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.