Leaf photosynthetic performance is not a key factor affecting grain yield in spring wheat subjected to heat and combined heat and drought stresses

The yield traits and physiological responses of three wheat genotypes were studied when subjected to heat and combined heat and drought stress at anthesis under either aCO(2) (400 ppm) or eCO(2) (800 ppm) in a greenhouse. The heat treatment was 7-days at day/night 35/28 degrees C, and the combined heat and drought was withholding irrigation from the heat-stressed plants until the photosynthetic rate reached <5 mu mol m(-2) s(-1). The LM62 genotype had higher photosynthetic rate compared with LM19, though no significant difference in grain yield was found. eCO(2) increased photosynthesis at 35 degrees C and significantly lowered the electron transport rate at high intercellular CO2 concentrations. Maximum velocity of Rubisco carboxylation (V-cmax) and the maximum velocity of RuBP regeneration in leaves (J(max)) increased in 35 degrees C compared with 25 degrees C, though when normalized to 25 degrees C both V-cmax and J(max) decreased in the heat-stressed plants, indicating that an inhibition had occurred. The maximum photochemical efficiency of photosystem II (F-v/F-m) decreased under heat, which correlated with the yield loss caused by the stress. F-v/F-m also decreased under combined heat and drought, though it did not related to the declined yield. A small leaf area prolonged the drying period of Gladius, and it was the genotype with the lowest decrease in yield because of stress. It is concluded that the effects of heat and combined heat and drought stress on the gas exchange and photosynthetic capacity on leaf area basis are not directly linked to the yield performance among wheat genotypes, while the morphological characteristics of the plants are important determinants of grain yield in response to those abiotic stresses.