Xylem conduction and cavitation in Hevea brasiliensis

Clones of Hevea were studied in an attempt to discover the reasons for differences in the hydraulic performance of xylem. Differences between clones were determined, including hydraulic conductivity and conduit width and length distributions. However, it has proved difficult to reconcile anatomical differences with physiological performance for use in future plant breeding programmes. When leaf relative water content (RWC) had been reduced from about 95% to 85%, the hydraulic conductivity of petioles decreased sharply to about 40% of the initial value. This value corresponded with xylem sap tensions of 1.8-2.0 MPa. Acoustic detection experiments revealed that this reduction in hydraulic conductivity coincided with the greatest occurrence of cavitation. It seems inescapable that the reduction in hydraulic conductivity was caused by embolization; thereafter gas bubbles blocked the flow of water inside many of the conduits. There was some indication that eventually such bubbles might be dissolved, because the hydraulic conductivity increased again if specimens were fully rehydrated. Apparently, the incidence of cavitation coincides with the entry of gas bubbles via ultramicroscopic pores into the conduits through the walls according to the air-seeding hypothesis. When a petiolate leaf is tested in a pressure chamber it is impossible to make satisfactory measurements of a balancing pressure beyond c. 1.8-2.0 MPa, because air bubbles, mixed with sap and escaping from the conduits, form a persistent froth. Xylem transport in Hevea seems to be disrupted relatively easily under water stress which is a feature of other tropical species adapted to rainforest-type environments.