Leaf mesophyll conductance and leaf hydraulic conductance: an introduction to their measurement and coordination

Two highly contrasting variables summarizing the efficiency of transport of materials within the leaf are recognized as playing central roles in determining gas exchange and plant performance. This paper summarizes current approaches for the measurement of mesophyll conductance to CO2 (g(m)) and leaf hydraulic conductance (K-leaf) and addresses the physiological integration of these parameters. First, the most common methods to determine g(m) and K-leaf are summarized. Next, novel data compilation is analysed, which indicates that, across diverse species, g(m) is strongly linked with gas exchange parameters such as net CO2 assimilation (A(area)) and stomatal conductance (g(s)), and with K-leaf, independently of leaf vein length per leaf area. Based on their parallel responses to a number of environmental variables, this review proposes that g(m) is linked to the outside-xylem but not to the xylem component of K-leaf. Further, a mechanistic hypothesis is proposed to explain the interactions among all these and other physiological parameters. Finally, the possibility of estimating g(m) based on this hypothesis was tested using a regression analysis and a neurofuzzy logic approach. These approaches enabled the estimation of g(m) of given species from K-leaf and leaf mass per area, providing a higher predictive power than from either parameter alone. The possibility of estimating g(m) from measured K-leaf or vice-versa would result in a rapid increase in available data. Studies in which g(m), K-leaf, and leaf mass per area are simultaneously determined are needed in order to confirm and strengthen predictive and explanatory models for these parameters and importantly improve resolution of the integrated hydraulic-stomatal-photosynthetic system.