Changes in stem and leaf hydraulics preceding leaf shedding in Castanea sativa L.

This paper describes changes in leaf water status and in stem, petiole and leaf blade hydraulics preceding leaf senescence and shedding in Castanea sativa L. (chestnut). Measurements of maximum diurnal leaf conductance to water vapour (g(L)), minimurn water potential (psi(L)), hydraulic conductance per unit leaf surface area of stems (K-SL), petioles (K-PL) and leaf blades (K-LL) and number of functional conduits and inside diameter distribution were performed in June, September and October 1999. In September, still green leaves had undergone some dehydration as indicated by decreased g(L) (by 75%) and psi(L) with respect to June. In the same time, K-SL decreased by 88%, while K-PL and K-LL decreased by 50% and 20% of the conduits of stems and 10% of the petioles (all belonging to the widest diameter range) were no longer functioning, causing a decrease in the theoretical flow by 82% in stems and 27% in petioles. Stem xylem blockage was apparently due to tyloses growing into conduits. We advance the hypothesis that the entire process of leaf shedding and winter rest may be initiated by extensive stem embolism occurring during the summer.