Field observations of individuals of Cupressus sempervirens (CS) in an arid region showed that they died at the end of the dry season with xylem water potential (Ψx) of only -6.5 MPa, despite CS’s more resistant xylem (Ψ50 ~-10 MPa). Thus, we hypothesized that the observed CS mortality was driven by other (than xylem tension) agents, and that CS has the potential to survive severer droughts. Our goal was to characterize the water potential and hydraulic failure that lead to mortality in CS.
We performed a terminal drought experiment in a greenhouse with different CS genotypes: K type- based on propagation from a forestry organization seed source; and R type- based on clones from cuttings from a robust tree in a dry area (Yatir forest). Water content, water potential, photosynthesis, membrane leakage, water loss by weight, leaf conductivity, and leaf non-structural carbohydrates (NSC) were measured throughout the experiment.
K type water loss was lower, which was reflected in lower mortality rates on day 18. Mortality of 14/30 and 19/30 individuals was observed following 40 days in R type and K type, respectively. Mortality corresponded with 40% water content and Ψx ~-10 MPa. Following rewatering significant differences were observed in membrane leakage but not in NSC between recovered and dead seedlings.
This work demonstrates that CS’s Ψx can approach its Ψ50 under drought. We also found that a water-saving genotype delayed mortality.