Sweet Cherry ( Prunus avium L.) Fruit Drop Reduction by Plant Growth Regulators (Naphthalene Acetic Acid NAA and Gibberellic Acid GA 3 )

This study was conducted during 2019/2020 on sweet cherry trees (Prunus Avium L.) (Bing and Hardy Giant) cultivar planted at Sergaya-Al_Zabadani area of Rural Damascus, to reduce fruit drop of sweet cherry. The experiment included 4 foliar applications: T1: control, T2: GA 3 (100 ppm), T3: NAA (20 ppm), T4: (100 ppm GA 3 + 20 ppm NAA). Fruit set and fruit drop percentage, fruiting factor, and yield were recorded. The results showed that treatment with (100 ppm GA 3 + 20 ppm NAA) recorded higher fruit set percentage (73.81% and 75.62%), and fruiting factor (48.38% and 50.04%) respectively; In addition to fruit yield (40.19 and 41.21 kg/tree) for both cultivars, compared to the control (9.13, 6.60 kg/tree). Therefore, it can be concluded that GA 3 + NAA treatment reduced Sweet cherry fruit drop better than other treatments, where fruit drop percentage didn’t exceed (63.11% and 62.01%) in both cultivars (Bing and Hardy Giant) respectively, compared to the control (80.92% and 80.64%).


Introduction
Plant growth regulators or plant hormones are organic substances that are naturally produced in high plants. They control and activate growth and physiologi-cal functions at low concentrations and far from their production site. Use of growth regulators has become an important way in agriculture, especially in fruit trees, such as spraying auxin and Gibberelline, which are widely used to reduce fruit drop and improve their quality [1].
The balance of internal hormones plays a main important role in nutrients distribution's in plant's organs and affects bud longevity. It has been confirmed that fruit drop is due to internal content of auxin by external application 2,4-D or NAA, where the transport of auxins by plant stays for a long time without any effect of ethylene [2], and it's found that gibberellic acid delays ripening and aging of fruits [3].
In an experiment on 3 sweet cherries varieties (Sweet Heart, Sunburst, Lapins) to improve fruitset, the trees were sprayed with a mixture of (10 mg/l GA 3 , 10 mg/l NAA) with a week intervals, where the first spray was at the end of flowering, The results showed that fruitset percent was improved by at least 2 times compared with control [4].
Many researches and experiments indicated that repeated application of gibberellic acid (GA 3 ) improved fruitset and reduced fruit drop of cherry trees in rainy years [5].
Tuan & Chung-Ruey [7] confirmed that foliar spray of apple trees (wax apple) with GA 3 (30 ppm) at the young bud and petal fall stages caused a significant increase in the fruitset percentage, this may be due to the increase in the availability of nutrients from the leaves by gibberellic, similar results were obtained on (wax apple) by Moneruzzaman et al. [8] as well as on sweet oranges [9].
The results of foliar application with a mixture of (GA 3 100 mg/l + 100 mg/l NAA) on olive trees showed a higher fruitset percentage (4.78%) compared to control (2.89%) [10].
Treatment of sweet orange trees (Citrus sinensis Osbeck.) Cv. Jaffa with three concentrations of NAA (20, 30, 40 ppm) showed that 20 ppm achieved the highest fruit drop reduction until harvest, the highest fruit retained percentage, the largest number of fruits/tree and the highest production/tree [12].
In an experiment on mandarin trees 10 years old, it was observed that application of NAA (10 mg/l) increased fruitset percentage (82.77%), the largest number of fruits/tree and the highest productivity [13]. As well as spray of navel orange trees with (GA 3 ppm 20 + NAA ppm 25) reduced the fruit drop percentage and increased productivity (kg/tree) [2].
Sweet cherry trees planted in the study area (Sargaya) suffer from fruit drop, which affects productivity, and according to the importance of growth regulators in increasing the fruitset, reducing fruit drop and increasing productivity, this research was conducted by application Naphthalene acetic acid (NAA) and Gibberellic acid (GA 3 ) to resolve this problem.

Materials and Methods
The search was carried out at Cherry orchard located in Sargaya, Al-Zabadani area in Rural Damascus, at about 1500 meters altitude from sea level on sweet cherry trees (Prunus avium L.), Bing and Hardy Giant, 20 years old, grafted on Prunus mahaleb. The treatment included: (T1: Control "sprayed with water only", T2: GA 3 at 100 ppm, T3: NAA at 20 ppm, T4: (100 ppm GA 3 + 20 ppm NAA), teen 20 was added 1 ml/l growth regulator solution. At white balloon and after fruitset stages for NAA application, and after fruitset for GA 3 application. The experiment was designed by the complete randomized method, with 4 treatments repeated 3 times and 1 tree was used for each treatment. 4 similar flowering branches on each tree were selected, and the number of flowers on each tagged branch for a particular treatment counted. All treatments were performed with a manual hand sprayer to the drip-point. The results were analyzed statistically and analysis of variance (ANOVA) was performed using the statistical program Genstat-12. The lowest significant difference LSD was calculated at the level of 5% coefficients to compare the mean and find out the significant differences, after angular conversion of percentages. The studied parameters were: • Mean of (total flowers/branch, fruitsets/branch, fruits drop/branch, and ripe fruits/branch) • Fruitset percentage: • Fruit weight Mean: A random sample (150 fruits) was taken from the two studied varieties, with 50 fruits of each replicate, and weighed. • Tree production (kg): Fruit trees weight was performed and mean fruit production was calculated for each treatment.

Effect of GA 3 and NAA on Fruitset Percentage
The results showed that fruitset percentage was increased with NAA treatment (76.43%) compared to control (72.76%) for the Bing variety. While it was (72.85%) with GA 3 without significant differences with control. For Hardy Giant variety fruitset was increased as well with GA 3 + NAA application (73.81%) compared to the other studied treatments, with no significant differences between control (71.59%) and GA 3 treatment (71.66%), Table 1.
These results are in agreement with the results of Stopar [4] who recommended the foliar spray with a mixture of (GA 3 10 mg/l + NAA 10 mg/l) as a technical application to increase fruitset percentage for all sweet cherry varieties.
These results agreed with Bubán [14] who indicated that using growth regulators in order to improve the final fruitset percentage in cherry trees is considered ineffective compared to other types of fruit trees such as pears, despite the presence of many previous studies confirming the possibility of using some growth regulators to obtain good fruitset percentage in the cherry trees too.

Effect of GA 3 and NAA on the Percentage of Fruit Drop and Fruiting Factor
The results showed the (GA 3 + NAA and GA 3   Similar results were obtained in previous experiments conducted by Stopar [4] on (Sunburst, Lapins and Sweetheart) cultivars, and by Webster et al. [5] who found that remaining fruits number on Colney's sweet cherry trees increased 2 times when sprayed with a mixture of (GA 3 10 mg/l + NAA 10 mg/l). On the other hand, this treatment did not have any effect on Stella cultivar, it would be concluded that Bing cultivar behavior differed to GA 3 Application than Hardy Giant, while the 2 cultivars behaved similar to NAA treatment, so varieties behavior differs to growth regulators application.
This result is in agreed with Sweety et al. [12] who attributed the higher fruiting factor in NAA (20 ppm) treatments to the reduction of fruit drop, where growth regulators treatment compensated the lack of endogenous auxin, thereby preventing the formation of the separation layer, possibly by inhibiting Enzymatic activity. Table 3 indicated as well a significant effect of GA 3 (7.447 and 8.800 g/fruit) compared to other treatments included the control for Bing and Hardy Giant, respectively. This result showed that the increase of fruit yield is due to the enhancement of fruit weight by GA 3 application, not for fruitset percentage. This is confirmed by Zhang [20] results, where GA 3 (100 mg/l) application had a positive effect on fruits weight of sweet cherry Bing cultivar, while the (200 mg/l) was more effective in increasing fruits weight by 15% compared to the control. Table 3 showed that (GA 3 , NAA and GA 3 + NAA) application enhanced Bing fruit yield (14.78, 28.89 and 40.21 kg/tree), and Hardy Giant fruit yield (11.06, 22.69 and 40.19 kg/tree), compared to the control (Bing and Hardy Giant) (5.93 and 9.13 kg/tree) respectively.

Effect of GA 3 and NAA on Fruit Weight and Yield
These results agreed with Stopar [4], who observed that sweet cherry trees' yield increased 2 times when sprayed with a mixture of (GA 3 10 mg/l + NAA 10  [23] who showed that spraying of orange trees with NAA and GA 3 increased trees yield compared to non-sprayed trees.
Yield increase in the studied treatments compared to the control could be attributed to the increase in fruits remained number at harvest time. The enhancement of fruit weight may be due to the activation and increase of cell division and cell elongation by NAA and GA 3 treatments [24].
Agrawal and Dikshit [25] showed that NAA treatment increased fruit weight and yield as a result of activating cell elongation, increasing vacuole size, flexibility, and cell wall elasticity. Stern et al. [26] also indicated that NAA treatment promotes cell elongation in mesocarp cells, thus increasing fruit size and yield.

Conclusion
It can be concluded that the application of the mixture of (20 ppm NAA + 100 ppm GA 3 ) on cherry trees had a positive effect in reducing fruit drop percentage and increasing fruiting factor compared to the control. NAA + GA 3 application was more effective than NAA alone. The efficacy of the GA 3 + NAA mixture should be further investigated.