WATER-STRESS PRECONDITIONING OF BLACK SPRUCE SEEDLINGS FROM LOWLAND AND UPLAND SITES

One-year-old seedlings from two pairs of upland -lowland populations w ere subjected to three cycles of drought preconditioning. During a sub sequent fourth cycle, preconditioned seedlings and seedlings that had been maintained under well-watered conditions were subjected to a fina l soil drought. During the drought periods, small but significant diff erences in gas exchanges and water relations were observed between upl and and lowland seedlings but the differences were not consistent eith er within each lowland -upland pair or over the four cycles. During th e final drought period, preconditioned seedlings maintained significan tly higher net photosynthesis rate, stomatal conductance, and transpir ation rate than unconditioned seedlings. In contrast, no significant d ifferences were found in midday water potential, total sugar content, or water relations parameters between preconditioned and unconditioned seedlings. The results suggest an absence of ecotypic variation with respect to drought tolerance in black spruce. They also suggest that p reconditioning in black spruce seedlings occurs mostly through the acc limation of stomatal and photosynthetic mechanisms to drought, without active osmotic adjustment.