Positive pressure does not efficiently repair xylem embolism after drought

Numerous plant species can actively pressurize water in their xylem. It has been hypothesized that regular peaks in positive pressure help plants to remove xylem embolism and to replenish water reservoirs. This may happen after rewatering when plants have suffered from soil water deficit (WD) and/or following diurnal cycles at well-watered (WW) conditions. Using X-ray microcomputed tomography (µCT), we tested these hypotheses in tomato plants by quantifying xylem functionality and internal water pools in stems of 35 plants subjected to WD intensities before and after rewatering and 14 WW plants. Positive xylem pressure was detected in only 30% of the WD plants after rewatering, with a majority of this 30% showing no or rare events of xylem embolism repair. However, all WD tomato plants restored their internal water pools after rewatering, independently of their ability to generate detectable positive xylem pressure. In WW plants, positive xylem pressure was consistently generated and allowed for repair of the few embolized vessels. The ability of tomato plants to rehydrate non-xylary parenchyma tissues after WD and to repair embolisms under WW conditions, sheds new light on alternative water-use mechanisms by herbaceous species to recover from drought.