Evaluating physiological traits to complement empirical selection for wheat in warm environments (Reprinted from Wheat:: Prospects for global improvement, 1998)

The response of spring wheat to heat stress has been determined in several hot wheat growing environments worldwide on different types of germplasm. Physiological data has been collected to identify potential traits to assist in the empirical breeding for heat tolerance. Initial studies focused on 10 established varieties to determine genetic diversity for heat tolerance, identify association between heat tolerance and traits measured, and evaluate genotype by environment interaction (G x E). Yields from over 40 hot environments were analysed for G x E, and relative humidity (RH) was identified as the major factor determining relative genotype ranking. Further analysis focused on 16 environments: those with low RH and relatively high yields, i.e., over 2.5 t ha(-1). For these environments, mean yield of lines correlated with a number of physiological traits measured in Mexico, including canopy temperature depression (CTD), membrane thermostability, leaf conductance and photosynthetic rate at heading, chlorophyll content during grainfilling, leaf internal CO2 concentration, and dark respiration. Morphological traits were measured in all environments and the following showed associations with yield: above ground biomass at maturity, days from emergence to anthesis and to maturity, grain number m(-2), and ground cover estimated visually after heading. Subsequent studies focused on breeding material, namely recombinant inbred lines derived from crosses between parents of contrasting heat tolerance, and 60 advanced breeding lines selected for performance under heat stress. The genetic basis for association between heat tolerance and CTD was established by demonstrating a correlation between the two traits in RILs (recombinant inbred lines). Data from RILs, as well as from the 60 advanced lines grown at several international locations, indicated CTD to be a powerful and robust selection criterion for heat tolerance.