Response of seedlings to heat-stress in cultivars of wheat: Growth temperature-dependent differential modulation of photosystem 1 and 2 activity, and foliar antioxidant defense capacity

Abstract

Impact of elevated temperature on the development of photosystems and foliar antioxidant activity in greening seedlings was examined for characterizing the response of wheat (Triticum aestivum L.) cultivars to heat-stress. Greening of seedlings for 60 h at temperatures above 25 °C induced alterations in photosystem (PS) 1 and 2 activities differentially in chloroplasts isolated from developing primary leaves. The electron flow driven by PS1 was stimulated in contrast to sharp inhibition of that mediated by PS2. Compared to HD1553, HD2329, and C306, heat-stressed seedlings of HD2307 had higher percentage of functional photosystem 2. In order to characterize the differential stability of both photosystems and the role of the antioxidant defense system, if any, in response to heat-stress among cultivars, the activity of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase enzymes associated with ascorbate-glutathione recycling and scavenging of H2O2 was analyzed. Heat-stress during greening led to stimulation in foliar superoxide dismutase activity, indicating a potential source for generation of its substrate, superoxide radical (O−2), in light. Ascorbate peroxidase, a key H2O2-metabolizing enzyme, was stimulated differentially within cultivars and maximally in HD2307. Moreover, the efficiency of this enzyme exhibited a linear rise with increase in growth temperature. H2O2-utilizing catalase enzyme was also stimulated in seedlings greened at non-lethal temperatures of 30 and 35 °C, but was inactivated markedly in 40 °C-stressed seedlings of different cultivars, except HD2307. The level of foliar ascorbate and glutathione increased in response to greening at elevated temperatures in cultivars, albeit markedly in HD2307. In summary, heat-stress during greening led to inactivation of PS2 vis-à-vis highturn over of both PS1 and superoxide dismutase, which allowed formation of superoxides and its dismutation product, H2O2, and reversible modulation of enzymatic H2O2-scavenging. This suggests a central role of H2O2 metabolism in seedling response to thermal stress. Heat-tolerance potential of cultivars, evident in HD2307 seedlings, co-relates to the potential of ascorbate peroxidase and catalase to dissipate H2O2 co-operatively and ameliorate photo-oxidative damage to heat-stressed seedlings. Ascorbate peroxidase, because of its temperature-dependent linear activation and stability over a broad range of elevated temperatures, is suitable for introducing tolerance to heat-stress in sensitive crops. This study signifies the importance of antioxidant evaluation in selection of tolerant lines as well as the role of H2O2 in heat-stress response of seedlings.