Abstract
In drought-stressed apple, leaf area decrease and stem growth cessation increase are key determinants of drought avoidance. Both the genotype and its possible phenotypic plasticity contribute to the variability of responses. Under soil water restriction, plant growth is impaired by the indirect negative impact on plant carbon balance of stomata closure and hydraulic failure. The relative contributions of these two phenomena have been scarcely explored in trees under different drought-stress intensities, and even less work is published which accounts for genetic variability and phenotypic plasticity. Working on 21 apple genotypes from the same progeny, we assessed the effects of two consecutive periods of soil drought, moderate and severe, on growth and functional patterns of leaf and stem. Leaf area decreased while temporary stem growth cessation increased under drought with strong variations depending on the genotype. These results suggested that both reduction of transpiring leaf area and leaf organogenesis are key determinants for drought avoidance in the apple. Results also confirmed the pivotal role of stomatal conductance (g s) in maintaining percent loss of conductivity of the stem xylem (PLC) under values for runaway embolism (ca. 14 % under severe drought). The sorting of genotypes according to their morphological response to drought showed that genotypes with high reduction of growth were characterized by similar g s but lower PLC than genotypes with medium and low reduction of growth. This suggests that for a given level of stomatal closure the drought-related reduction of leaf area could also limit the progression of cavitation in stem xylem. As a whole, the variability of morphological responses of apple genotypes to contrasted drought conditions indicated that both genetic variability and phenotypic plasticity are involved in the range of iso-anisohydry documented for this species.
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