Effect of plant bio regulators on fruit set and yield of cashew (Anacardium occidentale L.) cv. BPP-9

Introduction

The cashew tree is a tropical evergreen that produces an edible nut and apple. Cashew is Native to Brazil, it was introduced in India by Portuguese travellers in the 16 th century (Adeigbe et al., 2016) [1] . In India cashew occupied with an area 0.7 million ha with a production of 0.77 million tonnes, India is now a leading global cashew producer (Indian Trade Portal, 2022) [4] . Cashew cultivation is mainly concentrated in the coastal regions across several states of India like Maharashtra, Kerala, Karnataka, Tamil Nadu, Andhra Pradesh, Goa, Odisha, West Bengal, Telangana and some parts of North-Eastern region. In Telangana cashew is occupied an area of 5722.48 ha and production of 5221.15 metric tonnes. In Telangana, major cashew-growing districts include Bhadradri, Khammam, Mancherial and Kumuram Bheem Asifabad (Horticulture Area Production Statistics, 2022-23) [3] . In Telangana state, 90% of total cashew cultivable area is occupied with BPP (BPP-1 to BPP-9) cultivars. The cashew productivity is largely controlled by climate, which invariably cannot be controlled and hence efforts have to be directed to modulate the cashew phenology to suit unfavourable climatic conditions (Adiga et al., 2019) [2] . Under such circumstances, modulation of bud break, flowering and fruit set by spraying of various plant bio regulators is the best alternative to mitigate or reduce the climate change effect on cashew. Various studies in cashew have been standardized for profuse flowering and yield improvement by mitigating the adverse effects of climatic conditions. Among various vegetative bud breaking and flower promoting plant bio regulators, GA 3 is a plant hormone that regulate various developmental processes, including flower induction and flower development (Srivastava et al., 2002) [21] . GA 3 at 50 ppm effectively induces flowering in cashew by vegetative bud breaking (Jena et al., 2018) [10] . Ethrel at 50 ppm improves the fruit set per square meter, number of nuts per panicle), high flowering and sex ratio (Ghadage et al., 2016) [8] . Humic acids are organic molecules, which increased crop growth by increasing plant growth promoting hormones such as auxin and cytokinin (Billingham, 2012) [5] . Humic acid at 0.4% as foliar spray has effectively increased the number of panicles per unit area in cashew (Pandiyan et al., 2014) [15] . Among various fruit set and nut growth improving plant bio regulators NAA at 50 ppm double spray proved most effective for increasing number of perfect flowers, fruit retention, nuts per square meter and yield per square meter (Singh and Pariari 2012) [19] .

Materials and Methods

The experiment was conducted in an orchard at the Horticulture Research Station, Aswaraopet, SKLTGHU, Telangana. The experimental site was situated at an altitude of 166 m above mean sea level on 81 ̊ 13 ̊ East longitude and 17 ̊ 25 ̊ North latitude. It falls under a semi-arid tropical climate zone. The soil of the orchard selected is well drained deep sandy loamy with a pH of 6.7 and EC of 0.15 dsm -1 . Six-year-old cashew trees of the BPP-9 variety, which were well-grown and uniform, were selected for the experiment. The trees were planted in a square system with a spacing of 7 meters between them. In the experimental trees all cultural practices, including fertilizer, insecticide, fungicide application and irrigation were consistently applied. The trees for the treatment were chosen randomly and a randomized block design with a factorial concept was used for the layout of the experiment (Oliver, 1965) [14] . Factor 1: Vegetative bud-breaking and flower induction Plant Bio-Regulators (PBR's) (V) -Spraying was done at 20 days before bud break (319 phenophase). V 1: GA 3 @ 50 ppm, V 2: Humic acid @ 0.5%, V 3: Ethrel @ 50 ppm, V 0: Control (Water spray) Factor 2 (Fruit set and nut growth promoting PBR's) (F) -1 st spray was done at full bloom stage (617 phenophase) and 2 nd spray at fruit set (713 phenophase). F 1: NAA @ 50 ppm, F 2: ZnSO 4 @ 0.5% + Borax @ 0.15%, F 0: Control (Water spray). The detail of the treatments were T 1 -Spraying of GA 3 @ 50 ppm with NAA @ 50 ppm (V 1 F 1 ), T 2 -Spraying of GA 3 @ 50 ppm with ZnSO 4 @ 0.5% + Borax @ 0.15%(V 1 F 2 ), T 3 -Spraying of GA 3 @ 50 ppm with Control (Water spray) (V 1 F 0 ), T 4 -Spraying of Humic acid @ 0.5% with NAA @ 50 ppm (V 2 F 1 ), T 5 -Spraying of Humic acid @ 0.5% with ZnSO 4 @ 0.5% + Borax @ 0.15% (V 2 F 2 ), T 6 -Spraying of Humic acid @ 0.5% with Control (Water spray) (V 2 F 0 ), T 7 -Spraying of Ethrel @ 50 ppm with NAA @ 50 ppm (V 3 F 1 ), T 8 -Spraying of Ethrel @ 50 ppm with ZnSO 4 @ 0.5% + Borax @ 0.15% (V 3 F 2 ), T 9 -Spraying of Ethrel @ 50 ppm with Control (Water spray) (V 3 F 0 ), T 10 -Spraying of Control (Water spray) with NAA @ 50 ppm(V 0 F 1 ), T 11 -Spraying of control (Water spray) with ZnSO 4 @ 0.5% + Borax @ 0.15% (V 0 F 2 ), T 12 Absolute spray (V 0 F 0 ).

Results and Discussion 1. Fruits per panicle (at 715 stage)

The results on number of fruits per panicle of cashew cv. BPP-9 was influenced by plant bio-regulators showed in (Table 1) and (Fig 1). Among the vegetative bud-breaking and flower induction bio-regulators, the maximum number of fruits per panicle was produced in Ethrel @ 50 ppm (V 3 ) (12.71) followed by GA 3 @ 50 ppm (V 1 ) (11.52) and Humic acid @ 0.5% (V 2 ) (8.12). However, the minimum number of fruits per panicle was produced in Control (V 0 ) (6.69). Ethylene lead to the maximum number of fruits per panicle in this study, due to the increased production of bisexual flowers per panicle induced by Ethrel (Konhar and Mech 1988) [11] . A similar increase in fruits per panicle with Ethrel was earlier confirmed by Lakshmipathi et al. (2017) [13] in cashew. In fruit set and nut growth-promoting bioregulators, the maximum number of fruits per panicle was produced in ZnSO 4 @ 0.5%+ Borax @ 0.15% (F 2 ) (12.34), followed by NAA @ 50 ppm (F 1 ) (11.20). However, the minimum number of fruits per panicle was produced in control (F 0 ) (5.75). Zinc plays a crucial role in carbohydrate synthesis and auxin biosynthesis, while boron strengthens cell walls and aids in flower and fruit development as distinguished by Senthivel and Kaleeswaran (2019) [18] . The interaction between factors was significant. The maximum number of fruits per panicle was recorded in the combination of Ethrel @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V 3 F 2 ) (16.43) followed by Ethrel @ 50 ppm + NAA @ 50 ppm (V 3 F 1 ) (14.39), which was on par with GA 3 @ 50 ppm + ZnSO 4 + Borax (V 1 F 2 ) (14.24) fruits per panicle. However, the minimum number of fruits per panicle was recorded in Absolute control (V 0 F 0 ) (4.39). This synergistic effect of Ethrel, zinc, and boron enhanced fruit set and development, which was also earlier reported by Ghadage et al. (2016) [8] and Rajamanickam et al. (2023) [16] in cashew.

Percentage of fruit set

The results on fruit set percentage of cashew cv. BPP-9 was influenced by plant bio-regulators showed in (Table 1) and (Fig 1). Among the vegetative bud-breaking and flower induction bio-regulators, the maximum percentage of fruits set was recorded in Ethrel @ 50 ppm (V 3 ) (10.02) followed by GA 3 @ 50 ppm (V 1 ) (9.28), Humic acid @ 0.5% (V 2 ) (6.88). However, the minimum percentage of fruit set was recorded in Control (V 0 ) (5.77). Might be due to the higher fruits per panicle. In fruit set and nut growth promoting plant bio-regulators, the maximum percentage of fruit set was recorded in ZnSO 4 @ 0.5%+ Borax @ 0.15% (F 2 ) (10.10), followed by NAA @ 50 ppm (F 1 ) (9.16). However, the minimum percentage of fruit set was recorded in Control (F 0 ) (4.69). The interaction between the factors was significant. The maximum percentage of fruit set per panicle was recorded in V 3 F 2 (Ethrel @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15%) (12.97), followed by GA 3 @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V 1 F 2 ) (11.51). which is on par with Ethrel @ 50 ppm + NAA @ 50 PPM (V 3 F 1 ) (11.33). However, the minimum percentage of fruit set per panicle was recorded in Absolute control (V 0 F 0 ) (3.69). These results align with previous findings by Lakshmipathi et al. (2015) [13] and Gopikrishna et al. (2021) [9] confirming the positive effect of Ethrel, zinc and boron on increasing fruit set in cashew.

Number of nuts reached maturity per square meter (at 815 stage)

The results on number of nuts reached maturity per square meter of cashew (Anacardium occidentale L.) cv. BPP-9 as influenced by plant bio-regulators showed in (Table 1) and (Fig 1). Among the vegetative bud-breaking and flower induction bio-regulators, the maximum number of nuts reached maturity per square meter was recorded in Ethrel @ 50 ppm (V 3 ) (53.30), followed by GA 3 @ 50 ppm (V 1 ) (38.20) and Humic acid @ 0.5% (V 2 ) (19.00). However, the minimum number of nuts reached maturity per square meter was recorded in Control (V 0 ) (11.14). Ethrel increased hermaphrodite flowers, fruits per panicle, and fruit set percentage likely due to more nuts reaching maturity per square meter. Similar findings were reported by Lakshmipathi et al. (2015) [13] in cashew cv. Bhaskara. In fruit set and nut growth promoting plant bio-regulators, the maximum number of nuts reached maturity per square meter was recorded in ZnSO 4 @ 0.5%+ Borax @ 0.15% (F 2 ) (44.29) followed by NAA @ 50 ppm (F 1 ) (34.79). However, the minimum number of nuts reached maturity per square meter in Control (F 0 ) (12.15). Zinc enhances fruit retention by increasing auxin production, reducing abscission and improving nutrient mobilization, preventing premature fruit drop (Swietlik, 1999) [22] . Boron supports nut growth through sugar translocation, pollination and calcium mobilization (Rao et al., 2017) [17] . A similar increase in mature nuts per square meter was reported by Jena et al. (2018) [10] in cashew. The interaction effects of factors were found significant. The maximum number of nuts reached maturity per square meter was recorded in Ethrel @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V 3 F 2 ) (82.60) followed by Ethrel @ 50ppm + NAA @ 50 ppm (V 3 F 1 ) (60.59) which is on par with GA 3 @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V 1 F 2 ) (57.94). However, the minimum number of nuts reached maturity per square meter was recorded in Absolute control (V 0 F 0 ) (9.12). Might be due to Ethrel's enhancement of hermaphrodite flowers and fruit set combined with ZnSO 4 and Borax's role in boosting fruit retention and development.

The results on nut weight (g) of cashew cv. BPP-9, influenced by plant bio-regulators showed in (Table 1) and (Fig 2). Among the vegetative bud-breaking and flower induction bio-regulators, the maximum nut weight was recorded in Ethrel @ 50 ppm (V 3 ) (7.78) followed by GA 3 @ 50 ppm (V 1 ) (7.69), Humic acid @ 0.5% (V 2 ) (7.60) and the minimum nut weight (g) was recorded in Control (V 0 ) (7.40). Ethylene likely increased nut weight by improving nutrient mobilization and sugar translocation (Ghadage et al., 2016) [8] . In fruit set and nut growth treatments, the maximum nut weight was recorded in ZnSO 4 @ 0.5% + Borax @ 0.15% (F 2 ) (8.00 g) followed by NAA @ 50 ppm (F 1 ) (7.77 g), whereas, the minimum nut weight was recorded in Control (F 0 ) (7.08 g). Zinc enhances protein synthesis, carbohydrate translocation and auxin levels, promoting fruit growth and weight (Gopikrishna et al., 2021 in cashew) [9] . Boron stimulates hormonal metabolism, cell division and expansion, aiding water and sugar movement within fruits which boosts fruit weight (Singh et al., 2012 in aonla, Lakshmipathi et al., 2015 [20,13] in cashew). The interaction between factors was found significant. The maximum nut weight was recorded in Ethrel @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V F 2 ) (8.25) followed by GA 3 @ 50 ppm + ZnSO 4 +borax (V 1 F 2 ) (8.10). However, the minimum nut weight (g) was recorded in Absolute control (V 0 F 0 ) (7.01). This might be due to the synergistic effects of Ethrel's role in enhancing flowering and fruit set, combined with the nutritional benefits of zinc and boron, which optimize physiological and hormonal processes, resulting in heavier nuts.

Nut count per tree

The results on nut count per tree of cashew cv. BPP-9 influenced by plant bio-regulators showed in (Table 1) and (Fig 2). Among vegetative bud-breaking and flower induction plant bio-regulators. the maximum the maximum nut count per tree was recorded in Ethrel @ 50 ppm (V 3 ) (1221.22), followed by GA 3 @ 50 ppm (V 1 ) (1047.44), and Humic acid @ 0.5% (V 2 ) (971.03). Whereas, the minimum nut count per tree was recorded in control (V 0 ) (768.15). Ethrel increased the number of panicles, hermaphrodite flowers and extended flowering, leading to more fruits and nuts per tree (Gawankar et al., 2010) [7] . In fruit set and nut growth promoting plant bio-regulators, the maximum nut count per tree was recorded in ZnSO 4 @ 0.5%+ Borax @ 0.15% (F 2 ) (1339.36), followed by NAA @ 50 ppm (F 1 ) (1042.98). However, the minimum nut count per tree was recorded in Control (F 0 ) (623.54). The interaction between factors was found significant, the maximum nut count per tree was recorded in (1640.51), followed by Ethrel @ 50ppm + NAA @ 50 ppm (V 3 F 1 ) (1437.48). However, the minimum nut count per tree was recorded in Absolute control (V 0 F 0 ) (497.94). The combined effect of Ethrel and ZnSO 4 + Borax synergistically improved panicle numbers, fruit set, and nut maturity, leading to the maximum nut count was earlier confirmed by Lakshmipathi et al. (2015) [13] and Rajamanickam et al. (2023) [16] in cashew.

The results on nut yield per tree (kg) of cashew cv. BPP-9 as influenced by plant bio-regulators showed in (Table 1) and (Fig 2). Among vegetative bud-breaking and flower induction plant bio-regulators. The maximum nut yield per tree was recorded in Ethrel @ 50 ppm (V 3 ) (9.70) followed by GA 3 @ 50 ppm (V 1 ) (7.80), Humic acid @ 0.5% (V 2 ) (5.98) and the minimum nut yield per tree was recorded in Control (V 0 ) (4.75). Ethrel likely increased nut yield by enhancing fruit set and nut weight as noted by Gajbiye et al. (2007) [6] , Gawankar et al. (2010) [7] and Lakshmipathi et al. (2015) [13] in cashew. In fruit set and nut growth promoting plant bio-regulators, the maximum nut yield per tree was recorded in ZnSO 4 @ 0.5% + Borax @ 0.15% (F 2 ) (8.89), followed by NAA @ 50 ppm (F 1 ) (8.11). However, the minimum nut yield per tree was recorded in Control (F 0 ) (4.18). The treatment ZnSO 4 @ 0.5%+ Borax @ 0.15% (F 2 ) significantly improved nut development, increasing the number of fruits per panicle, improving fruit set percentage and resulting in a greater number of mature nuts per square meter and per tree. These factors, along with increased nut weight, led to the highest overall nut yield per tree by Jena et al. (2018) [10] . The interaction between factors was found significant. The maximum nut yield per tree was recorded in Ethrel @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V 3 F 2 ) (12.64) followed by Ethrel @ 50ppm + NAA @ 50 ppm (V 3 F 1 ) (10.65) which is on par with GA 3 @ 50 ppm + ZnsO 4 +borax (V 1 F 2 ) (9.60). However, the minimum nut yield per tree was recorded in Absolute (V 0 F 0 ) (2.73). The best results with the combination of Ethrel @ 50 ppm + ZnSO 4 @ 0.5% + Borax @ 0.15% (V 3 F 2 ) for nut yield per tree (12.64) can be attributed to Ethrel's effectiveness in increasing the number of fruits per tree and nut weight, combined with ZnSO 4 + Borax's role in enhancing fruit set and development. This synergistic effect resulted in the maximum number of fruits per panicle, improved fruit set percentage, increased mature nuts per square meter and greater nut weight leading to the optimal overall nut yield per tree. V1: GA3 @ 50 ppm F1: NAA @ 50 ppm V2: Humic acid @ 0.5% F2: ZnSO4 @ 0.5% + Borax @ 0.15% V3: Ethrel @ 50 ppm F0: Control (Water spray) V0: Control (Water spray) V1: GA3 @ 50 ppm F1: NAA @ 50 ppm V2: Humic acid @ 0.5% F2: ZnSO4 @ 0.5% + Borax @ 0.15% V3: Ethrel @ 50 ppm F0: Control (Water spray) V0: Control (Water spray)

Conclusion

Among the different plant bio regulators, Ethrel @ 50 ppm + ZnSO 4 @ 0.5%+ Borax @ 0.15% (V 3 F 2 ) significantly improved the fruits per panicle, percentage of fruit set, number of nuts reached maturity per square meter, nut weight, nut count per tree and overall nut yield per tree in cashew.

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