Differential Physiological and Metabolic Responses in Young and Fully Expanded Leaves of Aristotelia Chilensis Plants Subjected to Drought Stress

Drought stress reduces plant growth and crop yields. Plants activate defense mechanisms, such as stomatal closure, phenolic compounds biosynthesis, compatible solutes, protein, amino acids, and abscisic acid to cope with drought stress. Most studies on drought stress effects have been focused only on fully expanded leaves, while information on young leaves is scarce. In this study, we investigated the differential photosynthetic performance as well as the primary and secondary metabolic responses in young and fully expanded leaves of Aristotelia chilensis plants subjected to drought stress. We found that drought stress negatively affected net CO2 assimilation (AN), stomatal conductance (gs), and transpiration (E) more drastically in young leaves than fully expanded leaves of A. chilensis plants. On the other hand, young leaves accumulated higher total protein than fully expanded leaves of plants at day 10 of drought stress. Likewise, the total amino acid was increased in young leaves of stressed plants on days 5, 10, and 20, showing higher values at the end of the experiment. Starch was reduced in both young and fully expanded leaves of drought-stressed plants, while sucrose and fructose levels increased around 2-fold in young leaves at days 10 and 20 of drought stress. We observed that leucine and valine increased in young leaves at days 10 and 20 of drought stress, while in contrast, no changes were observed in the fully expanded leaves. PCA analysis clearly separated the young from fully expanded leaves at 10 and 20 days under drought stress, where anthocyanins and related genes (UFGT and NCED) were stimulated in fully expanded leaves, while in young leaves was amino acids, sucrose, and fructose. Our results demonstrate that young leaves stimulate branched-chain amino acid and phenylalanine accumulation, while fully expanded leaves stimulate anthocyanins and related genes, showing different physiological, biochemical, and molecular mechanisms to respond to drought stress.