promising tool for early and quick evaluation of water stress tolerance in apple genotypes

This preliminary study aimed to identify one or a cluster of physiological parameters (tools) potentially useful for phenotyping and selecting water stress tolerant genotypes within a progeny of apple plants. The "ideal" tool should be reliable, easy and fast (less than 30 s) to measure. Buds belonging to seventeen apple genotypes were grafted on Pajam 2 rootstocks and the plants were grown in pots. Three pots for each genotype were maintained at field capacity, while 3 other plants were subjected to water stress keeping their soil water content at about 40% of the total water available for transpiration. After a period of acclimation, leaf gas exchange and fluorescence parameters, leaf temperature, stem and leaf water potential were measured. A Principal Component Analysis (PCA) was performed considering all the measured parameters and only the water stressed samples. A set of PCAs were then carried out progressively removing the gas exchange and the water potential parameters. This procedure allowed us to test if a restricted number of parameters represented with significant accuracy the different water stress susceptibility among the 17 genotypes. Water stress reduced net photosynthesis (the principal indicator of plant growth potential) well before any visible effect on the plant. The results of the PCA considering only fluorescence and leaf temperature agreed with those including all parameters, suggesting the combination between the two gives quite reliable results about the water stress resistance of genotypes. These measurements are far less time consuming than photosynthesis (~ 20 s vs. ~3 min), which can be important where thousands of seedlings must be scored, and thus they are potential candidates to become a phenotyping physiological tool helping breeders in their selection work. Further studies aiming at (i) confirming these results in adult progenies, and (ii) testing this model in other fruit species are needed.