PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS ASSOCIATED WITH SPRING WHEAT YIELD UNDER HOT, IRRIGATED CONDITIONS

Sixteen spring wheat genotypes were grown under hot, irrigated, low latitude conditions, during the 1990-1991 and 1991-1992 winter cropping cycles in Mexico, Egypt, India and the Sudan, and in the 1990 and 1991 winter cycles in Brazil. The genotypes were chosen to represent a range of genetic diversity within modern wheat varieties. In addition to grain yield, in Mexico a number of morphological and physiological traits were measured throughout the growing season on two sowing dates (December and February), to evaluate their correlation with yield in the other countries. The morphological traits - above-ground biomass at maturity, grains m(-2), days to anthesis and days to maturity - measured on both sowing dates in Mexico showed generally significant correlations with yields measured at the other sites. Moreover, gr ound-cover estimates early in establishment, and at anthesis, also showed some association with performance at these sites, but only when measured on the hotter, February sowing date in Mexico. Membrane thermostability (MT), measured on field-acclimated flag leaves showed generally significant correlations with performance at all sites, and with MT measurements made on heat-acclimated seedlings of the same genotypes grown in growth chambers. Flag-leaf photosynthesis measured at boating, anthesis and during grain filling on both sowing dates, was generally significantly correlated with grain yield at all sites, as was rate of loss of leaf chlorophyll content during grain filling. Stomatal conductance was also significantly correlated with performance at all three stages. Canopy temperature depression (CTD) measured with an infrared thermometer was significantly positively correlated with performance at the international sites when measured between 1200 and 1600 hours, after full canopy establishment. The correlation of CTD with yield was not affected by the irrigation status of the crop under well-watered conditions. The possible use of these traits in selection for yield under hot conditions is discussed.