Vapour pressure deficit during growth has little impact on genotypic differences of transpiration efficiency at leaf and whole-plant level: an example from Populus nigraL

Poplar genotypes differ in transpiration efficiency (TE) at leaf and whole-plant level under similar conditions. We tested whether atmospheric vapour pressure deficit (VPD) affected TE to the same extent across genotypes. Six Populus nigra genotypes were grown under two VPD. We recorded (1) 13C content in soluble sugars; (2) 18O enrichment in leaf water; (3) leaf-level gas exchange; and (4) whole-plant biomass accumulation and water use. Whole-plant and intrinsic leaf TE and 13C content in soluble sugars differed significantly among genotypes. Stomatal conductance contributed more to these differences than net CO2 assimilation rate. VPD increased water use and reduced whole-plant TE. It increased intrinsic leaf-level TE due to a decline in stomatal conductance. It also promoted higher 18O enrichment in leaf water. VPD had no genotype-specific effect. We detected a deviation in the relationship between 13C in leaf sugars and 13C predicted from gas exchange and the standard discrimination model. This may be partly due to genotypic differences in mesophyll conductance, and to its lack of sensitivity to VPD. Leaf-level 13C discrimination was a powerful predictor of the genetic variability of whole-plant TE irrespective of VPD during growth. Transpiration efficiency of six genotypes of Populus nigraL. was up-scaled from instant values at leaf level (recorded from leaf gas exchange and 13C composition of soluble sugars) to whole plant level. The ranking of the genotypes observed at leaf level was maintained at whole plant level, demonstrating that variation of instant and intrinsic transpiration efficiency is reflected in variations of whole plant transpiration efficiency. Two levels of vapour pressure deficit induced a strong modulation of transpiration efficiency with nevertheless the same ranking of genotypes. In addition, some divergence in the relationship between 13C in soluble sugars and leaf gas exchange were interpreted as genetic differences in mesophyll conductance to CO2 among genotypes, that did not affect the genotype ranking for transpiration efficiency.