ACCLIMATION TO DROUGHT IN ACER-PSEUDOPLATANUS L (SYCAMORE) SEEDLINGS

A glasshouse experiment was conducted with well-watered and water-stressed seedlings of sycamore (Acer pseudoplatanus L.) grown in soil columns. Water was withheld when the seedlings were 82-d-old. Effects of soil drying on stomatal behaviour, water relations, xylem cavitation, and growth of leaves and roots were evaluated. Stomatal conductance declined well before any observable change in bulk leaf water potentials, and was correlated with soil water status. At seven weeks, osmotic potential had declined by 0.51 MPa and 0.44 MPa at full and zero turgor, respectively. Drought significantly increased both bulk elastic modulus and leaf dry weight to turgid weight ratio of water-stressed plants. Drought had no effect on relative water content at zero turgor. Water cavitation in the xylem was detected as ultrasonic acoustic emissions (AE). Water-stressed plants displayed significantly higher rates of AE than well-watered plants. Maximum rate of AE coincided with the minimum level of stomatal conductance and apparent rehydration of the leaves. Drought caused changes in the root distribution profile and it increased the root weight. The increase in root weight was mainly due to a substantial shift in assimilates allocated in favour of roots with total biomass being unaffected. Leaf growth was maintained for six weeks without any significant decline in expansion rate. However, the development of severe water stress reduced both leaf production and expansion.