Abstract
Heat and soil moisture stress account for serious abiotic constraint in black gram (Vigna mungo (L.) Hepper) production during spring-summer under Gangetic plains of Eastern India. Concurrence of these two can bring about early completion of phenophases that hampers normal metabolism of legumes by disrupting their defense mechanism, leading to poor seed set. The field experiment was conducted with two different sowing dates as the main plot, soil application of cobalt (Co) as subplots and foliar sprays of potassium (K) and boron (B) either alone or in combination as sub-sub plot treatment in a split-split plot design during spring-summer seasons of 2020 and 2021 with black gram (variety: Pant U 31). The study was aimed at evaluating the impact of sowing time and nutrients application alleviating adversities of abiotic stress during reproductive development of black gram. The March first week sown crop took significantly higher days to complete its life cycle compared to March third week sown one (82.0 vs. 78.2 and 81.8 vs. 78.8). This in turn relatively allowed a broader window for leaf area expansion, flowering, and seed filling in the first crop compared to the second one leading to the attainment of superior yield in the normal sown crop during the consecutive years. Crop growth rate (CGR), net assimilation rate (NAR), pod number per plant, seed yield, and harvest index were significantly higher (p <= 0.05) with soil Co @ 4 kg ha(-1) and foliar 1.25% K + 0.2% B applications through stress mitigation by stimulating chlorophyll biosynthesis, nitrate reductase activity, proline accumulation, and cell membrane stability, irrespective of the years. Fluctuations in per plant pod number explained about 96 and 94% variations in seed yield through linear regressions in respective years. Optimum sowing date along with soil Co application combined with foliar K+B sprays manifested immense potential to achieve higher black gram production. In addition, this nutrient schedule proved to be efficient enough to promote satisfactory growth and optimum seed yield of late sown blackgram through relief of stress during the spring-summer season.