Differential physiological and morphological responses of two hybrid Populus clones to water stress.

Hardwood cuttings of Populus clones Tristis #1 and Eugenei were grown in pots in a controlled environment chamber to observe early patterns of growth and water relations in response to changing conditions of water stress. Height and dry weight growth, dry matter partitioning, leaf area production, stomatal conductance and leaf water potential were measured periodically during the 73-day experiment. The two clones reacted in a similar way to an initial period of stress, showing reduced growth, stomatal conductance and leaf water potential. However, when stress was interrupted by ample watering and then reimposed, substantial differences between the clones were evident. Growth of Eugenei fully recovered after stress was relieved, especially leaf growth, but when water deficiency was reimposed, the plants wilted and some leaves died. Tristis #1, in contrast, showed a greater adaptation to changing stress conditions; it grew less than Eugenei after drought was interrupted, but showed little adverse effect of renewed water deficits. These responses were partially explained by the higher root/leaf weight ratios of Tristis #1 which enabled it to maintain a more favorable plant water status than Eugenei.