STOMATAL AND PHOTOSYNTHETIC ADJUSTMENT TO WATER-DEFICIT AS THE EXPRESSION OF HETEROSIS IN SUNFLOWER

Photosynthetic adjustment is a major mechanism of plant adaptation to drought. The objective was to determine the heterosis response to internal water deficit and to drought acclimation in sunflower (Helianthus annuus L.). Stomatal resistance, net photosynthesis, and water use efficiency were analyzed in relation to the leaf water potential reduction in acclimated and non-acclimated plants for two F-1 hybrids (Flamme and Viki) and their homozygous parent lines. The experiments were carried out in a controlled growth chamber on two series of plants previously acclimated or not acclimated to a 6-d drought period with a water deficiency resulting in leaf water potentials of - 2 MPa. The tolerance to internal water deficit in nonacclimated plants is a genotypic trait, but stomatal and photosynthetic adjustment appeared as a heterosis response shown by delayed stomatal closure from - 1.50 to - 1.91 MPa of the F-1 hybrid Viki and from - 1.52 to - 1.95 MPa for the F-1 hybrid Flamme. These F-1 hybrids were also characterized after acclimation by higher net photosynthesis and water use efficiency over a wide range of leaf water potentials during the leaf water deficit. Thus, stomatal adjustment appears to be closely associated with heterosis in sunflower which can develop a tolerance to internal water deficit in F-1 hybrids; the tolerance is at least equal to that of the best homozygous lines. The parental line performance thus cannot provide decisive information on the F-1 hybrid performance for this trait in breeding programs.