; Saleem, B.A. Heat units requirement of various citrus cultivars grown in Pakistan. J. Environ. Agric. 2016, 1, 30–34. [Google Scholar]
  13. Zekri, M. Factors Affecting Citrus Production and Quality; Citrus Industry: 2011. pp. 6–9. Available online: https://crec.ifas.ufl.edu/media/crecifasufledu/extension/extension-publications/2011/2011_Dec_factors_citrus.pdf (accessed on 3 July 2024).
  14. Balfagón, D.; Arbona, V.; Gómez-Cadenas, A. The future of citrus fruit: The impact of climate change on citriculture. Mètode Sci. Stud. J. 2022, 12, 123–129. [Google Scholar] [CrossRef]
  15. Abobatta, W.F. Challenges for Citrus Production in Egypt. Acta Sci. Agric. 2018, 2, 40–41. [Google Scholar]
  16. Khurshid, T.; Hutton, R.J. Heat unit mapping-a decision support system for selection and evaluation of citrus cultivars. Acta Hortic. 2005, 694, 265–269. [Google Scholar] [CrossRef]
  17. Hardy, S.; Khurshid, T. Calculating Heat Units for Citrus. NSW Government. Department of Primary Industries. Primefact. 2021. Available online: https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0011/218972/Calculating-heat-units-for-citrus.pdf (accessed on 5 May 2024).
  18. Łysiak, G.P.; Szot, I. The Use of Temperature Based Indices for Estimation of Fruit Production Conditions and Risks in Temperate Climates. Agriculture 2023, 13, 960. [Google Scholar] [CrossRef]
  19. Ikinci, A.; Mamay, M.; Unlu, L.; Bolat, I.; Ercisli, S. Determination of Heat Requirements and Effective Heat Summations of Some Pomegranate Cultivars Grown in Southern Anatolia. Erwerbs-Obstbau 2014, 56, 131–138. [Google Scholar] [CrossRef]
  20. Zaky, M.A. Relationship between climatic indicators and some physiological stages of navel orange trees in West Delta, Egypt. Middle East J. Appl. Sci. 2020, 10, 665–672. [Google Scholar] [CrossRef]
  21. Stenzel, N.M.C.; Neves, C.S.V.J.; Marur, C.J.; Scholz, M.B.D.S.; Gomes, J.C. Maturation curves and degree-days accumulation of orange trees. Sci. Agric. 2006, 63, 219–225. [Google Scholar] [CrossRef]
  22. Ortolani, A.A.; Pedro, J.R.; Alfonsi, R.R. Agro climatological of citrus. In Citrus Industry; Rodriguez, O., Viegas, F.C.P., Pompeu, J.R., Amaro, A.A., Eds.; Fundação Cargill: Campinas, Brazil, 1991; pp. 153–193. [Google Scholar]
  23. Nashwan, M.S.; Shahid, S. Symmetrical uncertainty and random forest for the evaluation of gridded precipitation and temperature data. Atmos. Res. 2019, 230, 104631. [Google Scholar] [CrossRef]
  24. Abdel-Sattar, M.; EL-Tanany, M.; EL-Kouny, H.M. Reducing mineral fertilizers by using organic manure to improve Washington navel orange productivity and sandy soil characteristics. Alex. Sci. Exch. J. 2011, 32, 372–380. [Google Scholar]
  25. Amaral, M.H.; McConchie, C.; Dickinson, G.; Walsh, K.B. Growing Degree Day Targets for Fruit Development of Australian Mango Cultivars. Horticulturae 2023, 9, 489. [Google Scholar] [CrossRef]
  26. Dhonju, H.K.; Bhattarai, T.; Amaral, M.H.; Matzner, M.; Walsh, K.B. Management Information Systems for Tree Fruit—2: Design of a Mango Harvest Forecast Engine. Horticulturae 2024, 10, 301. [Google Scholar] [CrossRef]
  27. Snedecor, G.W.; Cochran, W.G. Statistical Methods, 7th ed.; The Iowa State University: Ames, IA, USA, 1990; p. 593. [Google Scholar]
  28. SAS Institute Inc. The SAS System for Windows, version 9.13; SAS Institute Inc.: Cary, NC, USA, 2008. [Google Scholar]
  29. Wang, S.; Xie, W.; Yan, X. Effects of Future Climate Change on Citrus Quality and Yield in China. Sustainability 2022, 14, 9366. [Google Scholar] [CrossRef]
  30. Tso, G.K.; Yau, K.K. Predicting electricity energy consumption: A comparison of regression analysis, decision tree and neural networks. Energy 2007, 32, 1761–1768. [Google Scholar] [CrossRef]
  31. Khurshid, T.; Sanderson, G. Heat unit mapping and the effects of climatic conditions on citrus phenology with reference to developmental responses to climate change. Proc. Int. Soc. Citric. 2008, I, 819–820. [Google Scholar]
  32. Reuther, W.; Nauer, E.M.; Summers, L. Effects of seasonal temperature regimes on development and maturation of citrus fruits. Proc. Intl. Soc. Citricult. 1973, 3, 63–71. [Google Scholar]
  33. Lal, S.; Singh, D.B.; Sharma, O.C.; Mir, J.I.; Sharma, A.; Raja, W.H.; Kumawat, K.L.; Rather, S.A. Impact of climate change on productivity and quality of temperate fruits and its management strategies. Int. J. Adv. Res. Sci. Eng. 2018, 7, 1833–1844. [Google Scholar]
  34. Albrigo, L.G. Climatic Effects on Flowering, Fruit Set and Quality of Citrus—A Review. Available online: https://www.1stfruits.co.za/wp/wp-content/uploads/2022/03/Climatic-Effects-on-Flowering-of-Citrus.pdf (accessed on 10 March 2024).
  35. García-Luis, A.; Kanduser, M.; Guardiola, J.L. The influence of fruiting on the bud sprouting and flower induction responses to chilling in Citrus. J. Hortic. Sci. 1995, 70, 817–825. [Google Scholar]
  36. Makinde, A.A.; Afolayan, S.O.; Olaniyan, A.A.; Odeleye, O.M.O.; Okafor, B.N. Effect of climate on citrus yield in rainforest-savanna transitional zone of Nigeria. J. Agric. Biol. Sci. 2011, 2, 10–13. [Google Scholar]
  37. Abobatta, W.F. Influence of Climate Change on Citrus Growth and Productivity (Effect of Temperature). Adv. Agric. Technol. Plant Sci. 2019, 2, 180036. [Google Scholar]
  38. Omar, A.E.-D.K.; Belal, E.B.; El-Abd, A.E.-N.A. Effects of foliar application with compost tea and filtrate biogas slurry liquid on yield and fruit quality of Washington navel orange (Citrus sinenesis Osbeck) trees. J. Air Waste Manag. Assoc. 2012, 62, 767–772. [Google Scholar] [CrossRef]
  39. Zayan, M.A.; Sayed, R.A.; El-Shereif, A.R.; ElZawily, H.M.A. Irrigation and fertilization programs for “Washington navel” orange trees in sandy soil under desert climatic conditions. 1—Effect on soil properties, vegetative growth and yield. J. Agric. Res. Kafr El-Sheikh Univ. 2016, 42, 210–233. [Google Scholar]
  40. El-Khalifa, Z.S.; ElSheikh, M.H.; Zahran, H.F.; Ayoub, A. Evaluation of Washington navel orange economic indicators. Open J. Appl. Sci. 2022, 12, 481–490. [Google Scholar] [CrossRef]
  41. Saleh, M.A.; Hafez, O.M.; Thabet, A.Y.I. Enhancing of fruit set %, yield and fruit quality of “Washington” navel orange by different agrochemical foliar sprays in application times. Middle East J. Appl. Sci. 2022, 12, 83–90. [Google Scholar] [CrossRef]
  42. El-Aidy, A.; Esa, W.; Alam-Eldein, S. Effect of organic and bio-fertilization on vegetative growth, yield, and fruit quality of ‘Valencia’ orange trees. J. Product. Dev. 2018, 23, 111–134. [Google Scholar] [CrossRef]
  43. El-Shirbeny, M.A.; Ali, A.M.; Edriss, M.H.; Baghdady, G.A.; Dawood, A.S. The effect of water and vegetation vigor on citrus production in Egypt using remotely sensed data and techniques. Int. J. Plant Soil Sci. 2016, 11, 1–11. [Google Scholar] [CrossRef] [PubMed]
  44. Mossad, A.; Farina, V.; Bianco, R.L. Fruit Yield and Quality of ‘Valencia’ Orange Trees under Long-Term Partial Rootzone Drying. Agronomy 2020, 10, 164. [Google Scholar] [CrossRef]
  45. Badawy, H.E.M. Partial substitution of Valencia orange chemical fertilization by bioorganic fertilization conjoint with algae extract foliar spray. Middle East J. Appl. Sci. 2017, 7, 1016–1030. [Google Scholar]
  46. Yildiz, E.; Demirkeser, T.H.; Kaplankiran, M. Growth, yield, and fruit quality of ‘Rhode Red Valencia’ and ‘Valencia Late’ sweet oranges grown on three rootstocks in eastern Mediterranean. Chil. J. Agric. Res. 2013, 73, 142–146. [Google Scholar] [CrossRef]
  47. Dubey, A.K.; Sharma, R.M. Effect of rootstocks on tree growth, yield, quality and leaf mineral composition of lemon (Citrus limon (L.) Burm.). Sci. Hortic. 2016, 200, 131–136. [Google Scholar] [CrossRef]
  48. Abd-Elgawad, M.M.M. Managing nematodes in Egyptian citrus orchards. Bull. Natl. Res. Cent. 2020, 44, 2–15. [Google Scholar] [CrossRef]
  49. Ruiz-Camacho, W.; Villegas-Rivas, D.; Borjas-Ventura, R.; Alvarado Huamán, L.; Bello-Amez, S.; Arias-Heredia, G.; Sernaque-Auccahuasi, F.; Palacios-Sánchez, J.M.; Alzamora-Rivero, C.A.; Franco-Del Carpio, C.M.; et al. The Behavior of Valencia Orange Cultivation (Citrus x sinensis (L.) Osbeck cv. Valencia) in “Type Farms” in a Province in Central Jungle of Peru. J. Biol. Sci. 2023, 23, 307–312. [Google Scholar] [CrossRef]
  50. Arras, G.; Usai, M. Response of Murcott mandarins to storage temperature. Adv. Hortic. Sci. 1991, 5, 99–103. [Google Scholar]
  51. Samra, N.R.; Shalan, A.M.; Eltair, B.T. Efficacy of different edible coatings in improving “Murcott tangor” fruit qualities during chilled and ambient storage. J. Plant Prod. Mansoura Univ. 2014, 5, 1283–1302. [Google Scholar]
  52. Tucker, D.P.H.; Futch, S.H.; Gmitter, F.G.; Kesinger, M.C. Florida Citrus Varieties SP 102. UF/IFAS Extension; The University of Florida: Gainesville, FL, USA, 1998; p. 36. [Google Scholar]
  53. Ennab, H.A.; El-Shemy, M.A.; Alam-Eldein, S.M. Salicylic Acid and Putrescine to Reduce Post-Harvest Storage Problems and Maintain Quality of Murcott Mandarin Fruit. Agronomy 2020, 10, 115. [Google Scholar] [CrossRef]
  54. Fikry, A.M.; Radhi, K.S.; Abourehab, M.A.S.; Abou Sayed-Ahmed, T.A.M.; Ibrahim, M.M.; Mohsen, F.S.; Abdou, N.A.; Omar, A.A.; Elesawi, I.E.; El-Saadony, M.T. Effect of Inorganic and Organic Nitrogen Sources and Biofertilizer on Murcott Mandarin Fruit Quality. Life 2022, 12, 2120. [Google Scholar] [CrossRef]
  55. Hamdy, A.E.; Khalifa, S.M.; Abdeen, S.A. Effect of magnetic water on yield and fruit quality of some mandarin varieties. Ann. Agric. Sci. Moshtohor. 2015, 53, 657–666. [Google Scholar]
  56. Aly, M.A.; Ezz, T.M.; Harhash, M.M.; El-Sayed, H.A.; El-demerdash, I.M. Sunscald Reduction, Increasing Yield and Fruit Quality of Murcott Mandarin Fruits Using Different Compounds as Nano Form. J. Adv. Agric. Res. (JAAR) 2023, 28, 404–412. [Google Scholar] [CrossRef]
  57. Hamdy, A.E. Effect of pruning severity on yield and fruit quality of two mandarin cultivars. Acta Hortic. 2016, 9. [Google Scholar] [CrossRef]
  58. El-Sayed, F.S.; Ahmed, H.S.; Elazazy, A.M. Effect of Hand and Chemical Thinning on Regulating Alternative Bearing in Murcott Trees. J. Hortic. Sci. Ornam. Plants 2017, 9, 98–105. [Google Scholar]
  59. Ribeiro, L.D.O.; Girardi, E.A.; de Carvalho, H.W.L.; Bastos, D.C.; Soares Filho, W.D.S.; Passos, O.S. Evaluation of mandarin hybrids grafted on ‘Rangpur’ lime in the Coastal Tablelands, Brazil. Rev. Bras. Frutic. 2020, 42, 1–9. [Google Scholar] [CrossRef]
  60. Ülker, T.; Kamiloğlu, M.U. Influences of girdling and potassium treatments on fruit quality and some physiological characters of ‘Fremont’ mandarin variety. Folia Hortic. 2021, 33, 195–202. [Google Scholar] [CrossRef]
  61. Available online: https://www.tripletreenurseryland.com/product/bearss-seedless-lime (accessed on 10 March 2024).
  62. Singh, J.; Sharma, V.; Pandey, K.; Ahmed, S.; Kaur, M.; Singh Sidhu, G. Horticultural Classification of Citrus Cultivars. In Citrus—Research, Development and Biotechnology; IntecOpen: London, UK, 2021. [Google Scholar] [CrossRef]
  63. Nawaz, R.; Khan, M.A.; Hafiz, I.A.; Khanm, M.F.; Khalid, A. Climate variables effect on fruiting pattern of Kinnow mandarin (Citrus nobilis Lour × C. deliciosa Tenora) grown at different agro-climatic regions. Sci. Rep. 2021, 11, 18177. [Google Scholar] [CrossRef] [PubMed]
  64. Cojocaru, A.; Munteanu, N.; Petre, B.A.; Stan, T.; Teliban, G.C.; Vintu, C.; Stoleru, V. Biochemical and Production of Rhubarb Under Growing Technological Factors. Rev. Chim. 2019, 70, 2000–2003. [Google Scholar] [CrossRef]
  65. Monselise, S.P. Citrus. In Handbook of Fruit Set and Development; Monselise, S.P., Ed.; CRC Press: Boca Raton, FL, USA, 1986; pp. 87–108. [Google Scholar]
  66. Ahmed, K.F.; Ahmed, F.M. Effect of Irradiated Compost and Bio Fertilizer on Vegetative Growth and Fruit Quality of Valencia Orange. Egypt. J. Hortic. 2020, 47, 15–27. [Google Scholar] [CrossRef]
  67. Nawaz, R.; Abbasi, N.A.; Hafiz, I.A.; Khalid, A. Impact of climate variables on growth and development of Kinnow fruit (Citrus nobilis Lour x Citrus deliciosa Tenora) grown at different ecological zones under climate change scenario. Sci. Hortic. 2020, 260, 108868. [Google Scholar] [CrossRef]
  68. Siagian, I.C.; Nurmegawati; Puspitasari, M.; Miswarti; Dinata, K.; Mikasari, W.; Hartono, R.; Yuliasari, S.; Sastro, Y. Analysis of limiting fators of RGL citrus fruit productivity (study case in Pal VII Village, Bermani Ulu Raya Subdistrict, Rejang Lebong District). IOP Conf. Ser. Earth Environ. Sci. 2021, 648, 012008. [Google Scholar] [CrossRef]
  69. Roose, M.L.; Cole, D.A.; Atkin, D.; Kupper, R.S. Yield and Tree Size of Four Citrus Cultivars on 21 Rootstocks in California. J. Am. Soc. Hortic. Sci. 1989, 114, 678–684. [Google Scholar] [CrossRef]
  70. Georgiou, A. Performance of ‘Nova’ Mandarin on Eleven Rootstocks in Cyprus. Sci. Hortic. 2000, 84, 115–126. [Google Scholar] [CrossRef]
  71. Georgiou, A. Evaluation of Rootstocks for ‘Clementine’ Mandarin in Cyprus. Sci. Hortic. 2002, 93, 29–38. [Google Scholar] [CrossRef]
  72. Abdel-Sattar, M.; Al-Obeed, R.S.; Rihan, H.Z.; El-Badan, G.E. Genetic Diversity and Relationships among Indian Jujube (Ziziphus mauritiana Lamk.) Cultivars Using Morphometric Characteristics, matK Barcoding, and ISSR Markers. Diversity 2024, 16, 313. [Google Scholar] [CrossRef]
  73. Ahmad, S.; Firdous, I.; Jatoi, G.H.; Rais, M.U.N.; Mohsin, A.Q. Economic impact of climate change on the production of citrus fruit in Punjab province of the Pakistan. Sci. Int. 2017, 29, 413. [Google Scholar]
  74. Egypt Orange Production on the Rise. Available online: https://citrusindustry.net/2023/01/04/egypt-orange-production-on-the-rise (accessed on 10 March 2024).
  75. Shafqat, W.; Naqvi, S.A.; Maqbool, R.; Haider, M.S.; Jaskani, M.J.; Khan, I.A. Climate Change and Citrus; IntechOpen: London, UK, 2021; 24p. [Google Scholar] [CrossRef]
  76. Panwar, S.; Kumar, A.; Singh, K.N.; Paul, R.K.; Gurung, B.; Ranjan, R.; Alam, N.M.; Rathore, A. Forecasting of crop yield using weather parameters–two step nonlinear regression model approach. Indian J. Agric. Sci. 2018, 88, 1597–1599. [Google Scholar] [CrossRef]
  77. Bhattacharyya, B.; Biswas, R.; Sujatha, K.; Chiphang, D.Y. Linear regression model to study the effects of weather variables on crop yield in Manipur state. Int. J. Agric. Stat. Sci. 2021, 17, 317–320. [Google Scholar]
  78. Zandalinas, S.I.; Balfagón, D.; Arbona, V.; Gómez-Cadenas, A. Modulation of antioxidant defense system is associated with combined drought and heat stress tolerance in citrus. Front. Plant Sci. 2017, 8, 953. [Google Scholar] [CrossRef] [PubMed]
  79. Goldschmidt, E.E. Effect of Climate on Fruit Development and Maturation. Available online: https://irrec.ifas.ufl.edu/flcitrus/pdfs/short_course_and_workshop/citrus_flowering_97/Goldschmidt-Effect_of_Climate_on_Fruit_Development.pdf (accessed on 17 June 2024).
  80. Singh, R.S.; Vash1shtha, B.B.; Prasad, R.N. Micrometeorology of ber (Zyzyphus mauritiana) orchard grown under rained arid conditions. Indian J. Hortic. 1998, 55, 97–107. [Google Scholar]
  81. Ananthanaryanan, K.K.; Pillai, O.A.A. Studies on fruit maturity in mango. Indian J. Jori 1968, 25, 1–15. [Google Scholar]
  82. Bevington, K.B.; Castle, W.S. Annual root growth pattern of young citrus trees in relation to shoot growth, soil temperature and soil water content. HortScience 1986, 10, 840–845. [Google Scholar] [CrossRef]
  83. Jones, G.V.; White, M.A.; Cooper, O.R.; Storchmann, K. Climate change and global wine quality. Clim. Chang. 2005, 73, 319–343. [Google Scholar] [CrossRef]
  84. Isah, T. Stress and defense responses in plant secondary metabolites production. Biol. Res. 2019, 25, 39. [Google Scholar] [CrossRef] [PubMed]
  85. Ahmed, H.F. Phytohormones content and random amplified polymorphic DNA (RAPD) marker assessment of some Egyptian citrus cultivars. Afr. J. Biotechnol. 2012, 11, 15755–15762. [Google Scholar] [CrossRef]
Horticulturae 10 00712 g001
Figure 1. The mean of the recorded air temperature (mean, minimum, and maximum) from the year 2010 to the year 2022.
Figure 1. The mean of the recorded air temperature (mean, minimum, and maximum) from the year 2010 to the year 2022.
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Figure 2. The mean of the recorded air relative humidity from the year 2010 to the year 2022.
Figure 2. The mean of the recorded air relative humidity from the year 2010 to the year 2022.
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Figure 3. The mean of the recorded precipitation from the year 2010 to the year 2022.
Figure 3. The mean of the recorded precipitation from the year 2010 to the year 2022.
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Table 1. Start dates of both flowering and harvesting for citrus cultivars.
Table 1. Start dates of both flowering and harvesting for citrus cultivars.
Citrus CultivarsFlowering DateHarvesting Date
Washington Navel orange25 February30 January
Valencia orange1 March15 May
Murcott mandarin10 March30 March
Fremont mandarin15 March1 January
Bearss Seedless lime20 March15 July
Table 2. Example of DHUs calculations according to [17].
Table 2. Example of DHUs calculations according to [17].
DateMaximum Air
Temperature (°C)		Recalculated Maximum Air Temperature (for Air Temperatures > 35.0 °C)Minimum Air
Temperature (°C)		Average Air Temperature (Maximum + Minimum ÷ 2)Daily Heat Units (DHUs) (Average Air Temperature − Base Temperature of 13.0)Accumulated Heat Units (Negative Values for DHUs Are Not Used)
133.533.517.625.612.612.6
235.235.017.926.513.526.0
334.134.118.026.113.139.0
437.135.018.526.713.739.0
517.617.67.812.7−0.30.0
619.219.26.612.9−0.10.0
717.617.67.812.7−0.30.0
819.219.26.612.9−0.10.0
922.422.411.416.93.942.9
1028.328.39.018.75.748.6
1124.124.111.517.84.853.4
Table 3. The mean, standard deviation, and minimum and maximum number of days required for flowering to the harvesting of the investigated citrus cultivars in growing seasons.
Table 3. The mean, standard deviation, and minimum and maximum number of days required for flowering to the harvesting of the investigated citrus cultivars in growing seasons.
Statistical CarteriaCitrus Cultivars
Washington Navel OrangeValencia OrangeMurcott MandarinFremont MandarinBearss Seedless Lime
Mean339.3440.3384.2290.0482.2
Standard
deviation		±0.5±0.5±0.4±0.00±0.4
Minimum339440384290482
Maximum340441385290483
Table 4. The variation of citrus cultivars’ yield with the unit of tons/ha with production years.
Table 4. The variation of citrus cultivars’ yield with the unit of tons/ha with production years.
SeasonsCitrus Cultivars
Washington
Navel Orange		Valencia OrangeMurcott
Mandarin		Fremont
Mandarin		Bearss Seedless Lime
MeanStandard DeviationMeanStandard DeviationMeanStandard DeviationMeanStandard DeviationMeanStandard Deviation
2010/201147.6±0.866.7±0.442.9±0.450.1±0.447.6±0.3
2011/201252.4±0.864.3±0.745.4±0.652.4±0.642.9±0.4
2012/201357.5±1.056.1±0.740.6±0.448.8±0.642.9±0.5
2013/201452.3±0.663.1±0.844.2±0.354.7±0.645.2±0.7
2014/201546.3±0.957.1±0.447.7±0.454.8±9.547.6±0.3
2015/201654.8±0.554.7±0.645.2±0.452.4±0.545.2±0.5
2016/201759.7±0.662.0±0.845.2±0.653.4±0.247.6±0.4
2017/201859.7±0.759.5±0.646.6±0.647.6±0.350.0±0.4
2018/201945.7±0.564.4±0.947.6±0.654.1±0.548.8±0.3
2019/202049.6±1.653.8±0.539.8±0.450.0±0.847.6±0.4
2020/202142.7±0.347.6±0.447.7±0.347.6±0.443.6±0.3
Overall mean51.7 59.0 44.8 51.4 46.3
Standard deviation±5.8 ±5.7 ±2.7 ±2.7 ±2.5
Overall minimum42.7 47.6 39.8 47.6 42.9
Overall maximum59.8 66.7 47.7 54.8 50.0
Table 5. Variance analysis of citrus cultivars’ yields using ANOVA according to cultivars, growing season, and their interaction.
Table 5. Variance analysis of citrus cultivars’ yields using ANOVA according to cultivars, growing season, and their interaction.
Source of VariationDFAnova SSMean SquareF ValuePr > F
Replicates314.54.82.50.06
Cultivars45514.61378.60703.6<0.0001
Growing season10974.597.449.7<0.0001
Cultivars × growing season402541.363.532.4<0.0001
DF = degrees of freedom.
Table 6. Impact of citrus cultivars on yield with the unit of tons/ha for all seasons.
Table 6. Impact of citrus cultivars on yield with the unit of tons/ha for all seasons.
Citrus CultivarsMean Yield * (tons/ha) ± Standard Deviation
Valencia Orange59.0a ± 5.7
Washington Navel Orange51.7b ± 5.8
Fremont Mandarin51.5b ± 2.7
Bearss Seedless Lime46.3c ± 2.5
Murcott Mandarin 44.8d ± 2.7
LSD (5%)0.6
* Values with different letters next to them in the same column indicate significant differences (p = 5%).
Table 7. Impact of production growing seasons on citrus yield with the units of tons/ha.
Table 7. Impact of production growing seasons on citrus yield with the units of tons/ha.
Production Growing SeasonMean Yield * (tons/ha) ± Standard Deviation
2016/201753.6a ± 9.2
2017/201852.7b ± 8.3
2018/201952.1bc ± 7.6
2013/201451.9bc ± 7.7
2011/201251.5cd ± 4.9
2010/201151.0de ± 4.9
2014/201550.7e ± 7.3
2015/201650.50e ± 6.5
2012/201349.2f ± 7.5
2019/202048.2g ± 5.2
2020/202145.9h ± 2.5
LSD (5%)0.9
* Values with different letters next to them in the same column indicate significant differences (p = 5%).
Table 8. The correlation coefficient (r) between the climate factors and yield for the investigated citrus cultivars.
Table 8. The correlation coefficient (r) between the climate factors and yield for the investigated citrus cultivars.
Climate FactorsCitrus Cultivars
Washington
Navel Orange		Valencia OrangeMurcott
Mandarin		Fremont MandarinBearss Seedless Lime
Average daily precipitation0.053−0.5020.528−0.4850.121
Yearly average air relative humidity−0.046−0.5410.650−0.279−0.022
Yearly average maximum air temperature−0.4270.370−0.2260.1610.282
Yearly average minimum air temperature−0.4000.1180.1800.1950.399
Table 9. Regression statistics and significance, F, for the regression models (Equation (1)).
Table 9. Regression statistics and significance, F, for the regression models (Equation (1)).
Citrus Cultivars
Regression StatisticsWashington Navel
Orange		Valencia OrangeMurcott MandarinFremont MandarinBearss
Seedless Lime
Multiple R0.8750.5470.7760.6880.443
R Square0.7660.2990.6020.4740.196
Adjusted R Square0.610−0.1680.3360.123−0.340
Standard Error3.66.22.22.62.8
Observations1111111111
Significance F0.040.650.180.350.83
Table 10. Regression coefficients using Equation (1) for yield prediction based on weather parameters.
Table 10. Regression coefficients using Equation (1) for yield prediction based on weather parameters.
ParametersCoefficients SymbolsRegression Coefficients (Equation (1))
Washington Navel OrangeValencia OrangeMurcott MandarinFremont MandarinBearss Seedless Lime
Intercept a 0 852.8159.1−152.434.719.7
Average daily precipitation a 1 32.8−3.0−7.5−15.74.7
Yearly average air relative humidity a 2 −6.1−1.41.90.5−0.3
Yearly average maximum air temperature a 3 −20.5−1.62.8−3.00.4
Yearly average minimum air temperature a 4 9.01.70.44.91.9
R2 0.7660.2990.6020.4740.196
Table 11. Values of DHUs requirements for various citrus cultivars from flowering to fruit harvesting during the production seasons of 2010/2011 to 2021/2022 in Egypt.
Table 11. Values of DHUs requirements for various citrus cultivars from flowering to fruit harvesting during the production seasons of 2010/2011 to 2021/2022 in Egypt.
Citrus Cultivars
Growing SeasonWashington Navel OrangeValencia OrangeMurcott MandarinFremont MandarinBearss Seedless Lime
2010/20113327.13800.73403.63168.44455.6
2011/20122818.83310.82874.62780.74523.3
2012/20133140.13735.13332.43087.04521.0
2013/20143112.53688.23250.32975.14393.7
2014/20153109.63573.93215.53024.64273.8
2015/20163098.93762.03269.73017.34355.7
2016/20173169.63615.03235.13068.94339.8
2017/20182964.53609.13169.32894.24427.3
2018/20193250.93678.43284.73154.34455.1
2019/20203154.73590.23271.63110.04325.2
2020/20213099.83620.53215.82998.24371.9
2021/20223105.53556.13140.43052.14338.5
Average3112.73628.33221.93027.64398.4
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Abdel-Sattar, M.; Al-Obeed, R.S.; Aboukarima, A.M. Yield and Heat Unit Requirements for Several Citrus Cultivars over Several Seasons in Egypt. Horticulturae 2024, 10, 712. https://doi.org/10.3390/horticulturae10070712

AMA Style

Abdel-Sattar M, Al-Obeed RS, Aboukarima AM. Yield and Heat Unit Requirements for Several Citrus Cultivars over Several Seasons in Egypt. Horticulturae. 2024; 10(7):712. https://doi.org/10.3390/horticulturae10070712

Chicago/Turabian Style

Abdel-Sattar, Mahmoud, Rashid S. Al-Obeed, and Abdulwahed M. Aboukarima. 2024. "Yield and Heat Unit Requirements for Several Citrus Cultivars over Several Seasons in Egypt" Horticulturae 10, no. 7: 712. https://doi.org/10.3390/horticulturae10070712

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