Drought adaptation in Fraxinus excelsior L.: Physiological basis of the elastic adjustment

This study examines the drought-induced mechanisms in Fraxinus excelsior L. , a species that exhibits a great tolerance with respect to severe water sh ortage. Pressure-volume curves obtained from seedlings submitted to an arti ficially induced drought showed a decrease in the osmotic potential at full turgor and at zero turgor. This osmotic adjustment was also demonstrated i n stressed plants by the increase in the level of the main organic compound s, malate and mannitol. By calculating the elastic modulus (epsilon) of the foliar tissues, it was also shown that the cell walls of the stressed plan ts became less extensible. This adaptative response to water deficit was ea sily reversed after only 1 day of rehydration. A similar increase in epsilo n could be induced in short term experiments by the application of 20 mu mo l/L ABA to detached well-watered leaves, and counteracted in detached leave s from water-stressed seedlings by treatment with acidic pH buffer. From th ese data, it is suggested that bonding rearrangements rather than structura l modifications of the cell wall are involved in the elastic adjustment. Th e advantages of the elastic adjustment procedure are discussed in relation to plants water availability