Effect of foliar application of boron, zinc and potassium on flowering, fruiting and yield of avocado (Persea americana Mill.) cv. Hass

Introduction

Avocado (Persea americana Mill.) is an important subtropical fruit crop belonging to the family Lauraceae and is widely valued for its nutritional and economic importance. The crop originated in Mexico and Central America and has subsequently been introduced into several tropical and subtropical regions of the world. In India, avocado cultivation is mainly confined to Karnataka, Tamil Nadu, Kerala, Maharashtra, Sikkim and the north-eastern states, where its cultivation is gradually expanding. The reproductive biology of avocado is highly complex. The flowers exhibit Protogynous Diurnally Synchronous Dichogamy (PDSD), which influences pollination and fruit set. Although the trees produce an abundance of flowers, the fruit-set is only 0.1 per cent (Whiley et al., 1994) [1] . Consequently, poor fruit-set and excessive fruit drop are important constraints limiting avocado productivity. In addition to environmental conditions and pollination, the nutritional status of the tree plays a major role in flower development, fertilization, fruit-set and subsequent fruit retention. Boron is an essential micronutrient for reproductive development and plays an important role in pollen viability, pollen germination and pollen tube growth. Adequate boron nutrition can therefore improve fertilization and fruit-set and foliar application of boron together with zinc has been reported to enhance fruit set, yield and fruit quality in avocado. Zinc is another important micronutrient involved in several physiological and biochemical processes, including enzyme activation and auxin synthesis and adequate zinc nutrition is important for the normal growth and productivity of avocado. Potassium plays an important role in photosynthesis, enzyme activation, osmotic regulation and translocation of photosynthates. During the reproductive phase, balanced availability of these nutrients may improve flower initiation, fertilization, fruit set, fruit retention and subsequent yield. Considering the importance of balanced nutrient management during the reproductive phase of avocado, the present investigation was undertaken to study the influence of foliar application of boron, zinc and potas sium, individually and in combination, on flowering, fruiting and yield of avocado cv. Hass.

Materials and methods

The present investigation was conducted during 2025-26 at Varashree Farm and Nursery, Sakrebylu, Gajanuru, Shivamogga district, Karnataka, India. The experiment was conducted on four-year-old plants of avocado (Persea americana Mill.) cv. Hass. The experiment was laid out in a Randomized Complete Block Design with twentyone treatments and two replications. Each replication comprised of three plants. All experimental trees received a common recommended dose of fertilizer consisting of 400 g nitrogen, 120 g phosphorus and 240 g potash along with 30 kg farmyard manure per tree. The foliar nutrient treatments were applied in addition to this common fertilizer application. The experiment comprised twenty-one treatments, namely, T 1 -control (water spray), T 2 -boron (0.2%), T 3 -boron (0.3%), T 4 -zinc (0.2%), T 5 -zinc (0.4%), T 6 -potassium (0.5%), T 7 -potassium (1.0%), T 8 -boron (0.2%) + zinc (0.2%), T 9 -boron (0.3%) + zinc (0.4%), T 10 -boron (0.2%) + potassium (0.5%), T 11 -boron (0.2%) + potassium (1.0%), T 12 -boron (0.3%) + potassium (0.5%), T 13boron (0.3%) + potassium (1.0%), T 14 -zinc (0.2%) + potassium (0.5%), T 15 -zinc (0.2%) + potassium (1.0%), T 16 -zinc (0.4%) + potassium (0.5%), T 17 -zinc (0.4%) + potassium (1.0%), T 18 -boron (0.2%) + zinc (0.2%) + potassium (0.5%), T 19 -boron (0.2%) + zinc (0.2%) + potassium (1.0%), T 20 -boron (0.3%) + zinc (0.4%) + potassium (0.5%) and T 21 -boron (0.3%) + zinc (0.4%) + potassium (1.0%). The first foliar spray was applied fifteen days prior to flowerbud induction, while the second spray was applied fifteen days after anthesis.

Observations recorded

Number of panicles per square metre of canopy was recorded at full bloom using a one metre × one metre quadrat. The number of fruits per panicle was recorded from tagged panicles after fruit set. The number of fruits per tree was recorded by counting the fruits retained on each tagged tree until harvest. Fruit yield per tree was expressed in kilograms per tree, while fruit yield per acre was calculated from the plot yield and expressed in tonnes per acre.

Statistical analysis

The data recorded for different flowering, fruiting and yield parameters were subjected to analysis of variance appropriate for a Randomized Complete Block Design. Treatment means were compared using the critical difference (CD) at five per cent level of significance.

Results and discussion

Number of panicles per square metre of canopy

Foliar application of potassium and micronutrients significantly affected the number of panicles per square metre of canopy and the number of fruits per panicle in avocado cv. Hass. The highest number of panicles per square metre of canopy was recorded with T 8 [Boron (0.2 %) + Zinc (0.2 %)] with 8.40 panicles, which was statistically on par with T 18 [Boron (0.2 %) + Zinc (0.2 %) + Potassium (0.5 %)] with 8.16 panicles and T 19 [Boron (0.2 %) + Zinc (0.2 %) + Potassium (1.0 %)] with 7.72 panicles (Table 1). The lowest number was recorded in the control, T 1 , with 2.91 panicles. Zinc is required for tryptophan and auxin metabolism and supports leaf chlorophyll formation, protein synthesis and several enzyme systems. Boron supports meristematic activity, cell-wall development and carbohydrate movement. An adequate supply of both nutrients during the period preceding flowering may therefore have favoured the differentiation and emergence of more inflorescences per unit canopy area. The result is consistent with the observations of Mans (1996) [2] , Crowley et al. (1994) [3] .

Number of fruits per panicle

A similar response was observed for number of fruits per panicle.

Treatment T 8 recorded the maximum number of fruits per panicle (5.10), which was statistically on par with T 18 (4.86) and T 19 (4.62).

The minimum number of fruits per panicle was observed in T 1 (2.55) (Table 1). The superior response of T 8 suggests that the direct combination of boron and zinc at moderate concentrations was particularly effective in improving reproductive efficiency at the panicle level.

The increased number of fruits per panicle may be related to enhanced pollen viability, pollen germination and pollen-tube progression due to boron along with improved auxin synthesis and enzyme activity due to zinc. Abdel-Karim et al. (2015) [4] observed a positive synergistic effect of boron and zinc on fruit set in avocado.

Figure 1. Influence of foliar application of potassium and micronutrients on number of fruits per panicle

Number of fruits per tree

The maximum number of fruits per tree was recorded in T 18 with 143.50 fruits, which was statistically on par with T 8 with 138.60 fruits per tree, T 19 recorded 132.20 fruits per tree. The lowest number of fruits per tree was recorded in the control T 1 , with 80.75 fruits (fig 2). The superiority of the combined treatment may be attributed to the complementary functions of boron, zinc and potassium during flowering, fertilization, fruit set and subsequent fruit retention. The present response agrees with Jaganath and Lovatt (1995) [5] , Dixon et al. (2006) [6] .

Fruit yield per tree

Treatment T 18 recorded the highest fruit yield per tree (38.25 kg) and was statistically on par with T 8 (36.00 kg) and T 19 (34.74 kg). The minimum fruit yield per tree was recorded in the control T 1 , with 13.04 kg (Table 2). Boron probably improved fertilization and initial fruit set, zinc maintained metabolic activity and hormonal balance and potassium strengthened the source-sink relationship by facilitating assimilate synthesis and translocation towards developing fruits.

The findings also agree with El-Sherif et al. (2000) [7] , Abdel-Karim et al. (2015) [4] .

Fruit yield per acre

The maximum fruit yield was recorded in T 18 with 8.49 t per acre. This treatment was followed by T 8 with 7.99 t per acre and T 19 with 7.71 t per acre. The lowest fruit yield per acre was recorded in the control, T 1 , with 2.89 t per acre. The higher yield in T 18 may be attributed to the cumulative effect of increased fruit number per tree, fruit yield per tree and improved fruit growth. The present findings are in agreement with Dutta (2004) [8] , Banik et al. (1997) [9] .

Conclusion

The present investigation demonstrated that foliar application of boron, zinc and potassium significantly influenced flowering, fruiting and yield of avocado cv. Hass. Boron at 0.2 per cent combined with zinc at 0.2 per cent was the most effective treatment for improving the number of panicles per square metre of canopy and number of fruits per panicle. However, the combined foliar application of boron at 0.2 per cent, zinc at 0.2 per cent and potassium at 0.5 per cent was superior for overall fruiting and yield, recording the highest number of fruits per tree (143.50), fruit yield per tree (38.25 kg) and fruit yield per acre (8.49 t).

The results indicated that boron and zinc were particularly important for improving flowering and initial fruit-set, whereas the addition of potassium contributed to improved fruit retention and final yield. Therefore, foliar application of boron at 0.2 per cent + zinc at 0.2 per cent + potassium at 0.5 per cent was found to be the most effective nutrient combination for improving fruiting and yield of avocado cv.

Hass under the conditions of the present investigation.

Acknowledgement

AcknowledgmentsThe authors sincerely acknowledge the support provided by the College of Horticulture, Mudigere and College of Agriculture, Shivamogga for the facilities provided for conducting the present investigation. The authors also acknowledge Varashree Farm and Nursery, Sakrebylu, Gajanuru, Shivamogga district, for providing the experimental site and necessary cooperation during the study. The authors declare no conflict of interest related to this work.

References

1. Whiley AW, Schaffer B. Avocado. In: Schaffer B, Andersen PC. Handbook of Environmental Physiology of Fruit Crops. Vol. 2, Subtropical and Tropical Crops. Boca Raton, Florida: CRC Press; 1994. p. 3-35.
2. Mans CC. Effect of foliar feeding of 'Hass' at various stages of flowering. South African Avocado Growers' Association Yearbook. 1996;19:31-32.
3. Crowley DE, Smith W, Faber B. Zinc nutrition of avocado. California Avocado Society Yearbook. 1994;78:155-165.
4. Abdel-Karim HA, Nehad MA, El-Rouby KM, Roshdy KA. Effect of foliar application of boron and zinc on fruit set, yield and some fruit characteristics of 'Fuerte' avocado. Research Journal of Pharmaceutical, Biological and Chemical Sciences. 2015;6(5):443-449.
5. Jaganath I, Lovatt CJ. Efficacy studies on prebloom canopy applications of boron and/or urea to 'Hass' avocados in California. Proceedings of the World Avocado Congress. 1995;III:181-184.
6. Dixon J, Smith DB, Elmsly TA, Greenwood AC, Dixon EM. The impact of foliar applications of nitrogen and boron on 'Hass' avocado fruit set. New Zealand Avocado Growers' Association Annual Research Report. 2006;6:35-46.
7. El-sherif AA, Saeed WT, Nouman VF. Effect of foliar application of potassium and zinc on behaviour of Montakhab El-Kanatar guava trees. The Bulletin of Faculty of Agriculture, University of Cairo. 2000;51(1):73-84.
8. Dutta P. Effect of foliar boron application on panicle growth, fruit retention and physico-chemical characters of mango cv. Himsagar. Indian Journal of Horticulture. 2004;61(3):265-266.
9. Banik BC, Sen SK, Bose TK. Effect of zinc, iron and boron in combination with urea on growth, flowering, fruiting and fruit quality of mango cv. Fazli. Environment and Ecology. 1997;15(1):122-125.
10. Bergh BO. The origin, nature and genetic improvement of the avocado. California Avocado Society Yearbook. 1992;76:61-75.
11. Chanderbali AS, Soltis DE, Soltis PS, Wolstenholme BN. Taxonomy and botany. In: Schaffer B, Wolstenholme BN, Whiley AW, editors. The Avocado: Botany, Production and Uses. 2nd ed. Wallingford, UK: CABI; 2013. p. 31-50.
12. Coetzer LA, Robbertse PJ, Janse van Vuuren BPH. The role of boron in avocados: theory, practice and reality. South African Avocado Growers' Association Yearbook. 1993;16:2-4.
13. Fisher RA, Yates F. Statistical Tables for Biological, Agricultural and Medical Research. 6th ed. Edinburgh: Oliver and Boyd; 1963.
14. Hapuarachchi N, Kamper W, Wallace HM, Nichols J, Trueman SJ. Boron effects on fruit set, yield, quality and paternity of 'Hass' avocado. Agronomy. 2022;12(6):1479.
15. Lovatt CJ. Improving fruit set and yield of 'Hass' avocado with a spring application of boron and/or urea to the bloom. California Avocado Society Yearbook. 1994;78:167-173.
16. Robbertse PJ, Coetzer LA, Bessinger F. The influence of Solubor leaf spray on fruit production in avocado. South African Avocado Growers' Association Yearbook. 1991;14:83-84.
17. Salazar-García S, Cossio-Vargas LE, González-Durán IJL. Correction of chronic zinc deficiency in 'Hass' avocado. Revista Chapingo Serie Horticultura. 2008;14(2):153-159.
18. Schaffer B, Wolstenholme BN, Whiley AW. The Avocado: Botany, Production and Uses. 2nd ed. Wallingford, UK: CABI; 2013.