Living with cavitation: Sub-diurnal cycles of xylem cavitation and recovery in pine trees under seasonal drought
Xylem embolism and cavitation reduce hydraulic conductance and therefore bear major implications for plant functioning and survival. Yet the observed ability of some tree species to survive high levels of cavitation (percent loss of conductivity, PLC, up to 80%) suggests the existence of an efficient reversal mechanism. At the same time, it is currently unclear to what extent xylem cavitation and recovery are common phenomena among plants, or rather unique to a few species under extreme drought conditions. Hourly changes of hydraulic conductivity were studied in branches of mature Pinus halepensis trees in a semi-arid forest. Sap flow (SF) and leaf-scale transpiration (T) were measured simultaneously. The observations are discussed in relation to the species drought resistance and survival strategy, and in the broader perspective of hydraulic efficiency-safety-recovery tradeoff driving differential adaptations among plant species.
During the wet season T peaked at mid-day, and SF lagged by about 2 hr. But during the dry season T peaked at 9:00 with a minor peak later in the day, while SF lagged by up to 9.5 hours behind the T peak. Xylem cavitation (PLC of 30-40%) developed and reversed twice a day, in the morning and in the afternoon, coinciding with two peaks in stomatal conductance. Consequently, large water deficits (5 kg tree-1) developed, and up to 33% of the daily transpiration flux came from water storage. Based on these observations, we speculate that leaf conductance response is partly decoupled from changes in PLC, such that moderate cavitation and refilling of xylem in our extremely dry conditions are routine during daytime. Considering a hydraulic efficiency-safety-recovery tradeoff system, we hypothesize that the need for hydraulic recovery evolved exclusively in plant species where the hydraulic and stomatal capacities are limited (e.g. narrow xylem conduits; small and sparse stomata), and hence extending the range of stomatal conductance is the only way to increase efficiency. In xeric environments, these species are routinely exposed to hydraulic risk, and cavitation recovery developed to facilitate survival.