WHEAT CULTIVAR DIFFERENCES IN PHOTOSYNTHE TIC RESPONSE TO LOW SOIL-WATER POTENTIALS .1. MAINTENANCE OF PHOTOSYNTHESIS AND LEAF WATER POTENTIAL

Although many studies have shown cultivar differences in photosyntheti c response to water deficit, the understanding of detailed mechanisms is not sufficient. We examined the mechanisms of water stress-resistan ce in terms of photosynthetic performance under low soil water potenti al (Psi(soil)) with sixteen cultivars of wheat (Triticum aestivum L.) from different habitats, which had shown different drought resistance on a grain yield basis. Cultivar differences in photosynthetic mainten ance in response to decrease of Psi(soil) (water stress resistance), w ere found at all seedling, booting, and grain filling stages. Cultivar s with high drought resistance based on grain yield also showed high w ater stress resistance in photosynthetic performance. Water stress res istance (R(WS)) was caused more by tolerance (T-WS, maintenance abilit y of photosynthesis in response to decrease of leaf water potential, P si(L)) in some cultivars, which maintained relatively high photosynthe sis (P-N) in spite of decreases in Psi(L), while it was caused more by water stress avoidance (A(WS), maintenance ability of Psi(L) in respo nse to decreases in Psi(soil)) in other cultivars, which showed a rela tively high P-N by maintaining a relatively high Psi(L) under the same low Psi(soil). However, there was a positive correlation between R(WS ) and T-WS or between R(WS) and A(WS). It is suggested that avoidance and tolerance usually occur simultaneously in adaptation to low Psi(so il), although water stress resistant cultivars varied in the water str ess resistance mechanism.