Résumé
Transpiration restriction under high vapour pressure deficit (VPD), measured indoors with individual plants, increases water use efficiency (WUE). However, VPD is not the only factor driving transpiration, individual plants in a field rapidly become a canopy, and reports on the transpiration restriction versus WUE are scant. We analyzed the transpiration response to the evaporative demand (Penman-Monteith reference evapotranspiration, ET ref ) and WUE in sorghum canopies outdoors. These responses showed no plateau at high ET ref in 47 genotypes. The slope of the resulting linear relationship over the whole range of ET ref showed a large genetic variability. Unexpectedly, this slope was positively correlated with WUE in experiments with high ET ref . Conversely, a (classical) negative correlation was observed under low ET ref . Genotypes with high WUE and response to ET ref allowed maximum light penetration into the canopy, via more erect leaf orientation. VPD in the canopy was also lower than in open air when the leaf area index reached 2.5-3. We interpret that higher WUE related to a larger proportion of plant photosynthesis being contributed by lower level leaves that received light and faced lower VPD than leaves exposed to air VPD. This study opens new opportunities, agronomic and genetic, to improve WUE.