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

Abstract

Abstract 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 CO 2 , 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 CO 2 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 (Δ 13 C‐ g m ) into cell wall plus plasma membrane conductance (Δ 18 O‐ g m ) and chloroplast membrane conductance ( g cm ). A weak correlation was found between Δ 13 C‐ g m and k Leaf only when measured under growth conditions. However, Δ 18 O‐ g m was related to k Leaf under both short‐term environmental variation and growth conditions. Partitioning g m 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 CO 2 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.