Early effects of water deficit on two parental clones of Populus nigra grown under different environmental conditions

Global climate change is expected to induce a dramatic increase in the frequency and intensity of drought events in the Mediterranean region. Their effects might be particularly severe in short rotation forestry systems, such as poplar plantations, with high water demands. The aim of this study was to examine the clone-specific reaction of plant-water relations and growth to a dry-down cycle in two parental clones of Populus nigra L.: Poli, which is adapted to the dry/hot climatic conditions of southern Italy, and 58-861, which prefers the cooler and moister conditions typical in northern Italy. Plants were grown in controlled conditions in an airconditioned greenhouse, under three different irrigation regimes for 44 days. Drought stress resulted in a general decrease in plant size and predawn water potential in both clones. Although the control trees grew somewhat taller and retained leaves longer than those in other treatments, the two clones responded differently to water stress. Under severe stress conditions, Poli showed proline accumulation in old leaves to preserve plants from drought damage, without reduced stomatal activity, as shown by low values of delta C-13. In 58-861, the accumulation of ABA in roots during drought probably stimulated stomatal control, increasing drought avoidance in this drought-sensitive clone. Although in 58-861 the expression of aquaporin genes PIP1-2 and TIP1-3 was enhanced, in Poli gene expression was downregulated. We analysed only part of the aquaporins genes, but we assume that these clones exhibited contrasting water transport strategies during drought. Clone 58-861 seems to increase the permeability of the vascular tissue by overexpressing aquaporin genes, probably in order to facilitate water transport, and Poli appears to increase water conservation in the root cells by downregulating aquaporins.