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

Abstract

Despite increasing interest in tobacco as a plant model for analysing stress responses of transgenic lines, the drought behaviour of tobacco, in terms of avoidance or tolerance, has not been investigated. In an attempt to elucidate what strategy tobacco may express in response to a gradual and prolonged soil drying and to analyse underlying physiological mechanisms, young plants were subjected to progressive drought stress under controlled conditions. The specific drought procedure we used aimed to mimic the physiological conditions plants may experience in the field and allowed detection of any adaptive potential of the species. Tobacco was shown to be highly drought resistant as compared with other species previously studied under the same conditions. In a first phase of decreasing soil moisture, osmotic adjustment helped maintain plant turgor with a relative water content (RWC) above 90% and, associated with a steep increase in stomatal resistance from a threshold water deficit (<2%), characterizes a drought avoidance stategy. In a second drought period, despite abrupt falls in leaf water potential and RWC, enhancement of cell membrane stability (90% decrease in cell membrane relative injury) initiated a drought tolerance strategy, resulting in prolonged plant survival until severe soil desiccation. This is shown in the capacity for plant recovery upon rehydration after 2 months of drought. Such sequential expression of adaptive mechanisms, detected under progressive drought stress, provides a new tool that should increase understanding of changes in drought phenotypes of genetically engineered tobacco plants.