Résumé
The capacity of the apple tree to tolerate temporary abiotic constraints was investigated through leaf responses to different vapor pressure deficits (VPD). A ‘Starkrimson’ x ‘Granny Smith’ F1 progeny of 125 full-sibs whip-grafted onto Pajam® 2 rootstocks and pot-cultivated was studied (three replicates). After two months, plants were transferred into a controlled environment chamber ensuring five increasing VPDs, from 0.7 to 2.7 kPa, other conditions remaining constant. Leaf stomatal conductance (gsw), net CO2 assimilation (An) and transpiration (E) rates were measured, and water use efficiency (WUE, An/E) computed. The leaf responses were analyzed through a linear model including genotype as random effect for each VPD level separately. Whatever the VPD, broad sense heritability values estimated from this model were high for An and WUE, varying from 0.46 to 0.61 and from 0.36 to 0.54 respectively. This indicated a good repeatability of measurements in controlled conditions. Heritability values were moderate to high for gsw and E at most VPD levels. To investigate in-depth the genetic determinism of leaf traits, a QTL analysis was carried out using a consensus ‘Starkrimson’ x ‘Granny Smith’ genetic map. For each trait and VPD level, QTL detection was performed on the best linear unbiased predictor (BLUP) of genotypic values. Three genomic regions on LGs 8, 11 and 17 carried 3 QTL for E and gsw at the 2.2 kPa VPD level. For both traits, QTL mapped on LG 8 was also detected at 1.7 and 2.7 kPa, highlighting its stability with VPD variation. LOD scores ranged from 4.2 to 6.4, and each QTL explained 12.2 to 22.4% of the trait variability. No significant QTL were identified for A and WUE. Nevertheless, as LOD scores for A n n varied from 3 to 4 on LG 10, whatever the VPD level, this suggested a putative QTL for this trait. Potential value of ecophysiological traits for selection is discussed