Resistance to embolism is critical for postdrought recovery in resprouting Ceratonia siliqua

Xylem embolism persists long after drought has ended and should thus have a lasting effect on plants' ability to recover from severe drought. This assumption has rarely been tested, as quantifying the amount of embolism in individual intact trees is difficult, and following recovery would require long-term monitoring, specifically when studying trees. Our goal was to test the effect of embolism on recovery from drought in a broad population. We exposed 210 carob seedlings (Ceratonia siliqua), a species with high resistance to embolism, to a gradient of drought durations and monitored their ability to recover over five months with respect to their embolism level (assessed using micro-computed tomography). Seedlings that suffered 38% embolism had a 50% chance of dying. In the surviving seedlings, stomatal conductance was still inhibited a month after rehydration (ie 82% lower than in the irrigated control, even in plants that sustained only 10% embolism) but recovered to pre-drought levels after 5 months, regardless of the embolism levels. The hydraulic limitations were mostly noticed in the canopy size, as 5 months after rehydration, canopy area and the ability to resprout were strongly correlated with embolism level sustained during drought (but not with water potential). Our results suggest that embolism can be fatal even at levels below 50% and that maintaining the integrity of the hydraulic system is critical for rapid drought recovery.