Stomatal conductance and leaf water potential responses to hydraulic resistance variation in Pinus pinaster seedlings

In this study, tree hydraulic conductance (K-tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g(s)), net photosynthesis (A) and bulk leaf water potential (Psi(leaf)) in well watered 5-6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K (tree) was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO2 concentration. As expected, we found a strong and positive relationship between g(s) and K-tree (r = 0.92, P = 0.0001) and between A and K-tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Psi(leaf) to K-tree depended on the direction of change in K-tree: increases in K-tree caused Psi(leaf) to decrease from around -1.0 to -0.6 MPa, but reductions in K-tree were accompanied by homeostasis in Psi(leaf) (at -1 MPa). Both of these observations could be explained by an adaptative feedback loop between g(s) and Psi(leaf), with Psi(leaf) prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Psi(leaf) approached -1 MPa.