Genotypic differences in utilization of assimilate sources during maturation of wheat under chronic heat and heat shock stresses

Heat stress from chronic, prolonged exposure up to 32 ^°C or heat shock from brief exposure to 33 ^°C and above alters the source of assimilates for grain growth of wheat (Triticum asetivum L.). Our objectives were to identify genotypes that resist chronic heat stress and heat shock and to determine the relative contributions of photosynthesis and stem reserves to grain filling under both conditions. Twenty-eight genotypes were grown in controlled enviroments at 20/15 and 30/25 ^°C day/night in light and darkness during maturation in the first experiment, and six genotypes were grown in light at the same temperatures and at 40/35 ^°C followed by 20/15 or 30/25 ^°C in the second experimnet. Heat susceptibility indices (HSI) were calculated from grain yields of the genotypes in both experiments. The ratio of chlorophyll variable fluorescence to maximum fluorescence (Fv/Fm), a measure of the stability of photosynthesis, and carbohydrate reserves in the stems were measured in the second experiment. Photosynthesis provided 63 and 65% of assimilates in the grain at 20/15 and 30/25 ^°C, respectively, but both stable photosynthesis in some genotypes and high content of reserves in other genotypes were associated with low susceptibility to stress. The Fv/Fm ratio was decreased by heat shock and returned to normal values intolerant genotypes when the treatment was followed by 20/15 ^°C but not 30/25 ^°C. Grain yield was highly correlated among 20/15, 30/25, and 40/35 ^°C followed by 20/15 ^°C treatments, suggesting that similar plant traits were involved. We conclude that assimilates from either stable photosynthesis or high reserve levels provided for high grain yields during heat stress. Combining the two traits could improve heat tolerance of wheat but might not be feasible if other traits are impeded. This revised version was published online in July 2006 with corrections to the Cover Date.