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 (Δ¹³ C-g_m ) into cell wall plus plasma membrane conductance (Δ¹⁸ O-g_m ) and chloroplast membrane conductance (g_cm ). A weak correlation was found between Δ¹³ C-g_m and k_Leaf only when measured under growth conditions. However, Δ¹⁸ 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 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.