Responses of Nutritional Status and Productivity of Timor Mango Trees to Foliar Spray of Conventional and/or Nano Zinc

Abstract

The management of mango orchards is beset with a number of issues, including micronutrient deficits and significant fruit drop, which both contribute to decreases in mango tree output. Among these micronutrients, zinc is vital for increasing agricultural productivity, ensuring crop sustainability, and improving plant nutritional status during the growing season. To overcome zinc (Zn) deficiencies, this study was carried out during two successive seasons in an expected “Off” year (2022) and an expected “On” year (2023) on mature mango trees cv. Timor. To ameliorate this Zn deficiency, the effect of zinc in three forms—zinc oxide nanoparticles (ZnO NPs), sulfate (ZnSO₄), and chelated (Zn-chelated)—as a foliar sprayon leaves’ mineral, chlorophyll, total carotenoids, and total carbohydrate contents and productivity were studied. Ten spray treatments were used in this study, including nano zinc (100 ppm), zinc sulfate (0.1%), and chelated zinc (0.2%) on two occasions, 7 January and 4 weeks after the first application, either alone or in combination with each other as compared to the control. In both study seasons, the results showed that all the zinc forms in mineral, chelated, or nano form had a positive effect on the number of flowers per panicle, the percentage of fruit set, the number of fruits per panicle, and the number of fruits per tree, and it decreased the percentage of fruit drop. Furthermore, all zinc forms significantly increased the leaf N, P, K, Ca, Mg, Cu, Fe, Mn, B, and Zn contents (%), and all the treatments improved the chlorophyll, total carotenoid, and total carbohydrate contents compared with the other treatments. The most effective treatment was two applications of nano zinc at 100 ppm in terms of the nutritional status and productivity of Timor mango trees.

1. Introduction

The family Anacardiacea includes mainly tropical species and contains over 800 species classified into 70 genera [1]. However, the genus Mangifera comprises 69 species and is classified into 2 subgenera, i.e., Limus and Mangifera, and 11 uncertain position species [2]; at least 26 of these species bear edible fruits and are primarily found in Southeast Asia. The mango, Mangifera indica L., belongs to the order Sapindales and, which is a tropical tree fruit. The tree often blooms in the spring and bears beautiful fruits in June or July. Similar to many other tropical fruit trees, it does not blossom reliably [3]. Of 1000 cultivars of mango known worldwide, only 25 to 40 are of commercial importance [4]. As one of the most widely consumed fruits worldwide, mangos are grown in more than 100 nations. They are among the most well-liked and historically cultivated fruits in tropical and subtropical regions [5,6]. They are appreciated for their nutritional value, also attributable to the existence of health-enhancing substances, and they are considered a good source of ascorbic acid, proteins, amino acids, carbohydrates, carotenoids, dietary fiber, organic acids, phenolic compounds, polyphenolic pigments, and micronutrients [7,8].

The management of mango orchards is beset with several problems, with micronutrient deficiencies and heavy fruit drop [9,10] contributing to decreased mango tree productivity, which ultimately decreases production and exports [11]. Mango orchards have varying levels of production efficiency, which can be partially explained by changes in soil and leaf nutrients [12]. Mango cultivation has been taking place for around 40 years (recent) in Wadi Jizan, Saudi Arabia, and the indiscriminate use of chemical fertilizers means that fruit micro-nutrients, in particular, are still inconsistent and very variable from one grower to another [13], which leads to widespread micronutrient deficiencies in orchards, alternate bearing, mango malformation, fruit drop, spongy tissue, and susceptibility to major disease and pests, which affect tree growth and fruiting. Foliar micronutrients are critical for modern crop production, and the precise management of nutrient elements for sustainable agricultural production [14,15] is one tool that can be used to maintain or improve plant nutritional status during the growing season. Frequently, immediate effects are observed, and inadequacies can be addressed before yield or quality losses occur [16]. Foliar application has the advantage of involving the use of lower concentrations of fertilizers, a uniform distribution of nutrients, and the induction of a faster physiological response by the plant [17].

Among these micronutrients, zinc is of prime importance in regulating biochemical and physiological processes [18], with this micronutrient playing an important role in enhancing agricultural productivity, improving crop quality, maintaining crop sustainability, and maintaining or enhancing plant nutritional status during the growing season [16]. Zinc is a micronutrient that is necessary for plant metabolism, with it regulating various enzyme activities involved in biochemical pathways, such as protein metabolism, carbohydrate metabolism, growth regulator metabolism, and the maintenance of biological membrane integrity, and it is an integral part of the cell membrane [19,20,21,22,23]. Zinc is necessary for the activity of several enzymes, such as transphosphorylases, aldolases, RNA and DNA polymerases, dehydrogenases, and isomerases. It also plays a key role in membrane structure maintenance, cell division, tryptophan synthesis, and photosynthesis. Finally, zinc functions as a regulatory cofactor in the synthesis of proteins [15,24]. Due to its function in tryptophan production as an auxin precursor during biosynthesis, it also promotes pollen tube expansion [25]. Thus, a tree’s nutritional status and productivity are affected by the application of Zn fertilizer. Furthermore, zinc plays a part in fruit set and pollen formation as well as the synthesis of auxins and tryptophan [26,27]. The foliar feeding of zinc is more effective and has less of an environmental impact because of its low mobility in the soil and plant [28].

The majority of farmers mainly use either Zn sulfate (with its higher solubility and lower cost) or Zn chelates (but to a lesser extent) [29] for soil and foliar applications. Foliar sprays with synthetic fertilizers containing either zinc sulfate or EDTA-Zn chelate are convenient for field use, are sufficiently effective, and can improve crop growth. The development of crop production and agricultural practices has reached a tipping point with the current technological breakthroughs, meaning that new and innovative fertilizers with extremely high efficiency and minimal disadvantages can now be developed [30], such as nanotechnology fertilizers, which are effective in enhancing growth and yield and fruit quality parameters [31,32,33,34,35]. Because metal and metal oxide nanoparticles, like zinc, have distinct physicochemical properties, they can be used to develop novel fertilizers that perform as well as or better than their native macroparticulate compounds [36]. Many attempts were established to promote the productivity of mango trees including yield, fruit quality, and nutritional status by using conventional methods such as Zinc-containing compounds or unconventional methods such as Nanotechnology fertilizers which are effective in enhancing growth, yield, and fruit quality parameters [3,37,38,39,40]. However, many previous studies aimed to study the effect of zinc compared or combined with other compounds or factors and did not use more than two forms of zinc sources. So, there is a need to compare the effectiveness of more than two forms of zinc as a rapid method for the correction of Zn-deficient foliage in mangoes.

In a study by Khemira et al. [13], they reported that they discovered symptoms of zinc deficiency in several mango orchards in the Jazan region of Saudi Arabia. Therefore, the effect of zinc sulfate and zinc chelate on the leaves’ content of micronutrients and chlorophyll and its net photosynthesis rates were studied. However, the study ignored the effect of zinc in the form of ZnO NPs on the leaves’ content of macronutrients as well as crop parameters. In addition, the performance of a foliar Zn spray is determined by the solubility, pH, coverage of the spray solution, the age and anatomical structure of the leaves, relative humidity, wind, and air temperature at the time of spraying [41,42]. Furthermore, there is no systematic study available regarding the most effective method for ameliorating Zn deficiency using different sources of zinc, as the effects of the different types of zinc formulations (salts, chelates, and ZnO NPs and their application method on the nutritional status and yield of mango are still unexplored. Thus, our objective was to compare the influence of conventionally used zinc with ZnO NPs’ formulations to maximize zinc use efficiency enhancing the nutritional status in general, including the micro and macro elements, and the yield parameters of ‘Timor’ mango trees at harvest.

2. Materials and Methods

2.1. Experimental Site and Plant Materials

The current experiment was carried out during the two consecutive seasons of 2022 and 2023 on eight-year-old Timor mango trees (Mangifera indica L.) budded on Kutchineer seedling rootstocks. The trees were healthy, uniform, free of defects, spaced 2 × 4 m apart, and grown under a fertigation system in sandy soil at a private orchard in Jazan region, Kingdom of Saudi Arabia. All fertilizers were dissolved and injected into the irrigation system. The fertigation system comprised two laterals per row, one 0.5 m from each side of the row, and drippers (4 L/h) located one meter apart along the laterals.

Thirty healthy trees with as uniform growth and vigor as possible were chosen for the study, and they were exposed to the following treatments in three replicates for each treatment and one tree for each replicate (i.e., ten treatments × three replicates × one tree per replicate = thirty trees). The selected trees were sprayed with ZnO NPs at 100 ppm, zinc sulfate (ZnSO₄, 21% Zn) at a concentration of 0.1% (w/v), and chelated zinc (zinc EDTA, 12% Zn) at a concentration of 0.2% (w/v) on two occasions, 7 January and 4 weeks after the first application, either alone or in combination with each other during the two successive seasons of 2022 and 2023. The source of zinc nanoparticles were zinc oxide nanoparticles produced by Stem chemical, 7 Muliken Way, Newburyport, MA, USA; Zn chelated were Piosol Zn (chelated with Lignosulfonates) produced by Pioneers Chemicals Factory, Riyadh, Saudia Arabia; Karnataka Agro Chemicals, Bangalore, India manufactured zinc sulfate (Zn-21%). The trees were sprayed using a hand-pressure sprayer until saturation. To obtain good coverage and better penetration, Misr El-Dawliya’s Bio New film was added as a surfactant agent to all spray treatments, including the control, at a rate of 60 mL/100 L of water. Selected trees were divided into ten different treatments including the control as follows: control (water spray); A + A, ZnO NPs; B + B, chelated zinc; C + C, zinc sulfate; A + B, ZnO NPs + chelated zinc; A + C, ZnO NPs + zinc sulfate; B + A, chelated zinc + ZnO NPs; B + C, chelated zinc + zinc sulfate; C + A, zinc sulfate + ZnO NPs; C + B, zinc sulfate + chelated zinc.

The first foliar spray for treatments five and six contained ZnO NPs, while the second foliar spray contained chelated zinc and zinc sulfate. The first foliar spray for treatments seven and eight contained chelated zinc, while the second foliar spray contained ZnO NPs and zinc sulfate. The first foliar spray for treatments nine and ten contained zinc sulfate, while the second paper spray contained ZnO NPs and chelated zinc.

2.2. Measurements and Determinations

2.2.1. Nutritional Status

Through analysis of the mineral constituents and chlorophyll content of the leaves, the nutritional status of the mango trees was evaluated. A total of 40 matured leaves including the blade and petiole were collected from each mango tree at the terminal end of the 6–7-month-old branch (the fourth and fifth recently matured leaves from the terminal portion of the non-fruiting branch) in the first week of April 2, weeks before harvest [43], from each cardinal direction (north, south, east, and west) at working height (1–2 m from ground level) and combined into a single sample for each tree and packed in paper envelopes for the determination of the trees’ nutritional status. A sub-sample of both fresh leaves was immediately washed with water and used for chlorophyll content analysis according to the method of Li [44], which involved grinding 1 g of fresh leaves to a fine powder in liquid nitrogen in a pre-cooled mortar and then homogenization for 30 s in 5 mL of 95% acetone according to the method of Arnon [45]. According to the method of Helaly et al. [46], the extract was filtered, and then, the optical density of a fixed volume of filtrate was measured at a wavelength of 663 nm for chlorophyll a, 645 nm for chlorophyll b, and 440 nm for carotenoids using a spectrophotometer.

The remaining samples were washed with ultra-high pure water, then 0.1 N hydrochloric acid, distilled water, and lastly double-distilled water. The washed leaf samples were oven-dried at 70 °C for 48 h in a convection oven until a constant weight was obtained according to the method of Wilde et al. [47], and then, the samples were ground in a Wiley’s Stainless-Steel Micro Mill (Swedesboro, NJ, USA) on order to pass through a 40-mesh sieve to achieve homogeneous samples. The samples were stored in labeled, airtight, amber-colored glass bottles in a cool room (12 ± 2 °C) and analyzed in one batch to avoid variation from one batch to another during the analysis. To determine the leaf mineral contents of the samples, a 1 g sample of dried crushed leaf material from each tree was digested with sulfuric acid and hydrogen peroxide using the Evenhuis and De Waard procedure [48] and transferred quantitatively in calibrated flasks (100 mL). The nitrogen (N) and phosphorus (P) contents (%) of the digested solution were determined through the use of the micro-Kjeldahl method following the methods described by Chapman and Pratt [49], and total N and P were calorimetrically determined through the use of a spectrophotometer (9100UV-VIS, Manufacturer: PerkinElmer, Woodbridge, ON, Canada) according to the methods of Evenhuis [50] and Murphy and Riley [51], respectively. Potassium (K) content was determined using flame photometry (A&E-FP8501, A&E Lab (UK) Co., Ltd., London, UK), as described by Jackson [52]. The calcium (Ca), copper (Cu), magnesium (Mg), iron (Fe), and zinc (Zn) contents were determined using ionic chromatography plasma spectroscopy (Optical Emission Spectrometer; Perkin Elmer, Woodbridge, ON, Canada) according to the method of Donohue and Aho [53]. To determine the total carbohydrate contents of the leaves according to the method of Herbert et al. [54], a sample of dry tissue (0.2 g) was added to 10 mL H₂SO₄ (1 N); afterward, it was placed in a tube overnight in the oven at 100 °C, and total carbohydrates were determined through the use of the colorimetric method according to the method described by Smith et al. [55] at a wavelength of 490 nm using a spectrophotometer.

2.2.2. Productivity

Productivity was estimated by determining the fruit set, fruit drop, fruit retention, yield, and fruit yield increment. In the third week of January, bloom began. The majority of flowers were set on 3 March, and the fruitlet diameters were about 2 mm. Two primary branches (4.5 cm in circumference) growing in opposite directions were chosen and tagged during the flowering period in both seasons in order to calculate the following:

$$\mathrm{F}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{s}\mathrm{e}\mathrm{t} \mathrm{\%}=\frac{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t}\mathrm{s} \mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r} 15 \mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{u}\mathrm{l}\mathrm{l} \mathrm{b}\mathrm{l}\mathrm{o}\mathrm{o}\mathrm{m}}{\mathrm{p}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e}}\times 100,$$

$$\mathrm{F}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{r}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{\%}=\frac{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{r}\mathrm{e}\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{f}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{o}\mathrm{n} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{p}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e} \mathrm{u}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{l} \mathrm{h}\mathrm{a}\mathrm{r}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t}\mathrm{s} \mathrm{s}\mathrm{e}\mathrm{t} \mathrm{o}\mathrm{n} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{p}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{l}\mathrm{e} \mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r} 15 \mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{u}\mathrm{l}\mathrm{l} \mathrm{b}\mathrm{l}\mathrm{o}\mathrm{o}\mathrm{m}}\times 100,$$

Fruit drop (%) = 100 − Fruit retention.

When the fruits were fully ripe, they were harvested from each replication on 21 April in both seasons. The total yield of the trees was calculated by multiplying the weight of fruits per tree (kg) by the number of trees. The following formula was used to determine the increase in fruit yield when compared to the control:

$$\mathrm{F}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{i}\mathrm{n}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{\%}=\frac{\mathrm{F}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \left(\mathrm{K}\mathrm{g}\right) \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{e}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y} \mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t} - \mathrm{F}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \left(\mathrm{K}\mathrm{g}\right) \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}{\mathrm{F}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \left(\mathrm{K}\mathrm{g}\right) \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$$

2.3. Statistical Analysis

The study treatments were arranged in a randomized complete block design (RCBD) using the method of Gomez and Gomez [56]. All data obtained from both seasons were subjected to statistical analysis using SAS, version 9.13 [57], for the analysis of variance.The least significant difference (LSD) was used to compare the treatment means at a 0.05 probability level [58].

3. Results

3.1. Nutritional Status

The concentrations of several nutrients in the leaves of mango trees cv. Timor are shown in

Table 1. Effect of the sprayed sulfate, chelated, and ZnO NPs on the leaf mineral contents of Timor mango trees in the 2022 and 2023 seasons.

Season	Treatment	N (%)	P (%)	K (%)	Ca (%)	Mg (%)	Cu (ppm)	Fe (ppm)	Mn (ppm)	B (ppm)	Zn (ppm)
2022	Control	1.17 j	0.12 h	0.43 j	1.64 j	0.17 j	6.25 j	84.50 j	16.50 j	7.00 j	21.00 j
	A + A	1.98 a	0.36 a	0.98 a	2.72 a	0.79 a	22.75 a	192.00 a	97.25 a	28.75 a	120.00 a
	B + B	1.59 f	0.22 e	0.82 f	2.21 f	0.48 f	12.75 f	147.75 f	57.75 f	14.50 f	81.00 f
	C + C	1.34 i	0.16 g	0.56 i	1.96 i	0.27 i	7.50 i	123.50 i	29.00 i	9.50 i	58.75 i
	A + B	1.91 b	0.34 b	0.95 b	2.64 b	0.76 b	20.50 b	186.00 b	90.75 b	26.00 b	113.00 b
	A + C	1.85 c	0.33 b	0.93 c	2.55 c	0.72 c	18.50 c	179.00 c	85.25 c	24.50 c	105.00 c
	B + A	1.76 d	0.29 c	0.91 d	2.41 d	0.64 d	16.50 d	163.00 d	76.75 d	19.00 d	98.00 d
	B + C	1.54 g	0.19 f	0.75 g	2.14 g	0.42 g	10.50 g	137.00 g	45.00 g	12.50 g	73.00 g
	C + A	1.69 e	0.26 d	0.86 e	2.29 e	0.58 e	14.50 e	156.00 e	69.75 e	17.00 e	90.25 e
	C + B	1.42 h	0.17 g	0.68 h	2.05 h	0.37 h	8.75 h	130.25 h	38.25 h	11.25 h	66.50 h
	LSD0.05	0.020	0.014	0.010	0.021	0.013	0.879	2.095	1.637	1.148	1.498
2023	Control	1.16 j	0.07 i	0.40 j	1.49 j	0.16 j	6.00 j	81.75 j	14.75 j	6.25 j	21.25 j
	A + A	1.84 a	0.33 a	0.95 a	2.55 a	0.74 a	20.50 a	166.50 a	76.25 a	26.50 a	112.00 a
	B + B	1.52 f	0.18 f	0.75 f	2.14 f	0.39 f	11.25 f	134.25 f	44.00 f	14.00 f	80.25 f
	C + C	1.31 i	0.12 h	0.46 i	1.93 i	0.22 i	7.75 i	112.75 i	20.75 i	9.50 i	58.50 i
	A + B	1.75 b	0.29 b	0.93 b	2.46 b	0.68 b	18.50 b	160.00 b	72.50 b	24.00 b	107.50 b
	A + C	1.71 c	0.27 c	0.91 c	2.35 c	0.63 c	16.75 c	156.50 c	67.00 c	21.50 c	103.50 c
	B + A	1.63 d	0.22 d	0.85 d	2.28 d	0.55 d	14.00 d	146.75 d	59.00 d	18.00 d	95.00 d
	B + C	1.43 g	0.16 f	0.68 g	2.07 g	0.31 g	9.75 g	126.00 g	36.50 g	12.50 g	74.50 g
	C + A	1.57 e	0.20 e	0.82 e	2.21 e	0.47 e	12.50 e	141.00 e	51.00 e	16.50 e	89.75 e
	C + B	1.38 h	0.14 g	0.56 h	2.01 h	0.27 h	8.75 h	120.00 h	31.00 h	11.25 h	67.50 h
	LSD0.05	0.015	0.013	0.012	0.020	0.014	0.788	1.937	1.466	0.987	1.893

Mean values within a column for each season that are followed by different letters are significantly different at p ≤ 0.05. A: ZnO NPs; B: chelated zinc; C: zinc sulfate.

. The leaf minerals of Timor mango trees in terms of N, P, K, Ca, Cu, Mg, Fe, Mn, Zn, and B content were significantly affected by all treatments (p < 0.05) in the 2022 and 2023 seasons compared with the control. In both study seasons, as a result of foliar spraying with ZnO NPs treatment on two occasions, this treatment gave the highest leaf N, P, K, Ca, and Mg content (%) and Cu, Fe, Mn, B, and Zn content (ppm) compared with the other treatments followed by ZnO NPs + chelated zinc, ZnO NPs + zinc sulfate, chelated zinc + ZnO NPs, zinc sulfate + ZnO NPs, and then the applications of chelated zinc. The control had a significantly lower leaf mineral content than the other treatments in both seasons. Moreover, in 2022, no significant differences were found between the leaf concentration of phosphorus (P) in the leaves of the treatment with ZnO NPs + chelated zinc and ZnO NPs + zinc sulfate and also between the treatment with zinc sulfate + chelated zinc and foliar spray with zinc sulfate treatment twice. Furthermore, concerning leaf phosphorus content, no significant differences were found for the treatment of chelated zinc + zinc sulfate and foliar spray with chelated zinc twice in the second season.

Analysis of the data shown in Figure 1A–E revealed that all spraying treatments during the “on” or “off” year significantly increased leaf chlorophyll a, chlorophyll b, total chlorophylls, and total carotenoids (mg/g f.w) as well as total carbohydrates content (mg/g f.w) compared with the control. The data presented that the tree sprayed with ZnO NPs alone two had significantly higher averages of total carbohydrates, total carotenoids, total chlorophylls, leaf chlorophyll a, and chlorophyll b in both seasons when compared with the other treatments. The highest total carbohydrates, total carotenoids, total chlorophylls, leaf chlorophyll a, and chlorophyll b content were 78.0%, 0.95 mg/g f.w, 3.2 mg/g f.w, 1.91 mg/g f.w, and 1.29 mg/g f.w, respectively, in the 2022 season and, in the 2023 season, 63.50%, 0.86 mg/g f.w, 2.84 mg/g f.w, 1.64 mg/g f.w, and 1.21 mg/g f.w, respectively, were recorded with two applications of ZnO NPs alone. It was also found that the other treatments ranked behind two applications of nano zinc alone in descending order as follows: ZnO NPs + chelated zinc, nano zinc + zinc sulfate, chelated zinc + ZnO NPs, zinc sulfate + ZnO NPs, chelated zinc + chelated zinc, chelated zinc + zinc sulfate, zinc sulfate + chelated zinc, and zinc sulfate alone twice as compared to the control.

3.2. Productivity

The effects of the spray containing mineral, chelated, and ZnO NPs on the number of fruits/tree fruit retention (%), fruit set (%), and fruit drop (%) are presented in

Table 2. Effect of the tree sprayed with chelated sulfate and ZnO NPs on the fruit yield parameters of Timor mango trees in the 2022 and 2023 seasons.

Season	Treatment	Fruit Set (%)		Fruit Retention (%)		Fruit Drop (%)		Number of Fruits/Tree
		2022	2023	2022	2023	2022	2023	2022	2023
2022	Control	2.75 h	3.25 i	13.50 i	12.42 h	86.50 a	87.58 a	81.00 j	152.00 i
	A + A	9.25 a	10.00 a	24.80 a	27.60 a	75.20 i	72.40 h	132.25 a	200.00 a
	B + B	5.50 ed	6.25 fe	18.72 f	19.07 e	81.28 d	80.93 d	106.50 f	184.25 e
	C + C	3.75 g	4.75 h	16.00 h	16.55 g	84.00 b	83.45 b	96.00 i	172.25 h
	A + B	8.25 b	8.75 b	23.61 b	26.60 a	76.39 h	73.40 h	127.50 b	198.50 a
	A + C	7.50 b	8.00 bc	22.58 c	25.48 b	77.42 g	74.52 g	124.75 c	195.00 b
	B + A	6.50 c	7.25 dc	20.76 d	22.47 c	79.24 f	77.54 f	116.25 d	192.50 c
	B + C	4.75 ef	5.75 fg	17.05 g	18.55 ef	82.95 c	81.45 cd	104.25 g	181.50 f
	C + A	6.00 cd	6.75 de	19.82 e	20.11 d	80.19 e	79.89 e	112.00 e	186.50 d
	C + B	4.50 gf	5.25 hg	16.30 gh	17.63 f	83.70 bc	82.38 c	100.75 h	176.75 g
	LSD0.05	0.803	0.991	0.878	1.026	0.879	1.025	1.952	1.661

Mean values within a column for each season that are followed by different letters are significantly different at p ≤ 0.05. A: ZnO NPs; B: chelated zinc; C: zinc sulfate.

, with the results showing that, as a general trend, the applied treatments significantly increased the fruit set (%), fruit retention (%), and number of fruits/tree with respect to the untreated control in the 2022 and 2023 seasons (Table 2). In contrast, data from both seasons revealed that all treatments significantly decreased the percentage of fruit drop in comparison with the control. However, the most effective treatment was two applications of ZnO NPs at 100 ppm, which induced the highest number of fruits/tree and fruit set as well as the highest level of fruit retention (%), followed by ZnO NPs + chelated zinc as compared with the other treatments. Combining all foliar applications of ZnO NPs + chelated zinc and ZnO NPs + zinc resulted in a significantly higher fruit set percentage than chelated zinc+ ZnO NPs which did not significantly vary from zinc sulfate + ZnO NPs in the 2023 season. In both seasons, no significant difference was observed between trees sprayed with ZnO NPs + chelated zinc or ZnO NPs + zinc sulfate which did not vary significantly compared to trees sprayed with chelated zinc + ZnO NPs in the second season regarding the fruit set percentage. However, no significant difference was found in the second season regarding fruit retention (%) and the number of fruits/trees between the trees sprayed with nano zinc alone twice or ZnO NPs + chelated zinc.

The data representing the effects of trees sprayed with mineral, chelated, and ZnO NPs on the yield and yield increment of Timor mango trees in the 2022 and 2023 seasons are presented in Figure 2 and Figure 3. In both seasons, all treatments significantly enhanced the yield increment and yield (kg/tree) compared with the untreated control. In addition, the results presented in Figure 2 and Figure 3 show that all trees treated with ZnO NPs alone twice showed a significantly increased yield (kg/tree) and yield increment compared to the other treatments followed by ZnO NPs + chelated zinc, ZnO NPs + zinc sulfate, chelated zinc + ZnO NPs, and then chelated zinc alone twice, respectively; in comparison, the control showed the lowest values. In addition, the data indicated that Timor mango trees sprayed with ZnO NPs at 100 ppm on 7 January and 4 weeks after the first application produced the highest yield (kg/tree) and yield increment (%)—(52.10 and 117.41.27)—(76.27 and 76.44) in the “Off” year (2022), first, and “On” year (2023), respectively.

4. Discussion

Although the leaf nutrient content varies not only between cultivars but also depending on the type of soil, phenological stage, age, position, and orientation of the leaf, and climatic conditions, foliar analysis is the most useful tool for properly establishing a mango fertilizer program. Regarding nutritional supply and metabolic activity, fruit trees’ leaves are the most responsive and active plant organs [59]. As a result, the leaves can more accurately reflect the total nutritional health of fruit trees and using plant leaf mineral nutrition determination to examine the nutritional status of fruit trees is a useful method [60]. Since zinc is immobile in soil, foliar zinc treatment is recommended. Moreover, using zinc topically is more effective, and Zn is readily available as plants can absorb Zn through the surface of their leaves more rapidly than in the soil [17]. Therefore, the application of Zn foliar spray is a better approach to ameliorating deficiencies in leaf mineral content to improve overall fruit yield. Zn is required for dehydrogenase activity, aldolases, isomerases, transphosphorylases, and DNA and RNA polymerases [61,62]. It is vital in starch metabolism and works as a co-factor for various enzymes, influencing photosynthetic reactions, protein biosynthesis, and nucleic acid metabolism [17]. Its critical importance can be related to the fact that it synthesizes the amino acid tryptophan, which is a precursor of IAA, a phytohormone that significantly influences plant growth [63].

In the present study, it is clear from the data (Table 1 and Figure 1A–E) that chlorophyll a, chlorophyll b, total chlorophyll, total carotenoid, total carbohydrates, and the leaf mineral content, including nitrogen (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), iron (Fe), copper (Cu), and zinc (Zn), of the leaves were significantly increased in both seasons regardless of the application of the zinc forms. Zinc is one of the most essential micronutrients involved in many physiological functions that occur in plant cells that enhance the chemical composition of mango leaves [64]. Zinc plays a role in chlorophyll synthesis because of its effect on carbohydrate, protein, and energy metabolism [65]. It is commonly known that increased zinc intake boosts mesophyll cells’ chlorophyll content, which in turn improves photosynthesis [66].

Zinc fertilizers come in a variety of forms, such as Zn chloride (ZnCl₂); Zn sulfate (ZnSO₄); Zn oxide (ZnO); zinc oxide nanoparticles (ZnO NPs); Zn-coated urea/superphosphate; zinc chelate; Zn oxy-sulfate. Zinc sulfate, zinc oxide, and chelates are the most extensively used Zn fertilizers in the world, greatly improving Zn nutrition plant growth and yield [67]. Although Zn sulfate (ZnSO₄) has traditionally been the most commonly used Zn source, more recently, zinc chelate sprays are currently cheap and give satisfactory results. According to the previously presented results, it is necessary to use zinc chelate better than zinc sulfate. According to several researchers, positive effects of zinc nutritional status and mango productivity were reported by Merwad et al. [68] by the foliar application of zinc chelate at 0.2% (as EDATA 13% Zn, on Alphonso mango trees; Ahmad et al. [38] by foliar and soil applications of ZnSO₄ @ 300 g (0.5%) on Chaunsa (cv. white) mango trees; Maklad et al. [39] by the foliar application of 13% chelated zinc at 2.5 mL/L on mango Ewais cultivar; Elsheery et al. [3] by foliar spray of nan-chelate zinc oxide (50, 100, and 150 mg/L) on mango Ewais cultivar; Dhotra et al. [40] by the foliar application of 0.8% ZnSO₄ of mango cv Dashehari.

Zinc is a co-factor of over 300 enzymes and proteins and is a constituent of enzymes involved in leaf cell photochemistry such as ribulose-1,5-bisphosphate carboxylase oxygenase, and carbonic anhydrase [24,69]. It retains the structural alignment of phospholipids and membrane proteins while also maintaining ion transport systems [70]. Therefore, zinc has promising effects on biochemical processes in plant metabolism because it tends to form tetrahedral complexes with N-, O-, and S-donor ligands [24]. In addition, zinc is essential for the synthesis of the natural hormone IAA, activating certain enzymes for chlorophyll biosynthesis [17] and improving net photosynthetic assimilation rates [13].

Different zinc forms play a role in physiological processes such as enzyme promotion and photosynthesis product transfer, as well as cell division and elongation, which leads to increased growth and thus increased leaf mineral content and chlorophyll, total carotenoid, and total carbohydrate contents. The results concerning the influence of different zinc forms on vegetative growth and leaf mineral contents are in line with those reported by Ebeed et al. [71] and Abd El-Razek et al. [72], as spraying mango trees with zinc improved tree growth and increased the leaf mineral content of N, P, and K in their studies [71,72]. The importance of zinc in improving nutritional status may be due to its function in promoting photosynthesis, nucleic acid, protein synthesis, and lipid metabolism, the detoxification of reactive oxygen species (ROS), membrane stability, and the synthesis of tryptophan and auxins, and their transport to the leaves [26,73,74,75]. In addition, zinc may play a role in the cytoplasmic regulation of other nutrient contents such as iron (Fe) and magnesium (Mg), which are directly related to chlorophyll synthesis [65]. The noted increases in chlorophyll content are the result of zinc’s crucial role in plant metabolism. Zinc affects the activity of vital enzymes such as carbonic anhydrase, which comprises a zinc atom and catalyzes the hydration of CO₂ to facilitate carbon dioxide’s diffusion to plant carboxylation sites [24,69]. It is clear that foliar spray of conventional and/or nano zinc of mango trees encouraged a significant regulate the activities of antioxidant enzymes and an increase in the accumulation of most physiological behavior parameters of Timor mango transplants, i.e., total carbohydrates [26]. Similar results were obtained, demonstrating the important role of zinc in carbohydrate and protein metabolism by increasing photosynthetic activity and enhancing carbohydrate and by-product metabolism [3].

The data (Table 2 and Figure 2 and Figure 3) clearly show that fertilizing the trees with mineral, chelated, and nano zinc significantly improved the fruit set (%), fruit retention (%), number of fruits/trees, yield, and yield increment and decreased the percentage of fruit drop when compared to the control. The improvement in fruit set, fruit retention, number of fruits/trees, yield, and yield increment was associated with an improvement in the leaves’ mineral content, chlorophyll contents, total carotenoids, and total carbohydrates contents as a result of the application of the zinc forms compared to the untreated trees.

Zinc’s role in floral bud differentiation may explain the rise in mango tree production following its application [76]. According to Usenik and Stampar [77], Zn plays a significant role in auxin production and enhances metabolite translocation to the site of bud initiation or the bud itself. Zinc is therefore essential for fruit development and retention as well as overall fruit output and quality [78]; on the other hand, low zinc levels impair plant growth, resulting in smaller leaves and a lower yield overall [79]. Exogenous Zn treatment to mango trees improves the production of mango fruits (Table 2 and Figure 2 and Figure 3), which is reliant on the percentage of fruit retention and the number of fruit sets per panicle. This is due to the role of chlorophyll and these elements in improving photosynthesis and their role in improving flowering, fruit set, and fruit yield parameters. Zinc (Zn) plays a key role in fruit set, retention, and fruit yield [80]. Furthermore, Bally [81] revealed in their study that foliar applications of different Zn fertilizers increase flower induction, pollination, fruit set, and yield in mango trees. The positive effect of the foliar application of zinc in increasing mango productivity has been cited by Singh and Maurya [82], Ranjit et al. [83], and El-Gioushy et al. [17]. In addition, Tripathi and Kumar [84] revealed in their study that zinc improves the biochemistry of flowers, leading to an increased number of fruits per panicle and enhanced fruit retention percentage in mango trees. Mango cultivars treated with 1 g/L of nano zinc showed increased levels of leaf zinc and NPK [85].

Among the Zn compounds used as fertilizers, the Zn chelates are the most common over zinc sulfate in terms of their water solubility, gradual release, and absorption by plants when compared to zinc sulfate fertilizers [86,87]. The utilization of the most economically efficient Zn sources combined offers the possibility for a long-term reduction in production costs and better tree Zn nutrition [88]. Chelated fertilizers such as Zn-chelated have been developed to increase their use efficiency and protect them from oxidation and precipitation [86]. In addition, Zinc sulfate applied during the wheat growth stage was 1.4–1.7 times less effective than zinc chelate applied topically [89]. The zinc solubility stability of zinc chelate is more than zinc sulfate under different pH values and plant conditions [90]. Zinc solubility and stability depend on its type to allow for maximum up-take by foliar [91]. It was found that zinc chelate is more stable than zinc sulfate under different pH values and plant conditions [87]. In this regard, Crowley et al. [92] studied the retention and translocation of Zn in avocado leaves using foliar-applied ZnSO₄ and Zn metalosate. Experiments were carried out with 65Zn-radiolabeled materials applied as spots to either the top or bottom of the leaf surfaces of greenhouse-grown, containerized trees. The investigations found that the leaf surface retained less than 1% of the applied Zn following washing with soap and water followed by an acid wash. One exception was Zn metalosate, which retained 3.8% of the total material applied when applied to the leaf’s bottom side. Applying ZnSO₄ and Zn metalosate to the upper leaf surface increased Zn content by 35 and 72 µg·g⁻¹, respectively. These values fall within the same range as the trees treated with foliar fertilizer in the field studies. ZnSO₄ and Zn metalosate applied to the upper leaf surface did not show statistically significant differences. On the other hand, when ZnSO₄ and Zn metalosate were applied to the abaxial leaf surface as opposed to the adaxial surface, the amounts retained were considerably higher (6- and 24-fold, respectively).

Foliar applications of zinc improve productivity which ultimately increases yield because it aids in regulating stomatal openings, improves photosynthetic efficiency, and increases the chlorophyll formation of plants [93,94,95]. The reduction in fruit abscission could be attributed to the induction of enzymatic antioxidants in the fruit petiole [10]. The application of Zn improves antioxidant enzymatic activities and regulates stomatal conductance and photosynthesis, which facilitates the scavenging of reactive oxygen species (ROS) during fruit abscission [95]. Removing ROS helps to maintain biomolecules such as pigments, lipids, proteins, carbohydrates, and DNA, thus preventing cell death. Therefore, the increases in total chlorophylls described herein could be a sign of ROS elimination [10].

The highest values of productivity and nutritional status were significantly associated with Timor mango trees that were subjected to the treatment of ZnO NPs in this study. The results of this study suggest that foliar application of ZnO NPs at 100 ppm had a greater impact on the nutritional status and productivity of Timor mango trees than conventional Zn (ZnSO₄) and chelated Zn; this greater impact was likely due to its greater capacity to be absorbed by the leaves. The trees that received ZnO NPs were found to have considerably higher leaf mineral concentrations than those sprayed with the sulfate or chelated form of Zn.

The majority of farmers use either ZnSO₄ or EDTA-Zn chelate for soil and foliar applications; however, the retention period of Zn in the plant system is limited when using these fertilizers [96]. Spraying trees with ZnO NPs helps to release the required nutrients gradually in small amounts and improves the spraying efficiency of zinc compared to the sulfate or chelated forms [97]. Foliar applications of nano-fertilizers were demonstrated to be effective because smaller quantities are required, and they provide nutrients to plants in a gradual and regulated manner compared to conventional fertilization [98,99,100]. Furthermore, Kah et al. [100] revealed in their study that the effectiveness of nano-agrochemicals can surpass that of conventional products by up to 30%. In a study on fruit trees, the use of nanoparticles was found to improve vegetative growth and enhance reproductive growth and flowering, thereby increasing yield parameters and fruit quality [101]. Nano-fertilizers enhance the rate of photosynthesis in the plant, increasing carbohydrates and dry matter production and thus enhancing plant vegetative growth in general [102].

At present, nano-fertilizers are widely used in agriculture and horticulture as they can provide nutrients to plants gradually and under-regulated conditions, which is in contrast to the application of conventional fertilizers [103]. Nanoparticles offer greater potential for enhancing nutrient usage efficiency by providing more areas for different metabolic reactions to take place in plants, as well as an increase in the rate of photosynthesis, which leads to an increase in dry matter production and yield [102,104,105,106]. Our results were similar to those obtained by Elsheery et al. [3] for Ewais mango trees (Mangifera indica L.). The authors revealed that an-chelate zinc oxide (50, 100, and 150 mg/L) at full bloom and one month after salt-stressed “Ewais” mango trees significantly improved all the investigated plant growth, nutrient uptake, and carbon assimilation have enhanced with different levels of ZnO NPs compared with the control. Similar results were obtained by Genaidy et al. [107] reported that the exogenous applied nanoparticles at a rate of 100 and 200 ppm on Picual olive trees significantly improved the contents of total carotenoids, chlorophylls a and b, total chlorophylls, and nutrients specifically phosphorous, nitrogen, potassium, and magnesium in comparison to the control treatment. In addition, when compared to the control treatment, the application of zinc nanoparticles in various concentrations was able to influence the pigments and mineral content of leaves, fruit set percentage, fruit drop percentage, and fruit yield.

5. Conclusions

From the above results, it can be concluded that all zinc forms—ZnO NPs, chelated (Zn-chelated), and sulfate (ZnSO₄)—had a favorable impact on the leaf chlorophyll contents and leaf mineral constituents, fruit drop, fruit set, yield, and fruit yield increment and improved the fruit retention of Timor mango trees. Spraying mango trees with ZnO NPs at 100 ppm was found to be a beneficial treatment to enhance the growth vigor and nutritional status of mango trees in order to enhance productivity. These highlighted results will, in due course, enable farmers to achieve maximum productivity of this crop.