Leaf gas exchange and water potential responses to drought in nine poplar (Populus spp.) clones with contrasting drought tolerance

The dynamic responses of stomatal conductance (g(s)) net photosynthesis (A) and leaf water potential (Psi(leaf)) to a progressive drought were examined in nine poplar clones (Populus spp.) with contrasting drought tolerance from the Canadian Prairies, a region prone to frequent droughts. Plants were grown in a greenhouse and either well-watered or drought preconditioned (5-6 cycles of drought) for 8 weeks. At the end of the last cycle, plants were watered to saturation then progressively dried-down (-1.25 MPa Psi(soil)) during which A, gs and Psi(leaf) were measured. Drought tolerant Okanese reached the lowest combined Psi(leaf) while sensitive clones (Assiniboine and Imperial) had the highest (-1.6 vs. -1.1 MPa). Steady state g(s) (measured under well watered conditions) was lower in tolerant (Okanese and Tristis SBC# 1) than in sensitive clones. Preconditioning reduced steady state g(s) in all clones, lowered the threshold Psi(leaf) for stomatal closure and the minimum Psi(leaf) in most clones but did not affect the steady state A. Tolerant and some moderately tolerant clones maintained higher A at lower Psi(leaf) than the other clones. Stomatal closure was gradual in tolerant clones and in moderately tolerant Northwest but rapid in the other clones. Stomata in the sensitive clones closed at the highest Psi(leaf), Okanese closed at the lowest. The substantial range in gas exchange and Psi(leaf) responses observed here represented both drought tolerance and taxonomic (Aegiros or Tacamahaca sections) traits which could play a role in the survival and productivity in environments with limited water or during periods of drought.