John Sperry and Jarmila Pittermann University of Utah
Water transport in plants may be challenged by drought and/or freezing. When subjected to either phenomenon, the introduction of air into the water conducting tissue (xylem) may impair the movement of water from root to stem by what is commonly referred to as embolism. What are the costs associated with resistance to embolism under these stresses? The current research will answer this question by investigating the hydraulic efficiency and wood anatomy of members of the Pinaceae, Cupressaceae and Podocarpaceae families. Conifers are the ideal choice for this study because the structural simplicity of their wood, coupled with their broad distribution will allow for effective modeling and testing of their hydraulic response to drought and freezing. Currently, it is known that large xylem conduits are efficient conductors of water, but highly vulnerable to freezing-induced embolism. Also, drought resistant xylem exhibits high wood density, and is costly to construct. Hence, hydraulic conductance and wood density represent the currency of the cost-benefit tradeoffs associated with freezing and drought resistance, respectively. The predictions are that conifers inhabiting regions prone to water deficit and freezing, such as North American high deserts, will be much less vulnerable to these stresses, but at the cost of reduced hydraulic efficiency, and high wood density. Conversely, taxa inhabiting warm, mesic regions where freezing is rare, as in Australian tropical rainforests, will exhibit low tolerance to drought and freezing stress, but will enjoy efficient hydraulic conductance. Ultimately, conifers inhabiting any point on the drought-freezing spectrum will optimize their cost-benefit strategy to maximize fitness in their habitat. The goals of this research are to clarify not only the physiological significance of variation in xylem anatomy, but also the selective forces restricting current and past conifer distributions.
