Reversible decreases in the bulk elastic modulus of mature leaves of deciduousQuercusspecies subjected to two drought treatments

Changes in leaf water relations under water stress were examined. In experiment 1, water stress was imposed by withholding irrigation to potted seedlings of deciduous oak,Quercus crispulaandQ. serrata. Changes in the pressure–volume (P–V) curve in mature leaves were followed. The leaf water potential at turgor loss (Ψl,tlp) significantly decreased after 13 d of drought treatment. The bulk elastic modulus (ɛ) significantly decreased, which contributed to the maintenance of cell turgor together with the decrease in osmotic potential. In experiment 2, water stress was imposed by notching a branch of aQ. serratatree. After the notching, the daily minimum leaf water potential (Ψl) decreased, and a significant decrease in Ψl,tlpwas observed 15 d after notching. The osmotic potential at water saturation (Ψπ,sat) did not decrease significantly until 25 d after notching whereas,ɛhad already decreased significantly within 15 d after notching and increased promptly after substantial precipitation. It was confirmed thatɛof mature leaves decreased reversibly in water stress. This response ofɛwas more rapid than that of the osmotic potential and, thus, effectively maintained cell turgor when water stress was suddenly imposed on the leaves.