A COMPARISON OF WATER AND PHOTOSYNTHESIS BEHAVIOR OF CORN AND SUNFLOWER UNDER WATER-STRESS - INFERENCE ON WATER-USE EFFICIENCY

Various indices of water stress sensitivity lead to contradictory conc lusions as to the choice of irrigated crop between corn and sunflower. We propose an additional analysis of leaf gas exchanges after floweri ng to field analysis on dry matter and crop water use. Gas exchanges i ndicated that soil water depletion induces stomatal closure in both pl ants. For corn, this stomatal closure was accompanied by a reduction i n photosynthesis when predawn leaf water potential decreased below - 0 .2 MPa. Without water stress, stomatal conductance and transpiration w ere between two and three times higher for sunflower than for corn. Th e reduction in leaf photosynthesis in sunflower began at a predawn lea f water potential below - 0.5 MPa. For a given value of stomatal condu ctance, corn leaf photosynthesis was always higher than that of sunflo wer. Complementary information on physiological mechanisms at leaf lev el was derived; this should contribute to a better irrigation efficien cy at field level.