Hydraulic conductance and water potential gradients in squash leaves showing mycorrhiza-induced increases in stomatal conductance.

Stomatal conductance (gs) and transpiration rates vary widely across plant species. Leaf hydraulic conductance (k leaf) tends to change with g (s), to maintain hydraulic homeostasis and prevent wide and potentially harmful fluctuations in transpiration-induced water potential gradients across the leaf (Delta Psi leaf). Because arbuscular mycorrhizal (AM) symbiosis often increases gs in the plant host, we tested whether the symbiosis affects leaf hydraulic homeostasis. Specifically, we tested whether k leaf changes with gs to maintain Delta Psi leaf or whether Delta Psi leaf differs when gs differs in AM and non-AM plants. Colonization of squash plants with Glomus intraradices resulted in increased gs relative to non-AM controls, by an average of 27% under amply watered, unstressed conditions. Stomatal conductance was similar in AM and non-AM plants with exposure to NaCl stress. Across all AM and NaCl treatments, k leaf did change in synchrony with gs (positive correlation of gs and k leaf), corroborating leaf tendency toward hydraulic homeostasis under varying rates of transpirational water loss. However, k leaf did not increase in AM plants to compensate for the higher gs of unstressed AM plants relative to non-AM plants. Consequently, Delta Psi leaf did tend to be higher in AM leaves. A trend toward slightly higher Delta Psi leaf has been observed recently in more highly evolved plant taxa having higher productivity. Higher Delta Psi leaf in leaves of mycorrhizal plants would therefore be consistent with the higher rates of gas exchange that often accompany mycorrhizal symbiosis and that are presumed to be necessary to supply the carbon needs of the fungal symbiont.