Kadambot H. M. Siddique

Ambient temperatures have increased since the beginning of the century and are predicted to continue rising under climate change. Such increases in temperature can cause heat stress: a severe threat to wheat production in many countries, particularly when it occurs during reproductive and grain-filling phases. Heat stress reduces plant photosynthetic capacity through metabolic limitations and oxidative damage to chloroplasts, with concomitant reductions in dry matter accumulation and grain yield. Genotypes expressing heat shock proteins are better able to withstand heat stress as they protect proteins from heat-induced damage. Heat tolerance can be improved by selecting and developing wheat genotypes with heat resistance. Wheat pre-breeding and breeding may be based on secondary traits like membrane stability, photosynthetic rate and grain weight under heat stress. Nonetheless, improvement in grain yield under heat stress implies selecting genotypes for grain size and rate of grain filling. Integrating physiological and biotechnological tools with conventional breeding techniques will help to develop wheat varieties with better grain yield under heat stress during reproductive and grain-filling phases. This review discusses the impact of heat stress during reproductive and grain-filling stages of wheat on grain yield and suggests strategies to improve heat stress tolerance in wheat.

Drought is a major environmental stress threatening wheat productivity worldwide. Global climate models predict changed precipitation patterns with frequent episodes of drought. Although drought impedes wheat performance at all growth stages, it is more critical during the flowering and grain-filling phases (terminal drought) and results in substantial yield losses. The severity and duration of the stress determine the extent of the yield loss. The principal reasons for these losses are reduced rates of net photosynthesis owing to metabolic limitations—oxidative damage to chloroplasts and stomatal closure—and poor grain set and development. A comprehensive understanding of the impact of terminal drought is critical for improving drought resistance in wheat, with marker-assisted selection being increasingly employed in breeding for this resistance. The limited success of molecular breeding and physiological strategies suggests a more holistic approach, including interaction of drought with other stresses and plant morphology. Furthermore, integration of physiological traits, genetic and genomic tools, and transgenic approaches may also help to improve resistance against drought in wheat. In this review, we describe the influence of terminal drought on leaf senescence, carbon fixation, grain set and development, and explain drought resistance mechanisms. In addition, recent developments in integrated approaches such as breeding, genetics, genomics, and agronomic strategies for improving resistance against terminal drought in wheat are discussed.