Two measures of leaf capacitance: insights into the water transport pathway and hydraulic conductance in leaves

The efficiency and stress tolerance of leaf water transport are key indicators of plant function, but our ability to assess these processes is constrained by gaps in our understanding of the water transport pathway in leaves. A major challenge is to understand how different pools of water in leaves are connected to the transpiration stream and, hence, determine leaf capacitance (C(leaf)) to short-and medium-term fluctuations in transpiration. Here, we examine variation across an anatomically and phylogenetically diverse group of woody angiosperms in two measures of C(leaf) assumed to represent bulk-leaf capacitance (C(bulk)) and the capacitance of leaf tissues that influence dynamic changes in leaf hydration (C(dyn)). Among species, C(bulk) was significantly correlated with leaf mass per unit area, whereas C(dyn) was independently related to leaf lignin content (%) and the saturated mass of leaf water per unit dry weight. Dynamic and steady-state measurements of leaf hydraulic conductance (K(leaf)) agreed if C(dyn) was used rather than C(bulk), suggesting that the leaf tissue in some species is hydraulically compartmentalised and that only a proportion of total leaf water is hydraulically well connected to the transpiration stream. These results indicate that leaf rehydration kinetics can accurately measure K(leaf) with knowledge of the capacitance of the hydraulic pathway.