![]() Īlthough leaf ultrastructural changes may reflect the effects of HT stress, there is scant description of the anatomical and ultrastructural changes in wheat leaves under HT stress. Similarly, the thylakoid membranes of an Arabidopsis mutants deficient in ω-6 fatty acid unsaturation ( fad6) showed increased stability at HT, and decreased lipid unsaturation in tobacco caused by silencing an ω-3 desaturase gene rendered the plants more tolerant to HT. In soybean, a mutant deficient in fatty acid unsaturation showed strong tolerance to HT. The significance of membrane fluidity in temperature tolerance has been elucidated by mutation analysis and transgenic and physiological studies. Temperature-induced changes in membrane fluidity is one of the immediate consequences of temperature changes, including HT stress, in plants. Several studies indicate that HT stress increases membrane damage and decreases antioxidant levels in wheat at the seedling stage, anthesis stage, or grain filing stage. ![]() Earlier studies have shown that triacylglycerols (TAGs) are accumulated under HT stress however, the changes in the saturation index of these TAG under HT stress are not known. Similar to ROS, oxidized lipids may act as signaling molecules that initiate stress responses in plants. In Arabidopsis, oxidized lipids are produced enzymatically through the action of lipoxygenase as well as non-enzymatically through the action of ROS. Additionally, membranes may serve both as sources of ROS during plant stress and as reservoirs to take up ROS. ![]() The fate of these lipid species under HT stress is not fully understood. In wheat, galactolipids (monogalactosyldiacylglycerol MGDG and digalactosyldiacylglycerol DGDG) are the major chloroplast lipids, and trienoic species of MGDG and DGDG are highly vulnerable to peroxidation by ROS and by lipoxygenase. The thylakoid membranes are the location for the light-dependent reactions for photosynthesis. The membrane plays important roles in sensing environmental change, signal transduction and substance metabolism. Membranes are the targets of HT stress and membrane lipid composition is a crucial factor for temperature tolerance or susceptibility. The impacts of HT stress on leaf level photosynthesis is known, however, the mechanisms driving tolerance or susceptibility based on various membrane lipid species (polar lipids, triacylglycerol, oxidized lipids and acylated galactolipids), ultrastructure of cell organelles and its association with oxidants and antioxidant enzyme activity are not well understood. Elevated temperatures decrease the activities of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT) and peroxidase (POX). Apart from this, HT increases production of reactive oxygen species (ROS) including superoxide radical (O 2 −) and hydrogen peroxide (H 2O 2) and increase lipid peroxidation and cause membrane damage. HT stress causes damage to thylakoid membranes and decreases photosystem II (PSII) quantum yield and photosynthesis. Keeping in view the predicted increase in growing season temperature in wheat producing areas, it is important to understand mechanisms of HT tolerance in wheat to maintain and improve yield potential. The optimal temperature (OT) for anthesis and grain filling ranges from 12 to 22 ☌ for wheat, and grain yield is significantly reduced with HT. Wheat is very sensitive to HT during its floral development and grain-filling phase. An increase in number of hot days and temperature variability is also predicted. In fact, global mean surface air temperature has increased by 0.8 ☌ in the twentieth century and is predicted to increase further by 3–5 ☌ by the end of twenty-first century. ) is grown in about 30% of the world’s area cultivated with cereals, occupying over 220 million hectares worldwide of which 50% of the area experiences high temperature (HT) stress.
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