Cavitation Limits the Recovery of Gas Exchange after Severe Drought Stress in Holm Oak (Quercus ilex L.).

Holm oak (<italic>Quercus ilex</italic> L.) is a Mediterranean species that can withstand intense summer drought through a high resistance to cavitation far beyond the stomatal closure. Besides stomatal limitations, both mesophyll and biochemical limitations to CO₂ uptake could increase in holm oak under drought. However, no studies have addressed how hydraulic and non-hydraulic factors may limit the recovery of photosynthesis when re-watering after inducing 50% loss of hydraulic conductivity. We measured photosynthetic traits, xylem embolism, and abscisic acid (ABA) in holm oak with increasing levels of drought stress and seven days after plant re-watering. Drought stress caused a sharp decrease in net CO₂ assimilation (<italic>A</italic>_N), stomatal and mesophyll conductance (<italic>g</italic>_s and <italic>g</italic>_m), and maximum velocity of carboxylation (<italic>V</italic>_cmax). The stomatal closure could be mediated by the rapid increase found in ABA. The high level of xylem embolism explained the strong down-regulation of <italic>g</italic>_s even after re-watering. Therefore, only a partial recovery of <italic>A</italic>_N was observed, in spite of non-hydraulic factors not limiting the recovery of <italic>A</italic>_N, because i/ABA strongly decreased after re-watering, and ii/<italic>g</italic>_m and <italic>V</italic>_cmax recovered their original values. Therefore, the hydraulic-stomatal limitation model would be involved in the partial recovery of <italic>A</italic>_N, in order to prevent extensive xylem embolism under subsequent drought events that could compromise holm oak survival. [ABSTRACT FROM AUTHOR]