Influence of foliar-applied boron and Gibberellic Acid (GA₃) on fruit retention in Rambutan (Nephelium lappaceum L.)

Introduction

Rambutan (Nephelium lappaceum L.) is an important tropical fruit crop belonging to the family Sapindaceae and is believed to have originated in the Malay Archipelago, particularly Indonesia, Malaysia and Southern Thailand. The crop is widely cultivated in Southeast Asia and is also grown in other tropical regions, including India. In India, rambutan cultivation is mainly concentrated in humid tropical regions of Kerala, Tamil Nadu and Karnataka. The attractive appearance, pleasant flavour and nutritional value of its fruits have contributed to its increasing market demand and commercial importance (Tripathi et al., 2014; Jahurul et al., 2020) [21,8] . Despite its commercial potential, rambutan productivity is often constrained by poor flowering, low fruit set, excessive flower and fruit drop, inadequate fruit retention and irregular fruit development. These problems may arise due to nutritional imbalance, inadequate hormonal regulation and environmental conditions, ultimately affecting yield and fruit quality. Improving reproductive performance during flowering and early fruit development is therefore important for enhancing productivity in rambutan (Hang and Chau, 2019, Uddin et al., 2024, Gomasta et al., 2026) [7,22,6] . Boron is an important micronutrient involved in several physiological processes, particularly those associated with reproductive development. It plays a vital role in pollen germination, pollen tube growth, fertilization, cell wall formation and carbohydrate translocation. Adequate boron nutrition can therefore improve flowering, fruit set and fruit retention, while its deficiency may result in poor fertilization, increased fruit drop and reduced yield (Roygrong et al., 2006, Shukla, 2011, Ullah et al., 2012) [15,17,23] . Similarly, gibberellic acid is an important plant growth regulator that promotes cell division, cell elongation and fruit development and has been reported to influence flowering, fruit retention and yield in several fruit crops (Rao and Rao, 1963, Singh and Lal, 1980, Chaudhary et al., 2018) [14,20,3] Foliar application provides an efficient means of supplying micronutrients and plant growth regulators during critical stages of crop development. The combined application of boron and gibberellic acid may provide complementary effects by supporting reproductive processes through improved pollen performance and fertilization while promoting subsequent fruit development.

Materials and Methods

The investigation was undertaken on six-year-old rambutan trees maintained at 5 m × 5 m spacing at Kedoor, Brahmavar, Udupi, Karnataka, during 2025-26. A randomized complete block design (RCBD) was adopted with nine treatments and three replications. Treatments included boron at 0.1 and 0.2 percent, GA 3 at 50 and 75 ppm, their combinations and an untreated control. Boron sprays were given before flower initiation, whereas gibberellic acid was applied at 50 percent fruit set. Treatments were T 1 control, T 2 boron 0.1%, T 3 boron 0.2%, T 4 GA 3 50 ppm, T 5 GA 3 75 ppm, T 6 boron 0.1% + GA 3 50 ppm, T 7 boron 0.1% + GA 3 75 ppm, T 8 boron 0.2% + GA 3 50 ppm and T 9 boron 0.2% + GA 3 75 ppm. Flowering time, panicle production, fruit development, fruit number and fruit set were assessed. Data were analysed under RCBD and treatment differences were tested using CD at 5 percent.

Results and Discussion

The treatments showed considerable variation in the time taken for the onset of flowering. The earliest first flowering was recorded in T 6 (boron 0.1% + GA 3 50 ppm) at 120.18 days, followed by T 9 (boron 0.2% + GA 3 75 ppm) at 123.85 days and T 7 (boron 0.1% + GA 3 75 ppm) at 125.33 days. The control (T 1 ) recorded the latest first flowering at 159.40 days. The advancement in flowering with boron and gibberellic acid application attributed to improved physiological activity during the reproductive phase. Boron plays an important role in reproductive development, while gibberellic acid influences plant growth and developmental processes. Similarly, Pandey et al. (2012) [12] demonstrated that foliar feeding with a combination of ZnSO₄ (0.5%), borax (0.5%), and GA 3 (10 ppm) significantly reduced the days to flowering initiation and days to 50% flowering in ber, indicating an acceleration in the flowering process. The duration required to attain 50 percent flowering was shortest in T 9 (boron 0.2% + GA 3 75 ppm), which reached this stage at 151.93 days. T 7 and T 8 recorded 153.87 and 158.37 days, respectively, whereas the control required 197.35 days. Thus, the sequential application of boron and gibberellic acid resulted in earlier attainment of 50 percent flowering compared with the untreated control. The earlier completion of flowering under the combined treatments could be related to better regulation of reproductive growth. Boron supports reproductive processes, while gibberellic acid modifies growth and developmental responses. The increase in number of flowers per shoot might be due to the effect of gibberellic acid in accelerating the differentiation of inflorescence and rapid elongation of peduncle, leading to full development of flower buds having functional reproductive parts Parouissi et al., 2002 [13] . The result is in conformity with the findings of (Anwal et al., 2015) [2] in pomegranate, (Singh, 2017) [18] in phalsa and (Lal et al., 2013) [11] in guava. The number of panicles produced per square metre increased with the application of boron and gibberellic acid. T 9 (boron 0.2% + GA 3 75 ppm) recorded the highest number of panicles (17.48 m⁻²), followed by T 8 (15.08 m⁻²) and T 7 (14.43 m⁻²). The minimum number of panicles was observed in T 1 (control), with 9.93 panicles m⁻².The greater panicle production under sequential application treatments associated with improved nutrient availability and enhanced growth of reproductive shoots. Boron is particularly important for reproductive development, while gibberellic acid promotes cell division and elongation and modifies plant growth. The stronger response of T 9 suggests that the higher boron concentration together with gibberellic acid 75 ppm provided a favourable condition for panicle development, Singh et al. (2017) [18] found similar results with phalsa. [18] who observed that plant growth regulators, particularly gibberellic acid, significantly reduce the crop duration and promote early maturation in various fruit crops. Similar trend was also reported by Ferree et al., 2004 [5] .

The highest number of fruits per square metre was recorded in T9 (boron 0.2% + GA 3 75 ppm), with 64.10 fruits m⁻². This was followed by T 7 (boron 0.1% + GA [10,1] in phalsa and Shah et al., 2015 [16] in grapes, the application of gibberellic acid at 150 ppm produced the highest number of fruits per node and fruiting nodes per shoot in phalsa. [19] who found that applying gibberellic acid and 2,4, 5-T to phalsa significantly increased fruit set. The present findings are in agreement with Dixit et al. (2013) [4] , who reported that boron application improved fruit set and retention by enhancing reproductive processes in litchi. Kaur (2017) [9] also observed improvement in fruit quality and reproductive characters through application of boron and growth regulators in litchi.

The foliar application of boron and gibberellic acid improved the reproductive performance of rambutan compared with the control. T 9 (boron 0.2% + GA 3 75 ppm) was particularly effective in advancing 50 percent flowering, increasing panicle and fruit production and reducing the time required to reach the marble-size stage and recorded the highest fruit-set value among the treatments for which observations were available. These findings indicate that the sequential use of boron and gibberellic acid can favour reproductive development and early fruit establishment in rambutan under the conditions of the present study.

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