Drought induced responses in hybrid and balsam poplars

Six hybrid poplar clones and balsam poplar (Populus balsamifera L.) were subjected to different water availability regimes to determine differences in their drought resistance mechanisms and recovery capacity. Plants were subjected to mild drought stress (Psisoil -0.05 MPa), severe drought stress (Psisoil -2.0 MPa), and drought recovery following re-watering at 30 minutes, one day and (or) two days (Psi soil -0.004 MPa). A functional coordination by a timed response of the stomatal aperture to reduced stem hydraulic conductivity was likely the main factor associated with preventing extensive loss of vessel conductivity due to induced cavitation in these poplars. The studied poplars reached a relatively high percentage of loss of conductivity at lower xylem water potential values compared with other tree species. It was noted that, although stomatal regulation was involved in preventing xylem cavitation, xylem architecture also played an important role in resistance to xylem cavitation. In addition, most of the poplars displayed high cell wall elasticity, maintaining cell turgor pressure at lower tissue water contents. Accumulation of carbohydrates and proline were not a significant factor in drought resistance, and varied between the clones and tissues. Root hydraulic conductivity also differed between treatments and poplar clones. Increases in conductivity were observed after the re-watering treatment pointing to the importance of root water transport during drought recovery. In general, combined drought-resistant mechanisms and a recovery capacity were found in these poplar clones: Northwest showed lower vulnerability to xylem cavitation, lower cell wall elasticity and developed lower leaf water potential to maintain a positive water potential gradient for water uptake from drying soil. Walker and balsam poplar displayed a drought tolerance strategy maintaining open stomatas, relatively high root hydraulic conductivity and moderate resistance to xylem cavitation. The Berlin clone was relatively sensitive to cavitation and had poor stomatal control suggesting that the plants of this clone may be susceptible to drought stress conditions. Assiniboine was more sensitive to xylem embolism at relatively high stem water potentials. Finally, Okanese and P38P38 showed an avoidance strategy characterized by stomatal closure to maintain highPsistem values since their xylem was found to be relatively vulnerable to cavitation. 2...