**1. Introduction**

Sugar beet (*Beta vulgaris* L.) is one of the main raw materials for sugar production in many countries. It is considered to be the second most important crop in the world, after sugarcane, for the production of sucrose. The total area cultivated with sugar beet in Egypt during the 2018/2019 season was about 255,725.6 hectare (with an increase of 23.5% over the 2017/2018 season), producing about 12,247,170 tons (62.2% of national sugar production), with an average root yield of 47.89 t/ha [1]. Despite the importance of sugar beet as an industrial cash crop, some farmers still do not have great experience in its production; therefore, it is necessary to pay great attention to this and search for safe stimulating growth and untraditional natural substances that have a marked influence on plant growth parameters [2] that can increase plant growth and maximize productivity [3].

Generally, one of the most important questions for sugar beet growers is how much nitrogen fertilizer is needed to achieve maximum net profit. Consequently, the effects of nitrogen fertilization on the quality and production of sugar beets is one of the main concerns in the management of sugar beet production. Many studies have been conducted where it was concluded that fertilizing sugar beet with too little nitrogen resulted in the reduction of root tonnage and, conversely, the application of too much resulted in reduced sucrose concentrations and purity percentage [4–7]. Although deficient nitrogen content in the soil can reduce sugar beet root yield, excess amounts of N can decrease sucrose content while lowering sucrose recovery due to higher nitrate impurities [8,9]. In England, sugar beets are fertilized using 100–110 kg N/ha as an equilibrating rate between fertilizer prices and beet value [10]. In Germany, a maximum yield of sugar beet was achieved by adding an amount in the range of 100–125 kg N/ha [11], while, in Greece, maximum

yield was expected from using 252.5 kg N/ha because it showed a quadratic response to nitrogen levels [12]. Moreover, Hosseinpour et al. [13], in Iran, showed that fertilizing sugar beet using zero N significantly increased sugar percentage in the first season, while it was not influenced by N levels in the second season. Application of nitrogen fertilization is considered as an important practice that determines sugar beet growth and production [14]. In this regard, El-Sarag and Moselhy [15] in Egypt concluded that all sugar beet yields of root, top, and gross sugar were significantly affected by the addition of nitrogen, where each increase in nitrogen level caused a significant increase in these yields. Moreover, Abdelaal and Tawfik [16] reported that fertilizing sugar beet plants with 105 kg N/fad produced the highest values of root diameter and length, root and foliage fresh weights, and root yield/fed. Meanwhile, the highest means of sucrose and apparent purity percentages resulted from 0 kg N/fed (control treatment). Mekdad [17] stated that increasing the nitrogen level to 140 kg N/fad significantly increased root length and root diameter as well as root and top fresh weight. Additionally, it whas been stated by Mekdad and Rady [18] that, except for purity percentage and harvest index, all parameters, including root length and diameter, root and top fresh weights, and root and biological yield, were significantly increased by the application of 350 kg N/ha compared to 200 kg N/ha. Moreover, it was reported that raising nitrogen levels from 69 to 92 and 115 kg N/fad significantly increased root diameter, root length, root weight, and foliage fresh weight/plant, while it significantly decreased total soluble solid (TSS), sucrose, and purity percentages [19]. Tarkalson et al. [20] reported that nitrogen rates did not affect sugar beet yields compared to manure application treatments, where manured treatments increased root yields by 12 and 36% compared to nitrogen treatments in both seasons, respectively. Increasing the rate of nitrogen fertilizer from 56 to 224 kg/ha led to a linear increase in sugar beet root yield; however, sucrose concentration and purity percentage decreased [21]. Later, Zarski et al. [22] recorded a greater yield of sugar and roots in the fertilized sugar beet plants with a high nitrogen rate.

Optimal use of plant growth regulators with appropriate concentrations is considered one of the most effective practices for increasing sugar beet yields. It can improve growth regulation and the development of plants [23], and it may also be a solution for achieving a balance between growth and sucrose content in roots. Gibberellic acid (GA3) is one of the most important plant growth regulators used for agronomic and scientific research [24,25]. Previously, El-Taweel et al. [26] reported that the application of GA3 at a concentration of 300 mg/L significantly increased root length, root diameter, root weight, total soluble solids, and sucrose percentages. Root weight/plant and root length as well as root and sugar yields/fad were significantly increased with the increase of gibberellic acid concentrations from zero to 100, 200, and 300 mg/L. Conversely, it decreased TSS, sucrose, and purity percentages in both seasons [27]. In addition, Selim et al. [28] reported that foliar application of GA3 at 200 mg/L led to a significant increase in root length in the first season, root diameter, root weight, and purity percentage in the second season, and sucrose percentage and root and sugar yields in both seasons. Ibrahim et al. [29] stated that increasing the GA3 concentration from 50 to 100 or 150 mg/L significantly increased root and sugar yields. Recently, it has been revealed that foliar application of GA3 at 300 mg/L achieved 819.8 and 853.8 g root weigh/plant, 26.5 and 26.5 cm root length/plant, 20.0 and 19.7 tons root yield/fad, 4.8 and 4.6 tons top yield/fad, 23.1 and 22.3% TSS percentage, and a 3.6 and 3.5 ton sugar yield/fad in the first and second seasons, respectively [30]. Foliar spraying with GA3 was found to be more effective in enhancing root yield, sugar content, and leaf area index by increasing the activities/levels of non-enzymatic and enzymatic antioxidants [31].

Agricultural practices applied at specific times can enhance sugar beet growth, final root yield, and quality attributes. Several studies have been conducted to determine the effect of growth regulator types and concentrations on growth and productivity, while there have only been a few studies concerning their application time. Earlier, it was noted that foliar application of growth regulators 3 to 6 weeks before harvest time is more effective for enhancing sugar content in sugar beet roots [32]. However, Nelson and Wood [33] reported that applying gibberellic acid at 100 mg/L on the same dates (3 to 6 weeks before harvest time) decreased sucrose percentage. Peterson [34] found that applying potassium salt and GA3 at concentrations of 10 and 100 mg/L to the foliage early in the growing season had little effect on either sucrose content or root yield. El-Fiki et al. [35] indicated that spraying GA3 at 300 mg/L 70 days after sowing increased the TSS percentage by 18.9 and 14.2% and sucrose content by 24.1 and 12.2%, compared with the control treatment (without spraying) in the first and second seasons, respectively. In addition, it was concluded that foliar spraying of GA3 70 days after sowing had a significant effect on root length, the fresh weight of roots, sucrose percentage, and root and sugar yields/fad that surpassed the same treatment when it was added 140 days after sowing. Despite the superiority of spraying GA3 70 days after sowing compared with spraying 140 days after sowing, there were no significant differences on TSS and purity percentages in either season [27].

Most studies conducted on sugar beet crops were aimed at increasing root and sugar productivity per unit area. Therefore, many researchers have studied the effects of different fertilization levels and/or different growth regulators. Meanwhile, a limited number of these studies explored the effects of growth regulator application time on yield and quality traits. Therefore, the aim of this study is to determine the effect of nitrogen fertilizer levels and GA3 concentrations and spraying times, as well as their influence on sugar beet growth, productivity, and quality to specifically reduce the gap between sugar production and consumption in Egypt.

#### **2. Materials and Methods**

The present investigation was conducted during the two successive winter seasons of 2014/2015 and 2015/2016 at Aweesh Al-Hagar Village, center of Mansoura, Dakahlia Governorate, Egypt. The crop for the previous two years had been Maize. From the experimental field area, soil samples were randomly taken at a depth of 0–30 cm of soil surface to estimate the soil's mechanical and chemical properties (Table 1).

**Soil Analysis 2014/2015 2015/2016** Mechanical analysis Sand (%) 21.55 21.90 Silt (%) 29.84 30.29 Clay 48.60 47.80 Texture Clay Clay Chemical analysis Soil reaction pH 7.50 7.60 ECe (dsm<sup>−</sup>1) 1.37 1.33 Organic matter (%) 1.15 1.20 Available N (ppm) 45.80 46.50 Available P (ppm) 1.40 1.55 Exchangeable (ppm) 120.20 135.30

**Table 1.** Soil properties (mechanical and chemical) of the experimental sites (0–30 cm) during the 2014/2015 and 2015/2016 seasons.

The purpose of this was to study the effect of different nitrogen fertilizer levels and foliar applications of gibberellic acid (GA3) and its application time as well as their influence on growth, yield, and quality of sugar beet, cv. Kawemira. A split-split-plot design with 4 replicates was used. The main plots (84 m2) were assigned to three nitrogen fertilizer levels, i.e., 165, 220, and 275 kg N/ha. The sub-plots (21 m2) were restricted to four GA3 concentrations, i.e., 0 (tap water), 80, 160, and 240 mg/L, and the sub-sub plots (10.5 m2) were sprayed once using a knapsack sprayer either at 60 or 120 days after planting (DAP). Nitrogen fertilizer was applied in the form of Urea (46% N), which was added in two equal doses after thinning (at the first and second irrigations). The experimental unit contained 5 ridges, which were 60 cm wide and 3.5 m long. The experimental field was well prepared through three ploughings followed by leveling. Both phosphorus fertilizer in the form of Calcium Superphosphate (15.5% P2O5) and potassium fertilizer in the form of Potassium

Sulphate (48% K2O) were added during seed bed preparation, before ridging. Dry sugar beet balls were sown manually in dry soil at a rate of 3–4 balls per hill during the first week of November in both seasons. The experimental field was irrigated immediately after cultivation. Plants were thinned to secure one plant per hill, 30 days after sowing. All other agricultural practices were done in the same way that farmers usually do them in their fields.

At harvest, ten guarded plants were randomly chosen from each plot to decide the following characteristics: Root length (cm) was measured from the crown to the base of the root by a steel tape, root diameter (cm) was measured at the widest part of the proper root by a vernier caliper, and root and foliage fresh weights/plant were recorded separately in grams. All plants of the two inner ridges of each plot were harvested and cleaned. Roots and tops were separated and weighed in kilograms, then converted to estimate root and foliage yields in ton/ha. Quality parameters, including sucrose, TSS, and purity percentages were also estimated as follows: Total soluble solids percentage (TSS%) in roots was measured in the juice of fresh roots by using a hand refractometer, sucrose percentage was determined polarimetrically on a lead acetate extract of fresh macerated roots according to the method of Carruthers and Oldfield [36], apparent purity percentage was determined as a ratio between sucrose% and TSS% of the roots [36], and sugar yield (t/ha) was estimated by multiplying root yield (t/ha) by the sucrose percentage.

All collected data were statistically analyzed as the procedures of the split-split-plot design according to Gomez and Gomez [37] using the statistical analysis system (SAS) computer program. The bayesian least significant difference (BLSD) method was used to evaluate the differences between means at a 5% level of probability, as mentioned by Waller and Duncan [38].

#### **3. Results**

#### *3.1. Impacts of Nitrogen Fertilizer Levels on Sugar Beet Growth and Yield Parameters*

The results listed in Table 2 show that increasing nitrogen fertilizer level from 165 to 220 and 275 kg/ha significantly increased root length, root diameter, root fresh weight/plant, foliage fresh weight/plant, and root yield/ha in both seasons. The highest values of root length (32.9 and 32.8 cm), root diameter (11.5 and 11.2 cm), root fresh weight (919.8 and 876.8 g/plant), foliage fresh weight (535.9 and 492.5 g/plant), and root yield (75.9 and 72.6 t/ha) were obtained by adding 275 kg N/ha in the first and second seasons, respectively. On the other hand, fertilizing sugar beet plants with 165 kg N/ha resulted in the lowest values of these traits. However, N-fertilizer at 275 kg/ha significantly increased root length by 14.2 and 13.1%, root diameter by 17.3 and 17.9%, root fresh weight/plant by 25.5 and 21.8%, foliage fresh weight/plant by 40.4 and 37.8%, and root yield/ha by 25.7 and 22.3% compared with the application of 165 kg N/ha in the first and second seasons, respectively.

Foliage yield/ha, sucrose, TSS, and purity percentages, as well as sugar yield/ha were markedly affected by different nitrogen fertilizer levels (Table 3). The obtained results showed that adding 275 kg N/ha resulted in the highest values of foliage yield (43.9 and 40.5 t/ha) and sugar yield (13.544 and 13.059 t/ha), but at the same time it decreased sucrose percentage (17.9 and 18%), TSS percentage (24.2 and 23.6%), and purity percentage (73.8 and 76.3%) in the first and second seasons, respectively. Results showed that foliage and sugar yields/ha were significantly increased with each increase in nitrogen level, while the exception (no significant differences) on sugar yield was detected between the rate of 165 and 220 kg N/ha in the first season only. On the other hand, increasing the nitrogen fertilizer level from 165 to 275 kg/ha markedly decreased percentages of sucrose by 12.7 and 11.3%, TSS by 8 and 7.8%, and purity by 5.4 and 3.5% in the first and second seasons, respectively.


**Table 2.** Means of root length (cm), root diameter (cm), root fresh weight (g/plant), foliage fresh weight (g/plant), and root yield (t/ha) as affected by nitrogen fertilizer levels and gibberellic acid (GA3)spraying concentrations and its application time in the 2014/2015 (I) and 2015/2016 (II) seasons.

\* and NS indicate significance at 5% level of probability, and not significant, respectively. Means followed by the same letter(s) is/are not significantly differ at 5% level of probability; DAP = Days after planting.
