Drought Stress Responses in a Populus Hybrid Complex in Southern Alberta

Abstract

Stomatal response of narrow leaf poplar (P. angustifolia; (PA)), balsam poplar (P. balsamifera; (PB)), cottonwood (P. deltoides; (PD)), and a P. angustifolia X P. balsamifera hybrid (PX) was studied in the field and in controlled environments. Mild water stress was occurring in both field studies, as evidenced by pre-dawn Ψ values of no less than -0.3 MPa; controlled experiments were more severe, with values dropping below -1.0 MPa. Stomatal conductance patterns differed for each species, and suggest that in terms of tolerating drought stress, PX > PA > PB > PD. These patterns were significantly influenced by microclimate conditions; severe conditions caused partial or complete stomatal closure, even in the presence of low water stress. Preconditioning plants through brief, repeated exposure to water stress resulted in stomatal closure at approximately -1.0 MPa, a higher level Ψ than is otherwise indicated for members of the genus Populus. It may thus be a valuable component of drought resistance for these species. Dieback studies indicated dissimilar patterns of dieback between PA and PB; PA experienced dieback on a large number of small branches while that of PB occurred on a small number of main branches. These results suggests that the drought stress response may differ between the two species, and may further indicate that the pattern of dieback is reflective of the physiological response to drought stress.