Leaf hydraulic conductance and mesophyll conductance are not closely related within a single species

Stomata represent one resistor in a series of resistances for carbon and water exchange between the leaf and the atmosphere; the remaining resistors occurring within the leaf, commonly represented as mesophyll conductance to CO2, g(m), and leaf hydraulic conductance, k(Leaf). Recent studies have proposed that g(m) and k(Leaf) may be coordinated across species because of shared pathways. We assessed the correlation between g(m) and k(Leaf) within cotton, under growth CO2 partial pressure and irradiance treatments and also with short-term variation in irradiance and humidity. g(m) was estimated using two isotopic techniques that allowed partitioning of total g(m) (Delta C-13-g(m)) into cell wall plus plasma membrane conductance (Delta O-18-g(m)) and chloroplast membrane conductance (g(cm)). A weak correlation was found between Delta C-13-g(m) and kLeaf only when measured under growth conditions. However, Delta O-18-g(m) was related to k(Leaf) under both short-term environmental variation and growth conditions. Partitioning gm showed that g(cm) was not affected by short-term changes in irradiance or correlated with k(Leaf), but was strongly reduced at high growth CO2 partial pressure. Thus, simultaneous measurements of g(m), k(Leaf) and g(cm) suggest independent regulation of carbon and water transport across the chloroplast membrane with limited coordinated regulation across the cell wall and plasma membrane.