Physiological responses of Norway spruce (Picea abies) seedlings to drought stress

Four-year-old seedlings of Picea abies [L.] Karst (Norway spruce) were grown in semi-controlled conditions with three watering regimes. The seedlings in the control group (c) were watered to prevent any dehydration effect. The two remaining groups were subjected to mild (ms) and severe water stress (ss), respectively. The following physiological variables were monitored until ss seedlings began to die: leaf water potential (psi(L)), stomatal conductance (g(s)), CO(2) exchange (P(N)), free proline content (Pro), total chlorophyll (a + b) concentration (Chl(t)) and the maximal photochemical efficiency of photosystem II (F(v)/F(m)). The results indicate that not all observed physiological parameters display the same degree of sensitivity to dehydration. After Day 12 of dehydration, psi(L) of ss seedlings was already significantly lower than that of the two other groups. On Day 26, significant differences in psi(L) were recorded among all treatments. Decreasing values of water potential were accompanied by early changes in P(N), g(s) and Pro. A significant decrease in Chl(t) and F(v)/F(m) were only observed during the more advanced stages of dehydration. These results demonstrate that the drought response of P. abies seedlings include a number of parallel physiological and biochemical changes in concert, enhancing the capability of plants to survive and grow during drought periods, but only to a point.