Thermal Response of Spring–Summer-Grown Black Gram (Vigna mungo L. Hepper) in Indian Subtropics

The thermal environment of a crop is one of the prime factors enhancing growth and production by regulating its physiological processes at different phenophases. To study the impact of thermal regime on spring–summer-grown black gram (variety Pant Urd 31), an experiment was conducted with different sowing dates (from the first to the third week of March), soil application of cobalt (Co) and foliar sprays of potassium (K) and boron (B) in various combinations in the split–split plot design during 2020 and 2021. The first-week-of-March-sown crop recorded more accumulated growing degree-days (GDD), photothermal units (PTU) and heliothermal units (HTU) with a longer duration than the later sown crop. Higher daily mean temperature during the reproductive stage of the later sown crop compelled it to complete the phenophases earlier than the normally sown crop, leading to yield reduction. Soil application of Co at 4 kg ha−1 and foliar sprays of K at 1.25% and B at 0.2% mitigated the adversities of excess heat irrespective of sowing dates. Variations in GDD and HTU, respectively, explained variations of about 75.8% and 87.3% in the final dry matter accumulation and of 72.9% and 84.8% in seed yield through polynomial regressions in the respective years. The maximum mean thermal use efficiency (TUE) for biomass production (0.24 g m−2/°C/day−1) and seed yield (0.11 g m−2/°C/day−1) were observed with Co soil application and combined foliar sprays of K and B due to higher dry matter production or seed yield with lower heat units accumulation in the first sown crop.