Hydraulic and chemical mechanisms in the response of pinus pinaster A i t. To conditions of water stress

The increase of temperatures at a global scale constitutes a threaten, among the others, for forest ecosystems, and imposes a deeper comprehension of plant behaviour towards drought stress, in order to better plan a sustainable management of water resources. Young plants of Pinus pinaster A i t. were exposed to prolonged drought stress, during which their xylem ABA concentration, leaf water potential and stomatal conductance were measured at regular intervals on the top branches; the same measurements were taken on a sample of control, non stressed plants. At the same time, plant transpiration rate was measured in response to different ABA concentrations and light regimes. Plant responses to variations of environmental parameters were measured, too: leaf T° vs. atmospheric T°; "E" vs. VPD; "A" vs. (CO2). The data suggest that the plant final response to drought stress, i.e. stomatal closure, is mediated by a combination of hydraulic and hormonal regulation; the mechanical one is triggered on the short period and is more quickly reversible, while the chemical one acts on the medium-long term, when the water stress tends to chronicize.