Plant Growth Regulators and Mango Fruit Retention: GA3, NAA and BAP on Bombay Green Mango in Sarlahi, Nepal

Which plant growth regulator improves mango fruit retention?

In mature Bombay Green mango trees at the Tropical Horticulture Centre (THC), Nawalpur, Sarlahi, Nepal, GA₃ at 35 ppm gave the highest fruit retention (7.71% against 4.87% in untreated trees). The same trial found that different regulators were best for different goals: BAP at 15 ppm gave the highest yield per tree (124.67 kg), NAA at 35–50 ppm gave the sweetest fruit (highest total soluble solids and TSS:acid ratio), and GA₃ at 45 ppm produced the largest fruits.

The study, "Effect of Different Concentrations of Growth Regulators on Fruit Retention and Total Soluble Solids in Mango (Bombay Var.) in Nawalpur, Sarlahi", was published in Integrated Urban Food Systems, pp. 67–75 (DOI 10.26480/iufs.01.2026.67.75). Bishal Neupane is first and corresponding author, with Binita Pandey and Pragya Bhurtel as co-authors. The authors acknowledge Prof. Shiva Chandra Dhakal, who supervised the original field study.

Why does mango fruit drop matter?

Only a small fraction of the flowers on a mango panicle set fruit and survive to maturity, and heavy fruit drop in the first weeks after pollination is a major cause of yield loss. The paper explains that fruit drop has several drivers, including nutrient shortage, competition among fruitlets, drought and pests, but that hormonal imbalance underlies much of the abscission in immature fruit, and that the problem worsens in older trees. Conventional orchard management (irrigation, nutrition and plant protection) does not correct this hormonal constraint.

Each of the three regulators tested acts through a different route. Auxins such as NAA help suppress abscission by maintaining auxin flow through the fruit stalk; gibberellins such as GA₃ promote fruit growth and sink strength; and cytokinins such as BAP (6-benzylaminopurine) stimulate cell division. THC maintains around 35 mango varieties, most about 20 years old, and the Bombay variety there consistently suffers from severe fruit drop, low fruit set and irregular flowering, which made it a suitable test case.

How the experiment was designed

• Site: Tropical Horticulture Centre, Lalbandi-01, Nawalpur, Sarlahi, Madhesh Province; field data collected over about five months in 2025.
• Trees: grafted Bombay Green mango, 10–15 years old, uniform size, under the same routine orchard management.
• Design: RCBD, ten treatments, three replications, one tree per experimental unit (30 trees).
• Treatments: NAA at 15, 35 and 50 ppm; GA₃ at 20, 35 and 45 ppm; BAP at 15, 30 and 45 ppm; and an untreated control.
• Application: regulators dissolved in 0.1 N NaOH, diluted, and sprayed at about 10 L per tree with a power sprayer at three stages: panicle initiation, pea stage and marble stage.
• Measurements: fruit retention on eleven tagged panicles per tree; floral sex ratio and hermaphrodite flower percentage; fruit length, width and weight; yield; TSS, titratable acidity (TA) and TSS:TA.
• Analysis: ANOVA in R 4.4.5 with the agricolae and gvlma packages; means separated by Duncan's Multiple Range Test at P < 0.05.

Results: fruit retention, yield and fruit quality

Treatment significantly affected fruit retention (P < 0.01), yield (P < 0.05), fruit length and width, TSS, TA and TSS:TA (all P < 0.001 for the quality traits). Fruit weight, floral sex ratio and hermaphrodite flower percentage did not differ significantly. The table combines the article's Tables 3, 4 and 5; values sharing a letter in a column do not differ significantly (DMRT).

Source: Neupane, Pandey and Bhurtel (2026), Tables 3–5. CV was 10.11% for fruit retention and 30.26% for yield.

Fruit retention

GA₃ at 35 ppm gave the highest retention (7.71%), followed by NAA at 35 ppm (6.70%) and BAP at 30 ppm (6.68%). The control (4.87%) and BAP at 45 ppm (5.63%) gave the lowest. The authors link the GA₃ effect to relief of carbohydrate stress during heavy flowering, through activation of α-amylase and better nutrient mobilisation.

Yield

BAP at 15 ppm (124.67 kg tree^-1) and GA₃ at 35 ppm (115.67 kg tree^-1) gave the highest yields and were statistically comparable. The lowest yield came from NAA at 50 ppm (42.67 kg tree^-1), consistent with the abstract's observation that excessive NAA tended to reduce fruit number. Readers should note that yield had a high CV (30.26%) and that, by the DMRT letters, the two highest-yielding treatments were not statistically separated from the untreated control (92.67 kg tree^-1).

Fruit size and sweetness

The longest and widest fruits came from GA₃ at 45 ppm (9.86 cm long, 7.81 cm wide), and the heaviest fruit was also recorded there (276.19 g), although fruit weight differences were not significant. NAA at 35 ppm gave the highest TSS (17.80 °Brix), and NAA at 50 ppm combined high TSS with the lowest acidity (0.25%), giving the highest TSS:TA ratio (69.88). GA₃ treatments tended to give lower TSS, which the authors attribute plausibly to a dilution effect as rapid cell expansion increased fruit volume.

Why did no treatment change flower sex ratio?

Floral sex ratio and hermaphrodite flower percentage did not differ among treatments (CVs of 69.14% and 66.79%). The authors explain this by spray timing: floral sex expression is largely fixed before bloom, so sprays at panicle initiation, pea and marble stages came after the critical window. They recommend that future work apply regulators before bloom to test effects on floral sex expression.

What this means for mango growers

The paper proposes a concentration-specific hormonal strategy for ageing Bombay Green orchards: GA₃ at 35 ppm for fruit retention, BAP at 15–30 ppm for yield, and NAA at 35–50 ppm for fruit quality. The goal therefore decides the regulator. A grower whose main loss is early fruit drop would look at GA₃; one selling on sweetness would look at NAA, while avoiding the 50 ppm NAA rate if fruit number is the priority.

What this means for researchers

• With a single tree per plot and three replications, tree-to-tree variation is large; the high CVs for yield and floral traits suggest more replicate trees per treatment.
• Test pre-bloom applications to see whether floral sex expression can be shifted.
• Repeat across several seasons to separate treatment effects from year-to-year variation in flowering.

A related regulator study on a vegetable crop is summarised in the effect of GA3 and NAA on okra yield in Sunsari, and the Katahari okra trial covers Neupane's own NAA and GA₃ work. Students setting up the analysis can follow the RCBD and DMRT guide in R.

Limitations

The study ran over one season at one orchard. Variability was high for yield and some quality traits, which the authors attribute to the open-orchard setting, natural differences in canopy size and flowering intensity, occasional disturbance by monkeys and blue bull, and a four-to-five-month field window. Sprays were applied after floral differentiation, so effects on flower sex could not be assessed.

How to cite this study

Neupane, B., Pandey, B. and Bhurtel, P. (2026). Effect of Different Concentrations of Growth Regulators on Fruit Retention and Total Soluble Solids in Mango (Bombay Var.) in Nawalpur, Sarlahi. Integrated Urban Food Systems, 67–75. https://doi.org/10.26480/iufs.01.2026.67.75

See the publications page for Bishal Neupane's other articles and the CV for his research background. Supervisors and collaborators are welcome to get in touch through the contact page.