Heat and drought are the main abiotic constraints in harvesting potential yield of the crops on sustained basis. Climate change has a serious threat to cereal production levels required to feed future generations across the globe, hence create food insecurity. Global warming is predicted to increase the frequency and severity of heat waves in crops growing areas in the world. Heat stress to varying degrees at different phonological stages causes a substantial yield reduction, primarily because of growth acceleration, reduction in duration of phasic developmental stages and carbon starvation owing to reduced net assimilation. The probability of heat stress around flowering result in considerable yield loss is also predicted to increase significantly in coming years.

In major wheat growing regions, temperature exceeding 30ºC during reproductive growth and reduces grain yield and quality, which is due to a wide range of interlinked processes, including accelerated development, reduced photosynthesis; either via damage to photosystem-II or inhibition of Rubisco activase, increased respiration, disruptions of the respiratory mechanism and decreased starch synthesis in developing grain.

Determining mechanisms associated with heat tolerance and identifying screening methods are vital for improvement of heat tolerance in plants. The high grain filling rate and a high potential grain weight are major traits that can be useful to improve heat tolerance. Application of Ca2+ in the form of CaCl2 prior to the stress treatment elevated the content of lipid peroxidation product and stimulated the activities of super oxide dismutase and catalase, thus improve the crop performance. Heat stress reduced the grain size and with increase in temperature from 30 to 38°C during the reproductive growth phase, there was 20 to 44 per cent reduction in grain weight. It also influences the flour and bread quality and protein contents as well in wheat.

As the world water supply is declining, drought is also threatening the world food security. Drought-induced yield losses exceed the loss from all other causes, as both severity and duration of stress are critical. Drought stress reduces crop growth rate and yield regardless of the growth stage at which it occurs in arable crops. In the current scenario of water scarcity, drought is perhaps the major environmental hazard for plant productivity. Arid and semi-arid areas are the first and foremost victim of drought stress. Drought stress has pronounced effects on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in the form of physiological, biochemical, and molecular responses.

Exposure of plants to drought stress substantially decreases the leaf water potential, relative water content and transpiration rate, with a concomitant increase in leaf temperature. Although components of plant water relations are affected by reduced availability of water, stomatal opening and closing is more strongly affected. An important effect of water deficit is on the acquisition of nutrients by the root and their transport to shoots. Lowered absorption of the inorganic nutrients can result from interference in nutrient uptake and the unloading mechanism, and reduced transpirational flow.

Due to the lack of motility and immune system, plants have elaborated alternative defense strategies, involving the huge variety of secondary metabolites as tools to overcome stress constraints, thus adapt to the changing environment. The secondary metabolites are of major interest because of their multiple functions and impressive biological activities ranging from antimicrobial, antibiotic, insecticidal, hormonal properties to highly important pharmacological and pharmaceutical activities. These secondary metabolite including phenolics, brassinosteroids, jasmonates, salicylates, polyamines etc., are involved in resistance against several biotic and abiotic stresses. So, it is highly recommended that methods should be develop to use these metabolites to cope with these yield limiting stresses and to produce the large quantity of the food to meet the requirements of the people in the future.

The author is M.Sc. (Hons.) Agronomy, University of Agriculture, Faisalabad, Pakistan.

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