Improving the productivity and reducing the drop percentages of fruits in pear by the external application of some plant growth regulators

Abstract

Fruit drop from pear trees causes serious losses in income. However, the application of plant bio-regulators improves the internal physiology of developing fruit by ensuring that they receive an adequate supply of water, nutrients, and other compounds necessary for their proper growth and development, which leads to improved size, quality, and ultimately better yield in a variety of fruit crops. This study investigated the foliar application of three plant growth regulators: CPPU at 10, 15, and 20 ppm, GA₃ at 25, 50, and 75 ppm and NAA at 25, 50, and 75 ppm. The pear trees were sprayed four times: before flowering, full bloom, after three weeks, and after six weeks. The results showed that the spray of GA₃ at 50 and 75 ppm gave the highest effect in increasing the shoot length, shoot thickness, leaf area, and leaf total chlorophyll. The spraying of NAA at 50 and 75 ppm was the best treatment in increasing the fruit set percentages, fruit yield, fruit weight, and fruit dimensions as well as the fruit content from soluble solids, and fruit sugars, while they reduced the fruit drop percentages comparing with the other applied treatments.

INTRODUCTION

Pear (Pyrus communis L.) belongs to the Rosaceae family and the Pyrus genus, which includes twenty-two species found in Asia, Europe, and northern Africa. ‘Le-Conte’ is the essential pear cultivar grown in Egypt and it resulted from a hybridization between Pyrus communis x Pyrus serotina. It is one of the most important pear trees in the world, and it is cultivated in whole temperate-zone countries. The cultivated area in Egypt is approximately 5154 hectares, which has resulted in approximately 74,000 tons (FAO 2021). The productivity of pear cv. ‘Le-Conte’ in Egypt changes from year to year, and this may be as a result of a reduction in ovules viability and stigma receptivity, pollen development rates, ovule abortion, increased flower abscission, and reduced fruit reservation (Yehia and Hassan 2005). In general, pear is the 3^rdlargest crop from the cultivated area among the deciduous fruit trees.

Plant growth regulators (PGRs) play a vital part in improving horticultural crops’ yield and fruit quality (Velasquez et al. 2016). The exogenous spraying of PGRs is used often to improve the fruit size, increase the cell division, and minimize the fruit number by encouraging flower formation and reducing the flower and fruit shedding (Davenport 2011; Agustí et al. 2022). Additionally, they are organic chemical compounds, which regulate the plant’s physiological processes when they are applied in small concentrations, where they can induce the fruit set, minimize the fruit drop and raise the productivity and quality characteristics (Bons and Kaur 2020). PGRs are also signaling molecules that influence fruit growth, blooming rates, and plant cell division (Cutler and Nelson 2017; Talat et al. 2020).

CPPU (Sitofex) is a synthetic cytokinin that plays a paramount role in increasing fruit size by inducing cell division or cell increase in many fruits such as sweet cherry, apple, kiwifruit, grape, and pear (Zhang and Whiting 2011). Cytokinins have the ability to increase the fruit soluble solids and reduce the coloration of the exocarp. As cytokinins induce the growth of the floral meristem, it increases the flower number (Li et al. 2019). Aremu et al. (2020) found that cytokinins are critical for enhancing fruit development, floral and fruit growth, and fruit weight. They also help to preserve and enhance the texture, flavour, and aroma of fruit in a variety of fruits, including raspberries, kiwis, litchi, grapes, sweet cherries, and strawberries.

Gibberellins have the ability to ameliorate the fruit set percentages and growth in apple (Watanabe et al. 2008). Moreover, they are mostly applied to reduce the drop percentage and to increase the quality of fruit (Kumari et al. 2018). Aremu et al. (2020) reported that gibberellins enhance physiological functions such as fruit production, flower initiation, leaf elongation, and stem development. By promoting photosynthetic enzymes and increasing the efficacy of mineral utilization, they also enhance the process of photosynthesis. Additionally, they noticed that in grapes, sweet cherries, strawberries, kiwifruit, and raspberries, gibberellins improve fruit set percentages, growth, size, fruit preservation, and fruit quality attributes, such as fruit weight, hardness, length, and diameter.

As a broad-spectrum regulator of plant growth, NAA is an auxin analogue that stimulates cell division and expansion (Gill and Bal 2009). The synthetic auxin NAA helps to promote root development, vascular tissue differentiation, cell lengthening, apical control, fruit setting percentage, and the prevention of leaf or fruit loss (Mehraj et al. 2015). Additionally, the foliar spraying of NAA at 25 to 50 ppm on apple markedly enhanced the fruit set and retention (Osama et al. 2015). It is mostly utilized to improve the production of strawberries like improving the fruit from total sugars, ascorbic acid, and on the opposite side to lessen the titratable acidity (Bhople et al. 2020). So, the purpose of this study was to find out how foliar application of CPPU, GA₃, and NAA could improve vegetative growth, and increase fruit set percentages, productivity, and quality while decreasing fruit drop percentages.

EXPERIMENTAL

Treatments, Location and Design

The experiment was conducted in 2022 and 2023 on 10-year-old pear trees (Pyrus communis L.) budded on Pyrus betulifolia rootstock. The trees were planted at 4*4 meters in sandy soil within a private orchard under drip irrigation, in Nubaria region, El-Beheira governorate, Egypt. The analysis of the soil is shown in Table 1 (Sparks et al. 2020).

Table 1. Physiochemical Soil Analysis

Sixty uniform trees, selected for their similar growth and size, received consistent agricultural practices throughout the two-year experiment. The trees were sprayed four times: before flowering, during full bloom, three weeks after the full bloom spray, and three weeks after the third spray, using water as the control, 10, 15 and 20 ppm CPPU, 25, 5 and 75 ppm GA₃ and 25, 50, and 75 ppm NAA. The trees were randomly distributed and arranged in a Randomized Complete Block Design (RCBD) in six replicates (six trees).

Vegetative Parameters

At the end of the vegetative time, the shoot diameter was in cm and the shoot thickness was measured by using a vernier caliper. Total chlorophyll (SPAD) was measured in fresh leaves by taken from 10 mature leaves located in the middle section of the shoots surrounding the trees. The average leaf area (cm²) was determined using the below equation (Mosa et al. 2022),

LA = 0.70 (L × W) – 1.06 (1)

where LA is the leaf area, L is the leaf length, and W is the leaf width.

Fruit Set Percentages and Fruit Drop Percentages

In March of 2022 and 2023, four carefully selected branches from each side of the experimental trees were labeled, and the number of blooms on each branch was recorded. The fruit set percentage was then calculated using Eq. 2 (El-Hady et al. 2007).

(2)

By computing the quantity of dropped fruits from fruit planting until the harvesting date in June of each year, pre-harvest fruit drops were estimated. Next, the fruit drop’s proportion was determined using the formula below.

(3)

Fruit Yield

During the July 2022 and 2023 seasons, the yield in kg for each tree was weighted and then by multiplying the yield of the tree * the number of trees in a hectare to calculate the yield of hectare in ton.

Fruit Quality

Fruit physical characteristics

Thirty fruits were randomly selected from each replicate (tree). Measurements were made of the average of their weight (g), fruit length, and fruit diameter by using a vernier caliper. Fruit firmness (Ib/inch²) was determined using a Magness and Taylor pressure tester equipped with a 7/18-inch plunger. Fruit size (cm³) was assessed by measuring the volume of displaced water after immersing the fruit.

Fruit chemical characteristics

Total soluble solids percentages were measured by using the hand refractometer (ATAGO Co. LTD., Tokyo, Japan). Ascorbic acid content (VC) in the juice was assessed through titration with 2,6-dichloro phenol-indo-phenol and expressed in milligrams per 100 mL of juice. Total and reducing sugars were quantified calorimetrically using the Nielsen’s method (Nielsen 2010). Non-reduced sugars percentage is the difference between total sugars and reduced sugars. Fruit acidity, expressed as a percentage of malic acid content, was determined in fruit juice through titration with 0.1 N sodium hydroxide, using phenolphthalein as an indicator (A.O.A.C. 2005).

Mineral content in the apricot leaves

From the middle part of the shoots, 30 leaves were collected in July after fruit picking from each tree (Arrobas et al. 2018) to analyze macronutrients (nitrogen, phosphorus, potassium) and micronutrients (iron, zinc, manganese, boron). The leaves were washed with tap water, then distilled water, and dried in an oven at 70 °C until they reached a consistent weight before being thoroughly crushed. The samples were then digested using 2 mL of H₂SO₄ and H₂O₂. The nitrogen content in the leaves was measured using the micro-Kjeldahl method (Wang et al. 2016). Phosphorus was determined by the Vanadomolybdate method (Weiwei et al. 2017), and potassium was assessed using a flame photometer (SKZ International Co., Ltd., Jinan Shandong, China) (Chapman 2021). The concentrations of zinc, manganese, and iron were measured with an Atomic Absorption Spectrophotometer, while boron was quantified using a spectrophotometer at 430 nm.

Statistical analysis

The results were statistically analyzed using One Way ANOVA for a Randomized Complete Block Design (RCBD). The means of the treatments were compared using the Least Significant Difference at 0.05 (LSD_0.05) (Snedecor and Cochran 2021).

RESULTS AND DISCUSSION

Vegetative Growth Parameters

Table 2 reveals that external application of CPPU, GA₃, and NAA on pear trees can enhance shoot length, thickness, leaf area, and total chlorophyll content compared to unsprayed trees. The highest significant improvements were observed with foliar spraying of GA₃ at 75 ppm, followed by 50 ppm. Moreover, spraying pear trees with 75 ppm NAA and CPPU at 20 ppm significantly increased these vegetative parameters. The differences between the influence of 75 and 50 ppm from GA₃ were not significant in the shoot length and leaf total chlorophyll in both seasons and also there were insignificant differences between them in the first season in shoot thickness, and leaf are but in the second seasons the differences were significant.

Table 2. Effect of the Foliar Spraying of CPPU, GA3, and NAA on Shoot Length, Shoot Thickness, Leaf Area, and Leaf Total Chlorophyll of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

Fruit Set and Fruit Drop Percentages, and Fruit Number

Table 3 indicates that the application of plant growth regulators such as NAA, GA₃, and CPPU enhanced both the fruit set percentages and the number of fruits while decreasing the rate of fruit drop across both experimental seasons. Furthermore, the application of 75 ppm NAA resulted in the most obvious increases in both fruit set percentages and fruit numbers, proving to be the most effective treatment across both experimental seasons.

Table 3. Effect of the Foliar Spraying of CPPU, GA3 and NAA on Fruit Set and Fruit Drop Percentages and Fruit Number per Tree of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

This treatment also notably reduced the percentage of fruit drop during the two testing periods. Additionally, significant improvements in fruit quantity and fruit set ratio were achieved compared to untreated trees through the foliar applications of 50 ppm NAA, as well as 75 and 50 ppm GA₃, and 20 ppm CPPU throughout both seasons. The reduction in fruit drop percentage by 50 and 75 ppm NAA was similarly effective across the two seasons.

Fruit Weight and Fruit Yield

Table 4 shows that, across both seasons, foliar applications of 75, 50, and 25 ppm NAA, 75 and 50 ppm GA₃, and 10, 15, and 20 ppm CPPU significantly increased fruit weight and yield, both in kg per tree and tonnes per hectare when compared to untreated trees.

Additionally, the spraying of 75 ppm NAA was the superior treatment that gave the highest notable increments in both studying seasons over the other sprayed treatments. The spraying of 50 ppm NAA, 75 GA₃ and 20 ppm CPPU were more efficient in increasing the fruit weight, and fruit yields in kg or ton rather than the other sprayed treatments.

Table 4. Effect of the Foliar Spraying of CPPU, GA3, and NAA on Fruit Weight, Fruit Yield in kg and in ton per Hectare of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

Fruit Quality

Physical fruit characteristics

According to Table 5, the application of 75 and 50 ppm NAA, 75 and 50 ppm GA₃, and 20 and 15 ppm CPPU significantly enhanced the physical characteristics of the fruits, including size, length, and firmness, compared to trees that were not sprayed across the two seasons.

The most significant increments in these parameters were significantly noticed when the trees were sprayed at 75 ppm NAA in the two seasons. The fruit diameter was remarkably increased by the spraying of the pear trees with 75, 50, and 25 ppm NAA and 20 ppm CPPU significantly improved it in the two seasons.

Table 5. Effect of the Foliar Spraying of CPPU, GA3, and NAA on Fruit Size, Length, Diameter and Firmness of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

Fruit chemical characteristics

In comparison to the trees that were not sprayed throughout the two seasons, the findings in Table 6 showed that the foliar spraying of NAA, GA₃, and CPPU significantly increased the fruit content from TSS percentages and from vitamin C. Additionally, the treating of pear trees with 75 ppm NAA, 75 ppm GA₃ and 20 ppm CPPU gave the highest remarkable values in the fruit content from soluble solids in the two seasons. The application of 75 and 50 ppm from NAA and also 20 ppm CPPU extremely increased the fruit content from vitamin C during two test seasons. Concerning the fruit content from acidity, it was noticed that it was notably minimized by the spraying of 25, 50, and 75 ppm NAA, 50 and 75 ppm GA₃, and 15 or 20 ppm CPPU with respect to not treated trees.

Table 6. Effect of the Foliar Spraying of CPPU, GA3 and NAA on Fruit Content from Soluble Solids, Acidity and VC of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

Total and reduced sugars percentages were markedly increased by spraying the pear trees with 50 and 75 ppm NAA, 50 and 75 ppm GA₃, and 15 and 20 ppm comparing with untreated trees in 2022 and 2023 (Table 7). Moreover, the notable and most significant percentages were obtained by the spraying of pear trees with 75 ppm NAA, then by 50 NAA, 75 ppm GA₃, and 20 ppm in both experimental seasons. Moreover, the differences between the effects of 50 ppm NAA, 75 ppm GA₃, and 20 ppm CPPU were not significant on the fruit content from total and reducing sugars content in both testing season. Non-reducing sugars content in pear fruits were not significantly affected by the spraying of the foliar application of the tree applied PGRs compared to untreated trees.

Table 7. Effect of the Foliar Spraying of CPPU, GA3 and NAA on Fruit Content from Total, Reduced, and Non-Reduced Sugars of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

Mineral Content from Macro and Micronutrients

Leaf mineral content from macronutrients such as nitrogen, phosphorous, and potassium was positively affected by the spraying of NAA, GA₃ and CPPU in the two seasons (Table 8).

Table 8. Effect of the Foliar Spraying of CPPU, GA3 and NAA on the Leaf Mineral Content from Nitrogen, Phosphorous and Potassium of Pear Trees during 2022 and 2023

The treatments with the same letters in each column do not significantly differ from one another.

The most significant increment was obtained by the spraying of 75 ppm NAA compared to untreated trees. Spraying the pear trees with 25 and 50 ppm GA₃, 50 and 75 ppm, as well as 15 and 20 ppm significantly increased the leaf content of nitrogen compared to the control. The phosphorous concentration in the leaves of pear was markedly increased by the spraying of 75 ppm GA₃ and 20 ppm CPPU as compared to the control. The potassium percentage was remarkably increased by the spraying of 20 ppm CPPU, 50 and 75 ppm GA₃, and 50 ppm NAA as compared to untreated trees.

Table 9 demonstrates that the foliar application of 50 and 75 ppm NAA, 50 and 75 ppm GA₃, and 20 ppm CPPU significantly enhanced the leaf content from Fe, Zn, Mn, and B compared to untreated trees. The most effective treatment, yielding the best results, was the application of 75 ppm NAA, which outperformed the other treatments. Conversely, the effects of spraying 25 ppm NAA, 25 ppm GA₃, and 15 and 10 ppm CPPU on increasing leaf nutrient content were not significant in the years 2022 and 2023.

Table 9. Effect of the Foliar Spraying of CPPU, GA₃, and NAA on the Leaf Mineral Content from Iron, Zinc, Manganese, and Boron of Pear Trees during 2022 and 2023

Spraying of plant growth regulators (PGRs) greatly reduced the fruit drop and thus improved the obtained yield and its productivity per hectare.

* The most notable influence resulted from the spraying of 75 ppm NAA in improving the fruit set percentages, reducing the fruit drop percentages, and raising fruit productivity in the 2022 and 2023 seasons.

* The exogenous application of 50 NAA, 75 GA₃ and 20 ppm CPPU were also effective treatments in reducing the fruit drop and raising the fruit yield compared to the not sprayed trees.

ACKNOWLEDGMENTS

The authors extend their appreciation to the Researchers Supporting Project number (RSP202

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