XYLEM CAVITATION IN SCOTS PINE AND SITKA SPRUCE SAPLINGS DURING WATER-STRESS

Xylem cavitation and sap flow rates, stomatal conductance and shoot water potential were observed in irrigated and drought-treated saplings of Sitka spruce (Picea sitchensis (Bong.) Carr.), a drought-susceptible species, and Scots pine (Pinus sylvestris L.), a drought-resistant species. In Scots pine, cavitation rates increased as shoot water potential decreased below a threshold of around -0.55 MPa. In Sitka spruce, cavitation rates were inversely related to shoot water potential in irrigated saplings but not in drought-treated saplings. Sap flow rates declined sharply in both species as shoot water potential fell during the drought treatment. Thus sap flow rate was positively associated with cavitation rate in Scots pine but not in Sitka spruce. Three days after the drought treatment ended, shoot water potentials, stomatal conductances and sap flow rates in both species regained values observed before the drought treatment. In Sitka spruce, stem relative water content also returned to a value close to that observed before the drought treatment. Even 2 weeks after the end of the drought treatment, however, shoot relative water content of Scots pine remained significantly lower than it had been before the drought treatment.