Sequential expression of adaptive mechanisms is responsible for drought resistance in tobacco

Despite increasing interest in tobacco as a plant model for analysing stres s responses of transgenic lines, the drought behaviour of tobacco, in terms of avoidance or tolerance, has not been investigated. In an attempt to elu cidate what strategy tobacco may express in response to a gradual and prolo nged soil drying and to analyse underlying physiological mechanisms, young plants were subjected to progressive drought stress under controlled condit ions. The specific drought procedure we used aimed to mimic the physiologic al conditions plants may experience in the field and allowed detection of a ny adaptive potential of the species. Tobacco was shown to be highly drough t resistant as compared with other species previously studied under the sam e conditions. In a first phase of decreasing soil moisture, osmotic adjustm ent helped maintain plant turgor with a relative water content (RWC) above 90% and, associated with a steep increase in stomatal resistance from a thr eshold water deficit (<2%), characterizes a drought avoidance stategy. In a second drought period, despite abrupt falls in leaf water potential and RW C, enhancement of cell membrane stability (90% decrease in cell membrane re lative injury) initiated a drought tolerance strategy, resulting in prolong ed plant survival until severe soil desiccation. This is shown in the capac ity for plant recovery upon rehydration after 2 months of drought. Such seq uential expression of adaptive mechanisms, detected under progressive droug ht stress, provides a new tool that should increase understanding of change s in drought phenotypes of genetically engineered tobacco plants.