Terminal heat stress modulates growing degree days, heat use efficiency and yield stability index in brassica juncea

An experiment was planned to visualize the impact of terminal heat stress on yield potentials of late sown mustard cultivars under the changing climatic conditions. A set of fifteen B. juncea genotypes were evaluated at two planting dates during two winter seasons. Late planted genotypes faced heat stress during post anthesis/seed filling stage which negatively influenced the movements of photosynthates to developing sinks thus inhibiting synthetic processes, lowering seed weight and seed yield and may even hamper seed quality. Onset of flowering and siliquing were delayed but the completion of flowering and siliquae formation was earlier in the late planted genotypes. Seed filling was impeded by high temperature in late planting causing ≥50% decline in seed yield. Average accumulated growing degree days (AGDD) for phenological stages were higher in normal sown than the late sown crop. AGDD witnessed an increasing trend for flowering and fruiting behaviour in both the sowing dates. However, 50% flowering in late sown crop took 4 accumulated GDDoC days more. Average heat use efficiency was found to be significantly higher in normal sown genotypes for seed (0.95 kg ha-1 ºC-days-1) and biological (0.53 kg ha-1 ºC-days-1) yield relative to the late sown genotypes for seed (0.50 kg ha-1 ºC-days-1) and biological yield (2.71 kg ha-1 ºC-days-1). Seed yield under late sown condition had positive and significant correlation with siliquing duration (r= 0.600*) and reproductive phase (r= 0.558*) whereas, negative with flowering completion (r= -0.573*) and siliqua initiation (r= -0.519*). Based on the results IAN was the best genotype for delayed sowings.