The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Psi(pd)) and midday water potential were measured in transpiring (Psi(leaf)) and non-transpiring leaves (Psi(xyl)). Furthermore, photosynthesis (A), stomatal conductance to water vapour (g(s)) and sap flow (F-d) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil-plant air continuum (K-h) and whole tree hydraulic conductance (K-hsf) were calculated by using the simple analogy of the Ohm's law. K-h was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil-roots-stem-branches-leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Psi(l)(xyl)) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Psi(u)(xyl)) at the uppermost canopy. Predawn water potential (Psi(pd)) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of -0.81 +/- 0.03 and -0.75 +/- 0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil-plant-air (K-h) decreased progressively as water stress increases. Minimum values of K-h and K-hsf were recorded when Psi(pd) was lower. However, Q. petraea showed higher K-h than F. sylvatica for the same Psi(pd). The decrease in K-h with water stress was mainly linked to its fall from the soil to the lowermost canopy (K-srs). Nevertheless, a significant resistance in the petiole-leaf lamina (K-pl) was also recorded because significant differences in all dates were found on Psi between transpiring and non-transpiring leaves from the same shoot. The decline in K-h was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (g(s)) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.
