Responses of young apple trees to soil water restriction: Combining shoot morphology and leaf functioning over a range of genotypes

Adapting temperate fruit-tree cultivation to stressful environments is a considerable challenge for the breeder. This is especially true for water restriction (WR) which is likely to occur more frequently in the near future, not only in Mediterranean climates but also in several parts of the middle and high latitudes. Apple was chosen as an example of fruit tree distributed worldwide and also cultivated in semi-arid regions where irrigation is crucial for regular cropping. A range of apple genotypes sourced from a single-cross-population and trained as grafted one-year-old single shoots, grown in 4-L pots in controlled conditions were used. We investigated genotype variations in some morphological and leaf ecophysiological traits of shoots maintained in a severe WR in comparison to wellwatered (WW) shoots. A Principal Component Analysis performed on all variables showed that the effects of WR on the two components of vegetative growth (i.e., stem and leaf) and on leaf ecophysiology strongly varied depending on the water regime, and within each water regime on the genotype. From an ecophysiological point of view, well-watered genotypes were better discriminated by leaf "efficacy" (i.e., net photosynthesis, electron transport rate and growth) than by "efficiency" (i.e., water use efficiency), whereas the reverse was true under WR.