Efficacy of new generation plant growth regulators on fruit set, fruit drop and nubbins of mango (Mangifera indica L.) cv. Kesar under HDP

Introduction

The mango (Mangifera indica L.) a botanical cornerstone of the anacardiaceae family stands as the prominent horticultural icon of India and the global tropics. While the genus Mangifera encompasses 69 recognized species and an expansive global diversity of 11,595 cultivars. India remains the unparalleled custodian of its genetic heritage harboring a germplasm repository of approximately 1,000 distinct cultivars (Bose, 1999) [4] . In Gujarat, mango is cultivated on estimated area of 1.84 lakh hectare with 12.72 lakh metric tonne production and productivity of 6.92 t/ha (Anon., 2025) [2] . Mango serves as the backbone of Gujarat's commercial horticulture. Gujrat state maintains a diverse germplasm including traditional and regional cultivars such as Alphonso, Kesar, Rajapuri, Totapuri, Sonpari, Jamadar, khodi, Vanraj, Karanjio, Dadamio, Pachhatio, Sardar, Sinduriyo many of these exhibit limitations in specific economic traits. Consequently Kesar, Alphonso and Rajapuri have emerged as the three primary commercial cultivars. Among these Kesar is highly esteemed for its distinctive saffron colored pulp and green overtoned orange yellow exocarp which facilitates easy varietal identification. Nowadays mango growers faced a problem of low fruit setting, high fruit drop and more number of nubbins due to climate change. In mango fruit drop occurs at pea, marble and maturity stage that leads to low fruit yield. Number of factors are involved in pre mature fruit drop; such as irregular pattern of rain and high wind at the time of flowering and fruit setting due to climate change, nutrient deficiencies at embryo developmental stage, lack of pollination, serious pest and diseases. Post fertilization fruit drop occurs due the longer period of stress i.e., in the form of high temperature, low soil moisture content, low atmospheric humidity during flowering and fruiting that cause hormonal imbalance like low auxin and high ABA content. These leads to more fruit drop. In mango, flowering and fruit setting is crucial event.

Modulation of flowering and fruit setting by spraying of new generation plant growth regulators at flowering, pea and marble stage is the best alternative to mitigate climate change effect on mango. Triacontanol (TRIA) acts as a metabolic catalyst that enhances photosynthetic rate and nutrient acquisition. It is primarily used to minimize premature fruit drop (Neem and Khan, 2011) [9] . Brassinosteroids (BRs) function at low concentrations to promote pollen germination and tube growth ensuring higher reproductive success. By modulating hormonal level and stabilizing chloroplast membranes they enhance sink strength and fruit retention especially under abiotic stress and fluctuating temperatures (Seif et al., 2014; Yue et al., 2019) [13,15] . Salicylic Acid (SA) is natural signaling molecule that inhibits ethylene biosynthesis to prevent fruit abscission. It improves the hermaphrodite flower ratio and strengthens the plant's defense architecture against environmental stressors, boosting overall yield (Hayat et al., 2010 and Ding et al., 2001) [6,5] .

Materials and Methods

The study entitled "Efficacy of new generation plant growth regulators on fruit set, fruit drop and nubbins of mango (Mangifera indica L.) cv. Kesar under HDP" was conducted on 7 years old trees at Fruit Research Station, Sakkarbaug, College of Horticulture, Junagadh Agricultural University, Junagadh which is located in the South Saurashtra Agro-climatic Zone. The study was carried out during 2023-24 and 2024-25. The experiment was carried out in Randomized Block Design with three replications and 10 treatments comprised T 1 (Control), T 2 (Tricontanol 500 ppm), T 3 (Tricontanol 750 ppm), T 4 (Tricontanol 1000 ppm), T 5 (Brassinosteroid 1ppm), T 6 (Brassinosteroid 2 ppm), T 7 (Brassinosteroid 3 ppm), T 8 (Salicylic acid 1500 ppm), T 9 (Salicylic acid 2000 ppm) and T 10 (Salicylic acid 2500 ppm). This study aimed to increase fruit set and reduce fruit drop and nubbins, thereby addressing the major challenges faced by farmers due to climate change. The parameters recorded during the study included number of hermaphrodite flowers, number of fruits at grain stage, pea stage and marble stage, fruit set at grain stage, pea stage and marble stage, fruit drop at pea stage and marble stage and number of nubbins per 50 fruits at pea stage. The data collected from the experiment underwent statistical analysis using the method proposed by Panse and Sukhatme (1985) [10] . The significance of each character was assessed using the "F" test. Additionally, the Standard Error of Mean (S. Em.±) and Critical Difference (CD) were calculated at a 5% level of significance to evaluate the variation and significance among the different treatments The formulas adopted for the observations are detailed below.

Fruit set (%) at grain stage = Number of fruits at grain stage × 100 Total number of hermaphrodite flowers

Fruit set (%) at pea stage = Number of fruits at pea stage × 100 Number of fruits at grain stage

Fruit set (%) at marble stage = Number of fruits at marble stage × 100 Number of fruits at pea stage

Fruit drop (%) at pea stage = No. of fruits at grain stage-No. of fruits at pea stage × 100 No. of fruits at grain stage

Fruit drop (%) at marble stage = No. of fruits at pea stage-No. of fruits at marble stage × 100 No. of fruits at pea stage

Results and Discussion

The experimental results summarized in Tables 1 to 5 demonstrate a statistically significant enhancement in mango fruit set parameters following treatments with modern plant growth regulators. The fruit set parameters significantly influenced by new generation plant growth regulators whereas hermaphrodite flowers were not significantly influenced. The maximum number of fruits at the grain stage per panicle (74.98, 72.18, and 73.58), number of fruits at pea stage per panicle (21.58, 20.30 and 20.94) and number of fruits at marble stage per panicle (5.37, 4.96 and 5.16) were recorded with the foliar spray of brassinosteroid 1 ppm (T 5 ) during both experimental years as well as in the pooled analysis. The highest fruit set at grain stage per panicle (79.58, 78.50 and 79.04%), fruit set at pea stage per panicle (28.79, 28.25 and 28.52%) and fruit set at the marble stage per panicle (24.94, 24.45 and 24.70%) were noted with the foliar spray of brassinosteroid 1 ppm (T 5 ) during individual years and in the pooled data. The minimum fruit drop at pea stage per panicle (71.21, 71.75 and 71.48%) and fruit drop at marble stage per panicle (75.06, 75.55 and 75.30%) were recorded with the foliar spray of brassinosteroid 1 ppm (T 5 ) during both experimental years as well as in the pooled results. The significantly lowest number of nubbins per 50 fruits at pea stage (0.52) was recorded with the foliar application of brassinosteroid 1 ppm (T 5 ) during first year. The minimum number of nubbins per 50 fruits at pea stage (0.57 and 0.55) was recorded in brassinosteroid 1 ppm (T 5 ) during both experimental years and in the pooled analysis. However, poor performance was registered in treatment control (T 1 ) in above all the fruit set attributing characters during the year 2023-24, 2024-25 and in pooled data. Brassinosteroids increase the number of fruits at the grain stage and increase fruit set at grain stage might be due to it promote vascular differentiation within the panicle rachis which strengthens the physical connection between the hermaphrodite flower and the panicle rachis. In high temperature environments it provides a critical rescue effect by protecting pollen grain membranes from thermal damage and accelerating pollen tube growth to ensure successful fertilization before ovule senescence. Furthermore, at the grain stage the young ovaries are highly susceptible to oxidative stress. Brassinosteroids enhance the activity of antioxidant enzymes like Superoxide Dismutase (SOD) and Catalase (CAT) which protect these developing fruitlets from premature abortion. The present findings are in close conformity with the results of Ramani et al. (2016) [12] , Tepkaew et al. (2022) [14] , Almutairi et al. (2023) [1] and Attia and Shehata (2023) [3] in mango. Brassinosteroid increase the number of fruits at the pea stage, which ultimately leads to an improved final fruit set and reduce fruit drop per panicle at pea stage. Highest fruit set at the pea stage per panicle under brassinosteroid might be due to the stabilization of the developing embryo and the maintenance of a favorable endogenous hormonal balance. Brassinosteroid effectively suppress the activity of ACC synthase, thereby inhibiting ethylene biosynthesis and delaying the formation of the abscission layer at the fruit pedicel junction. By enhancing the plants antioxidant defense system through increased Superoxide Dismutase (SOD) and Peroxidase (POX) activity and improving water use efficiency through stomatal modulation brassinosteroids protect young fruitlets from oxidative stress and desiccation leading to an increased fruit set at the pea stage per panicle. The notable improvement in fruit set at pea stage recorded in the present investigation is Supported by the findings of Patel et al. (2021) [11] , Ramani et al. (2016) [12] , Tepkaew et al. (2022) [14] , Almutairi et al. (2023) [1] and Attia and Shehata (2023) [3] in mango, Mostafa and Kotb (2018) [8] in sugar apple and Isci and Gokbayrak (2015) in grapes.

The application of brassinosteroid significantly increases the number of fruits at the marble stage by enhancing the photosynthetic rate and ensuring a steady supply of nutrients which ultimately results in a higher final fruit set and reduction in fruit drop per panicle at marble stage. Besides this, brassinosteroid enhances metabolic sink strength prioritizing the flow of photo assimilates to the fruits and promotes vascular differentiation in the pedicel to provide mechanical support for the increasing fruit weight. Simultaneously, by upregulating the antioxidant defense system and modulating ethylene biosynthesis, brassinosteroid mitigate late season heat stress and prevent the formation of the abscission layer. The results recorded for the highest fruit set at marble stage in this study are validated by the research of Ramani et al. (2016) [12] , Tepkaew et al. (2022) [14] , Almutairi et al. (2023) [1] and Attia and Shehata (2023) [3] in mango. Brassinosteroid play vital role in reduction of number of nubbins (underdeveloped or seedless fruitlets) per 50 fruits at pea stage. Nubbins typically develop due to failure in fertilization or premature embryo abortion often triggered by physiological stress or hormonal deficiencies during the early post anthesis phase. When the embryo fails to develop the fruitlet loses its primary source of auxin signaling leading to a cessation of growth and subsequent abscission. Minimizing nubbins is primarily due to its role in ensuring successful embryo development. Brassinosteroid promote pollen tube elongation and stimulate active cell division within the newly formed embryo which prevents its premature abortion. Since a healthy developing seed serves as the physiological engine that provides the necessary hormonal signals to keep the fruit attached. The stabilization of the embryo by brassinosteroid ensures that the fruitlets do not revert to a nubbin state. Furthermore, it enhances the metabolic sink strength and the flow of metabolites toward the developing seed ensuring it has sufficient energy to complete its initial growth phase thereby significantly lowering the percentage of underdeveloped fruitlets. The reduction in number of nubbins at pea stage in the current study perfectly aligns with the findings of Tepkaew et al. (2022) [14] .

Conclusion

The findings of investigation lead to the conclusion that the foliar spray of brassinosteroid 1 ppm at full bloom, pea and marble stage improves the number of fruits as well as the fruit set percent at grain at grain, pea and marble stage, while reduce the fruit drop at pea and marble stage as well as the number of nubbins in mango cv. Kesar under HDP.

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