Effect of foliar nutrition on yield and quality parameters of green gram (Vigna radiata)

Introduction

Global pulse production reached 93.4 million tonnes in 2021, yet per-hectare yields of green gram have barely risen over the past two decades in southern Europe [1] . Green gram (Vigna radiata L. Wilczek) is valued for its short maturity, nitrogen-fixing ability, and high-protein grain, making it a good fit for rotations on the Iberian Peninsula [2] . But the crop's yield potential is often limited by transient nutrient deficiencies during flowering and grain filling, stages when root uptake alone may not meet demand [3] . Foliar fertilisation offers a quick route to correct these mid-season shortfalls. Urea sprayed on leaves at 2% concentration has been shown to raise leaf nitrogen status within 48 hours [4] . Zinc and boron, though needed only in trace amounts, are frequently deficient in calcareous Spanish soils and are linked to pollen viability and seed set [5,6] . Potassium applied through the leaf surface appears to regulate stomatal function and improve assimilate translocation to developing pods [7] . Combined multi-nutrient sprays remain less explored under Mediterranean conditions where high evaporative demand can affect spray retention [8,9] . This research was designed to evaluate the effect of single and combined foliar sprays on seed yield, quality parameters, and their interrelationships in green gram grown under semi-arid Mediterranean conditions [10,11] .

Material and Methods

Research area description

The trial was conducted at the experimental station of the Polytechnic University of Madrid (40°26′N, 3°44′W, 660 m elevation) during kharif 2022 (June-September). The site has a semiarid Mediterranean climate with 436 mm mean annual rainfall and summer temperatures around 24.8°C [12] . Soil was a sandy clay loam (pH 7.9, OC 0.73%, DTPA-Zn 0.54 mg kg⁻¹).

Material

Green gram variety Cristal (65-70 days maturity) was sown on 18 June 2022 at 25 kg ha⁻¹ in 30 cm rows. Foliar sources were urea (46% N), zinc sulphate heptahydrate (21% Zn), KH₂PO₄ (34.6% K₂O), and borax (11.3% B). A non-ionic surfactant (0.05% Tween 20) was added. Sprays were applied at flowering and early pod-fill using a knapsack sprayer at 500 L ha⁻¹.

Field experimental design

The experiment followed a randomized complete block design with three replications and seven treatments: T1 -absolute control (no spray), T2 -water spray, T3 -2% urea, T4 -0.5% ZnSO₄, T5 -1% KH₂PO₄, T6 -0.3% borax, and T7 -combined spray of T3+T4+T5+T6. Plot size was 4 m × 3 m with 1 m alleys between plots and 1.5 m between replications.

Methods

At maturity, five plants per plot were sampled for yield components. Seed yield was recorded from 4.8 m² net plot and converted to t ha⁻¹ at 12% moisture. Grain protein was estimated as Kjeldahl N × 6.25 [13] . Fe and Zn were measured by AAS after tri-acid digestion. Data were analysed by ANOVA in R (v4.3.1) with Duncan's test at p≤0.05 [14] . The combined foliar spray gave the highest seed yield (1.38 t ha⁻¹), 28.4% above the absolute control. Among individual nutrients, KH₂PO₄ was the most effective single spray (1.28 t ha⁻¹), followed by urea (1.24 t ha⁻¹). Water spray alone didn't differ significantly from the untreated control. Grain protein reached 24.9% under the combined spray, up from 21.7% in the control (Table 2). ZnSO₄ alone raised grain zinc from 28.4 to 36.8 mg kg⁻¹ and iron from 52.8 to 58.3 mg kg⁻¹.

Results

The combined treatment produced the highest Fe and Zn concentrations overall. The correlation heat map (Figure 2) shows strong positive associations between grain Fe and Zn concentrations (r = 0.81) and between pod length and seeds per pod (r = 0.76). Protein content was moderately correlated with both Zn (r = 0.72) and Fe (r = 0.67), suggesting that foliar micronutrient enrichment supports nitrogen metabolism in developing seeds.

Discussion

The 28.4% yield gain from combined foliar sprays agrees with reports from pulse crops in semi-arid regions, where multinutrient sprays at flowering raise seed yield by 20-35% [3,8] . KH₂PO₄'s superiority among single nutrients likely reflects potassium's role in phloem loading and assimilate movement toward reproductive sinks [7] . Zinc foliar application raised grain Zn by 29.6% over the control. This bio fortification effect has practical value given that zinc deficiency affects over 1.5 billion people globally [5,15] . The parallel rise in grain Fe with ZnSO₄ spray may stem from shared membrane transporters in developing seeds [16] . Protein improvement of 3.2 percentage points with the combined spray is attributable mainly to the urea component supplying nitrogen during grain filling [4] . The positive correlation between protein and micronutrient density hints at a shared sink-driven mechanism where enhanced grain filling draws in both amino acids and minerals [18,19] .

Conclusion

This research showed that combined foliar application of 2% urea, 0.5% ZnSO₄, 1% KH₂PO₄, and 0.3% borax at flowering and pod-fill stages raised green gram seed yield by 28.4% and grain protein from 21.7% to 24.9%. Among single nutrients, potassium dihydrogen phosphate was the most effective for yield while zinc sulphate was the most effective for grain micronutrient enrichment. Grain zinc and iron concentrations improved significantly, pointing to the biofortification potential of foliar Zn sprays in pulse cropping. For Mediterranean growers seeking both higher yield and better grain quality, a combined foliar spray at two key growth stages appears to be a simple and cost-effective intervention. Future work should test this strategy across different varieties and soil types.

References

1. FAO. FAOSTAT Crop Production Data. Rome: Food and Agriculture Organization; 2022.
2. Nair RM, Yang RY, Easdown WJ. Biofortification of mung bean (Vigna radiata) as a whole food to enhance human health. Journal of the Science of Food and Agriculture. 2013; 93(8):1805-1813.
3. Venkatesh MS, Basu PS. Foliar application of nutrients on growth and yield of pulses: a review. Agricultural Reviews. 2017; 38(4):250-260.
4. Mondal MMA, Puteh AB, Malek MA. Effect of foliar applied urea on growth and yield of mung bean. Bangladesh Journal of Botany. 2019; 48(2):291-297.
5. Cakmak I. Enrichment of cereal grains with zinc: agronomic or genetic biofortification? Plant and Soil. 2008; 302(1):1-17.
6. Sarker SK, Chowdhury AKMB, Dey K. Effect of boron on yield and seed quality of mung bean. Bangladesh Agronomy Journal. 2018; 21(1):73-80.
7. Tiwari KN. Potassium nutrition of pulse crops and its management. Indian Journal of Fertilisers. 2020; 16(2):130141.
8. Singh AK, Singh NP, Chaturvedi SK. Foliar nutrient management in chickpea under rainfed conditions. Indian Journal of Agronomy. 2016; 61(2):196-203.
9. Valenciano JB, Casquero PA, Boto JA. Influence of foliar fertilization on seed yield of common bean under Mediterranean climate. Communications in Soil Science and Plant Analysis. 2017; 48(11):1291-1299.
10. Kaur H, Sharma S, Kaur R. Grain quality and micronutrient profiling of green gram genotypes. Legume Research. 2020; 43(5):681-687.
11. Ram H, Singh G, Aggarwal N. Grain yield and zinc biofortification of mung bean with soil and foliar zinc application. Field Crops Research. 2013; 152:26-31.
12. AEMET. Climate Atlas of Spain, 1981-2020. Madrid: Spanish Meteorological Agency; 2021.
13. AOAC. Official Methods of Analysis. 18th ed. Washington DC: Association of Official Analytical Chemists; 2010.
14. Gomez KA, Gomez AA. Statistical Procedures for Agricultural Research. 2nd ed. New York: John Wiley and Sons; 1984.
15. Wessells KR, Brown KH. Estimating the global prevalence of zinc deficiency. PLoS ONE. 2012; 7(11):e50568.
16. Palmgren MG, Clemens S, Williams LE. Zinc biofortification of cereals: problems and solutions. Trends in Plant Science. 2008; 13(9):464-473.
17. Quddus MA, Rashid MH, Hossain MA. Effect of zinc and boron on yield and yield contributing characters of mung bean in low ganges river floodplain soil. Bangladesh Journal of Agricultural Research. 2011; 36(1):75-85.
18. Imran M, Rehim A. Zinc fertilization approaches for agronomic biofortification and estimated human bioavailability of zinc in wheat grain. Archives of Agronomy and Soil Science. 2017; 63(1):106-116.
19. Ullah A, Farooq M, Rehman A. Foliar application of boron and zinc improves seed yield and quality of lentil. Journal of Plant Nutrition. 2019; 42(16):1987-2001.