Gibberellic acid combined with hand pollination increases ‘Red’ and ‘Lessard Thai’ sugar apple fruit quality and produced parthenocarpic ‘Gefner’ atemoya fruits

ABSTRACT: The present research aimed to evaluate the effect of applying gibberellic acid (GA3) with hand (HP) or natural pollination (NP) on fruit set and the fruit quality of ‘Red’ and ‘Lessard Thai’ sugar apple and ‘Gefner’ atemoya fruits.This study was performed in an experimental orchard located in Homestead, Florida, USA. The experimental design included randomized blocks, with eight treatments, four replicates and four plants per plot. Treatments included: (1) HP; (2) HP + 10 mg L-1 GA3; (3) HP + 100 mg L-1 GA3; (4) HP + 1,000 mg L-1 GA3; (5) NP; (6) NP + 10 mg L-1 GA3; (7) NP + 100 mg L-1 GA3 and (8) NP + 1,000 mg L-1 GA3. The HP plus 1,000 mg L-1 GA3 promoted fruit setting above 90% over the 14 weeks for all genotypes evaluated. Significant increments for length and total fruit weight were observed. ‘Red’ sugar apple and atemoya had a reduced number of seeds per fruit. The NP plus GA3 (1,000 mg L-1) was effective in producing high quality seedless ‘Gefner’ atemoya fruits. This investigationdemonstratedthat GA3 plus hand pollination produced high quality sugar apple and atemoya seeded fruits and in association with natural pollination promoted seedless ‘Gefner’atemoya fruits.

Among the various management techniques for sugar apple and atemoya, hand pollination (HP) is unequivocally one of the most important since self-fertilization is negligible for this genus and natural pollination is not commercially effective. The HP ensures the formation of fruit with excellent commercial quality; its disadvantages include intensive labor requirements and a significant increase in the average number of seeds per fruit (PEREIRA et al., 2014a;RODRIGUES et al., 2016).
The development of technology for production of seedless fruits by applying plant growthregulators in Annonaceae has been investigated recently (FERRARA et al., 2014;PEREIRA et al., 2014a;SANTOS et al., 2016;TANG et al., 2015;MAHMOOD et al., 2016). Some studies with cherimoya (Annona cherimola) and the atemoya (A. cherimola x A. squamosa) suggested the possibility of using this technique to increase fruit set, produce parthenocarpic fruit and improve the physical and chemical qualities of the fruit (PEREIRA et al., 2014a;SANTOS et al., 2016).
The objective of this investigation was to determine the capacity of GA 3 to produce parthenocarpic fruits in sugar apple and atemoya. This research also aimed to verify the effect of gibberellic acid at different concentrations combined with hand pollination, on fruit set and the quality of 'Red' and 'Lessard Thai' sugar apple and 'Gefner' atemoya fruits, under South Florida conditions.
The commercial product used was 10% GA 3 in the form of a soluble powder (Abbott Laboratories, São Paulo, SP, Brazil). Plant growthregulator solutions contained 250 mL of non sterile, non-distilled water supplemented with 0.1% nonionic spreader sticker (mean pH of 8.17) were prepared on the day of each treatment application. First application of the solution targeted the female flowers (functionally pistillate). Each flower received the equivalent of 3 mL of solution using a spray bottle. HP was performed when the flowers reached the female stage by using pollen grains collected from A. squamosa and pollination was performed using a number 2 watercolor paintbrush, from 8:00 am to 9:00 am (TANG et al., 2015). The percentage of fruit set, physical characteristics and total soluble solids of the fruits were evaluated. Results were subjected to analysis of variance, and the means were compared using Tukey's test, at a 5% probability.
The HP treatment alone exhibited 95%, 100% and 75% fruit set for the 'Red' and 'Lessard Thai' sugar apple varieties and 'Gefner' atemoya, respectively ( Figure 1A, 1B and 1C). The use of NP alone and the combination of NP plus 10 and 100 mg L -1 GA 3 did not result in enough set fruit for fruit quality evaluations. The combination of NP plus 1,000 mg L -1 GA 3 resulted in mean fruit set of 5%, 25% and 90% for the 'Lessard Thai', 'Red' and 'Gefner', respectively ( Figure 1D, 1E and 1F). Studies have shown that fruit set of many species is a complex trait and associated with the capacity to produce gibberellins, auxins and cytokinins by fruits and seeds (ZHANG et al., 2007;DE JONG et al. 2009).The differences observed in the present research pointed to a wide genetic divergence among the varieties evaluated.
HP plus 1,000 mg L -1 GA 3 promoted a significant increase infruit weight of 'Red' sugar apple and greater length, diameter and fruit pulp weight (Table 1). The HP plus 1,000 mg L -1 GA 3 significantly increased fruit size of 'Lessard Thai' sugar apple and the 'Gefner' atemoya but fruit quality was similar (Table 1). 'Gefner' atemoya and 'Red' sugar apple fruits treated with NP plus 1,000 mg L -1 GA 3 resulted in significantly lower fruit length, diameter, and weight when compared with HP plus 1,000 mg L -1 GA 3 .
The NP plus 1,000 mg L -1 GA 3 produced seedless 'Gefner' atemoya fruits and this same GA 3 concentration with HP resulted in a smaller number of seeds per 100 g of fruit and pulp without affecting fruit quality (Table 1). A previous study reported the efficiency of GA 3 at 1,000 to 1,500 mg L -1 to produce seedless atemoya fruits (PEREIRA et al., 2014b). The authors described flowers exclusively treated with GA 3 and no hand pollinated had 65% to 80%fruit set. STIKIĆ et al. (2015) reported that gibberellins, when applied exogenously in plants, promotes parthenocarpy, and GA 3 is most commonly used for producing fruits quite similar to those produced in normal conditions. Our hypothesis is that the fruit set and the growth of non-pollinated GA 3 -induced atemoya ovaries is similar to seedy atemoya fruits; therefore, GA 3 triggers and maintains cell division in the ovary walls promoting fruit set and subsequent growth and development of 'Gefner' fruits in nonpollinated flowers.
'Red' sugar apple treated with HP plus 1,000 mg L -1 GA 3 significantly increased pulp weight and decreased seed weight and number of seeds per 100 g of fruit pulp (Table 1). As expected, NP plus 1,000 mg L -1 GA 3 treated 'Red' sugar apple had significantly smaller fruit and lower °Brix and percent pulp (~25% compared to ~35% for HP plus 1,000 mg L -1 GA 3 ). 'Lessard Thai' sugar apple treated with HP plus 1,000 mg L -1 GA 3 had similar °Brix and significantly larger fruit and fewer seeds per 100 g of pulp than all other treatments ( Table 1).
The HP plus 1,000 mg L-1 GA 3 resulted in 90%, 90%, and 95% fruit set of 'Gefner' atemoya and 'Red' and 'Lessard Thai' sugar apple fruits, respectively (Table 1). This treatment also resulted in the largest fruit when compared to all other treatments. HP plus 1,000 mg L -1 GA 3 applied to 'Gefner' atemoya may be an alternative for the production of high-quality seeded fruits. Results of the present research is in agreement with a previous investigation conducted with the same hybrid but under Brazilian semi-arid conditions (SANTOS et al., 2016). This region is characterized by two main seasons (dry and rainy) with an average precipitation of 830 mm, mean annual temperature of 24 °C, average maximum of 30 °C, and minimum of 20 °C. The present research was established in Homestead and the climate is classified as marine subtropical climate. Average precipitation is 1473 mm per year, with mean annual temperature of 24 °C, average maximum of 29 °C, and minimum of 19 °C. Despite of these climatic differences atemoya was very responsive in both regions.These studies suggested that split applications of GA 3 concentrations between 500 and 1,000 ppm after anthesis promoted a favorable increase in all physical characteristics of the fruit without the loss of sensory and post-harvest quality (PEREIRA et al., 2014b;SANTOS et al., 2016). The Annonaceae is characterized by producing fruits with a large number of seeds per fruit and seed weight is an important component, corresponding among 10 to 12% of total fruit weight. Another relevant result was the production of seedless 'Gefner' atemoya and the reduced seed weight per fruit in 'Red' sugar apple. Many investigations have reported that in general seedless fruits have lower weight, length and diameter compared to seeded fruits. Seedless fruit are highly desirable if fruit quality is not adversely affected. However, in many cases fruit size, shape and/or organoleptic characteristics are negatively affected by a lack of seeds.
Considering the findings of the present study and previous investigations, GA 3 plus HP can be considered as a new practice in order to improve fruit quality in sugar apple ('Red' and 'Lessard Thai') and 'Gefner' atemoya. The next step is to develop new studies to adjust this methodology to the field conditions. Some questions must be elucidated: (1) How to apply the GA solution efficiently in all flowers? (2) At which floral development stage will treatments be most efficient? (3) How many flowers must be treated in order to achieve an economic return? Although, methodological adjustments are still needed and more studies will be necessary in order to elucidate and understand the mechanism of the parthenocarpy in atemoya, today it is can be considered a potential practice for producing seedless fruits in 'Gefner'atemoya. Table 1 -Hand and natural pollination associated to GA3 in sugar apple and atemoya fruits on fruit diameter (DIA), fruit length (LGT), total fruit weight (TFW), pulp weight (PWE), total seed weight (TSW), number of seeds per fruit (NSF), number of seeds per 100 g pulp (NPU) and total soluble solids (SS) in Homestead, Florida, USA. TREATMENT LGT (     Data followed by the same letter in the column do not differ statistically at the 5% level of significance according to Tukey's test. CV: coefficient of variation.

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The use of NP alone and the combination of NP with concentrations of 10 and 100 mg L -1 GA3 did not result in a sufficient number of set fruit for conducting the analyses planned for the three varieties evaluated.