α-Tocopherol Application Modulates the Response of Wheat (Triticum aestivum L.) Seedlings to Elevated Temperatures by Mitigation of Stress Injury and Enhancement of Antioxidants

Wheat seedlings (4 days old) were subjected to varying temperatures of 25, 30, and 35 A degrees C for 7 days in a growth chamber under hydroponic conditions in the absence or presence of alpha-tocopherol (5 mu M). The growth of shoots and roots was inhibited severely at 35 A degrees C. The endogenous alpha-tocopherol increased in the shoots at 30 A degrees C over the controls but decreased significantly at 35 A degrees C over the previous temperature. The exogenous application of alpha-tocopherol elevated the endogenous levels in the heat-stressed plants, which were consequently able to maintain significantly greater growth associated with reduction in damage to membranes, cellular oxidizing ability, chlorophyll content, and photochemical efficiency in shoots. The relative leaf water content and stomatal conductance were not affected significantly with the application of tocopherol. The oxidative stress induced by high temperature (35 A degrees C) in terms of malondialdehyde and hydrogen peroxide contents was significantly lower in the presence of alpha-tocopherol. The enzymatic antioxidants such as superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase showed considerable reduction in their activities at 35 A degrees C compared to those at 30 A degrees C, with greater effects on APX and GR. The nonenzymatic antioxidants like ascorbate, glutathione, and proline increased at 30 A degrees C but decreased appreciably at 35 A degrees C, suggesting impairment in their synthesis at stressful temperatures. alpha-Tocopherol-treated plants, especially those growing at 35 A degrees C, had improved levels of enzymatic and nonenzymatic antioxidants. These observations provided evidence about the involvement of alpha-tocopherol in governing heat sensitivity in wheat and suggested manipulation of its endogenous levels to induce heat tolerance in this crop.