PHENOTYPING APPLE PROGENY FOR ECOPHYSIOLOGICAL TRAITS: HOW AND WHAT FOR?

In the context of (i) global changes and (ii) more limited water resources, we considered the capacity of apple species to tolerate temporary abiotic constraints. The variability of functional responses of apple foliage to the environment was studied on a ‘Starkrimson’ × ‘Granny Smith’ apple progeny, comprising 120 full-sib replicated genotypes. One-year-old apple trees grafted on M9 rootstock were cultivated in 4 L pots in a greenhouse, without water or mineral restriction. After a two-month growth period, each population replicate was transferred into a growth cabinet ensuring constant daily values of photon flux density (PFD, 650 micromoles m^-2 s^-1), leaf temperature (25°C), and atmospheric CO₂ (380 micromoles moles^-1). The effect of different leaf-to-air vapor pressure deficits (VPD, 7 to 27 hPa) on net CO₂ assimilation rate (A_n), stomatal conductance (g_sw), transpiration flux (E) and intrinsic water use efficiency (WUE_i = A_n/g_sw) was assessed. Leaf behavior of control plants proved to be stable along the diurnal period, and this allowed us to validate the procedure of functional phenotyping. The average response of apple leaves to a VPD increase from 7 hPa to 27 hPa was a 33.5% decrease of A_n and a 78.6% decrease of g_sw, while a linear and strong increase (+200%) of WUE_i was shown. As estimates of mean heritability broad sense were high, this suggested the potential value of leaf ecophysiological traits for selection. The genotypic variability among the segregating population was structured in 5 clusters according to independent leaf traits: A_n_max, WUE_i_max and g_sw sensitivity to VPD increase. Methods for phenotyping fruit tree progenies and potentialities of apple genotypes according to their ecophysiological traits are discussed.