Fast and reliable phenotyping of leaf functions: A tool for water stress tolerance evaluation

A protocol and a related physiological tool to discriminate drought tolerant apple genotypes in a reliable, fast way were assessed. Trials were carried out in two years and two sites: Bologna (Italy) and Montpelier (France). Potted trees from 17 genotypes grafted on Pajam 2 rootstocks were used. After sufficient shoot development, three pots genotype-1 were maintained at field capacity, while three other plants were subjected to water stress, by keeping their soil water content at about 40% of field capacity. Leaf net photosynthesis, stomatal conductance, electron transport rate via chlorophyll fluorescence, air and leaf temperature were measured on each plant. By a multi-variate semi-mechanistic approach, a new physiological index (IPL) was developed, tested and validated, which appeared strongly and linearly related to net photosynthesis. All the validation tests suggest IPL can be used as a reliable and fast (about 30" leaf-1) photo-assimilation index. The response of apple genotypes to water shortage was evaluated considering the percent reduction in leaf parameters when plants were subjected to water stress. These percentages were subjected to a Principal component analysis (PCA) and the case coordinates of the main components F1 and F2 were clustered via a K-means clustering. Each genotype was placed in one of 3 pre-defined classes: Drought susceptible, intermediate, tolerant. The same clustering was performed using only the IPL percent reduction. Using either F1, F2 or the less time consuming IPL as discriminant factors, about 53% of the genotypes were assigned to the same class in the four cases (2 years and 2 sites). IPL could be useful to assess leaf functioning under varying environmental conditions, like water availability. However, as only half of the genotypes behaved in the same way in the 4 cases, further research should be performed in order to set up a replicable water stress imposition protocol.