Leaf senescence and grain filling affected by post-anthesis high temperatures in two different wheat cultivars

High temperature is a major factor affecting grain yield and plant senescence in wheat growing regions of central and east China. In this study, two different wheat cultivars, Yangmai 9 with low-grain protein concentration and Xuzhou 26 with high-grain protein concentration, were exposed to different temperature regimes in growth chambers during grain filling. Four day/night temperature regimes of 34 degrees C/22 degrees C, 32 degrees C/24 degrees C, 26 degrees C/14 degrees C, and 24 degrees C/16 degrees C were established to obtain two daily temperatures of 28 and 20 degrees C, and two diurnal day/night temperature differences of 12 and 8 degrees C. Concentration of a lipid peroxidation product malondialdehyde (MDA), activities of the antioxidants superoxide dismutase (SOD) and catalase (CAT), chlorophyll concentration (SPAD) in flag leaves and kernel weight were determined. Results show that activities of SOD and CAT in leaves increased markedly on 14 days after anthesis (DAA) for the high-temperature treatment (34 degrees C/22 degrees C) and then declined. As a result, MDA concentration in leaves increased significantly under high temperature (34 degrees C/22 degrees C and 32 degrees C/24 degrees C). Compared with optimum temperature treatment, high temperature reduced the concentration of soluble protein and SPAD values in flag leaves. Grain-filling rate increased slightly initially, but decreased significantly during late grain filling under high temperature. As a result, final grain weight was reduced markedly under high temperature. Decreases in the activities of SOD and CAT and increases in MDA concentration in leaves were more pronounced with a 12 degrees C of day/night temperature difference when under high temperatures. Kernel weight was higher under 12 degrees C of day/night temperature difference under optimum temperatures (24 degrees C/16 degrees C and 26 degrees C/14 degrees C). The responses to high-temperature regimes appeared to differ between the two wheat cultivars with different grain protein concentrations. It is concluded that a larger diurnal temperature difference hastened the senescence of flag leaves under high-temperature conditions, but retarded senescence under optimum temperature treatments of 26 degrees C/14 degrees C and 24 degrees C/16 degrees C.