Food crops face rising temperatures: An overview of responses, adaptive mechanisms, and approaches to improve heat tolerance

The rising temperatures are resulting in heat stress for various agricultural<br/>crops to limit their growth, metabolism, and leading to significant loss of yield<br/>potential worldwide. Heat stress adversely affects normal plant growth and development<br/>depending on the sensitivity of each crop species. Each crop species has its own<br/>range of temperature maxima and minima at different developmental stages beyond<br/>which all these processes get inhibited. The reproductive stage is on the whole more<br/>sensitive to heat stress, resulting in impaired fertilization to cause abortion of flowers.<br/>During seed filling, heat stress retards seed growth by affecting all the biochemical<br/>events to reduce seed size. Unfavorable temperature may significantly affect photosynthesis,<br/>respiration, water balance, and membrane stability of leaves. To combat<br/>heat stress, plants acquire various defense mechanisms for their survival such as<br/>maintaining membrane stability, and scavenging reactive oxygen species by generating<br/>antioxidants and stress proteins. Thermo-tolerance can be improved by the accumulation<br/>of various compounds of low molecular mass known as thermo-protectants<br/>as well as phyto-hormones. Exogenous application of these molecules has benefited<br/>plants growing under heat stress. Alternatively, transgenic plants over-expressing<br/>the enzymes catalyzing the synthesis of these molecules may be raised to increase<br/>their endogenous levels to improve heat tolerance. In recent times, various transgenics<br/>have been developed with improved thermo-tolerance having potential benefits<br/>for inducing heat tolerance in food crops. Updated information about of the effects<br/>of heat stress on various food crops and their responses as well as adaptive mechanisms<br/>is reviewed here.