On the minimum leaf conductance: its role in models of plant water use, and ecological and environmental controls

When the rate of photosynthesis is greatly diminished, such as during severe drought, extreme temperature or low light, it seems advantageous for plants to close stomata and completely halt water loss. However, water loss continues through the cuticle and incompletely closed stomata, together constituting the leaf minimum conductance (g(min)). In this review, we critically evaluate the sources of variation in g(min), quantitatively compare various methods for its estimation, and illustrate the role of g(min) in models of leaf gas exchange. A literature compilation of g(min) as measured by the weight loss of detached leaves is presented, which shows much variation in this trait, which is not clearly related to species groups, climate of origin or leaf type. Much evidence points to the idea that g(min) is highly responsive to the growing conditions of the plant, including soil water availability, temperature and air humidity - as we further demonstrate with two case studies. We pay special attention to the role of the minimum conductance in the Ball-Berry model of stomatal conductance, and caution against the usual regression-based method for its estimation. The synthesis presented here provides guidelines for the use of g(min) in ecosystem models, and points to clear research gaps for this drought tolerance trait.