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