| Summary: | The morphological and physiological responses of tree seedlings to water stress and the significance of the non-hydraulic influence of root on shoot behaviour of the effects of soil drying were investigated. The experiments were conducted in a glasshouse and growth chamber, using sycamore (<i>Acer pseudoplatanus</i> L.) seedlings rooted in a soil mixture with high water holding capacity. The prime objective of the project was to evaluate the morphological and physiological components of drought tolerance that could be useful for isolation of plants with seedling characteristics acceptable for afforestation in drought-prone environments. Effects of drought on water relations and root growth were studied using long soil columns. Drought resulted in active osmotic adjustment in leaves, with decreases in osmotic potential at full and zero turgor, and it increased bulk elastic modulus and leaf dry weight to turgid weight ratio. Stomatal conductance declined well before any observable change in bulk leaf water potential and was correlated with soil water status. 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. Cyclic water stress treatment, induced major changes in sycamore seedlings, including osmotic adjustment, acclimation of photosynthesis and stomatal conductance to water stress, increased water use efficiency, and a substantial shift in biomass allocation pattern in favour of roots, with a consequent increase in root/shoot ratio. The acclimation of photosynthetic machinery was the major factor contributing to the acclimation of photosynthesis to water stress. These modifications were concluded to be important for improvement of seedling drought tolerance. Seedlings grown in soil columns and subjected to drought exhibited substantial reduction in stomatal conductance and a limitation in leaf expansion well in advance of any detectable change in shoot water relations. Root abscisic acid (ABA) concentration increased deeper in the soil profile in concert with the progressive soil drying, and it appeared to be a sensitive indicator of the soil water status around the roots.
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