Xylem embolism refilling revealed in stems of a weedy grass.

Plant hydraulic dysfunction by embolism formation can impair photosynthesis, growth, and reproduction and, in severe cases, lead to death. Embolism reversal, or "refilling," is a hypothesized adaptive process in which xylem functionality is rapidly and sustainably restored. This study investigated xylem embolism refilling during recovery from severe drought stress using entirely noninvasive measurements of the same plants. These results were considered in relation to functional traits to address long-standing gaps in understanding the consequences of severe drought stress. Leaf and stem xylem embolism as well as transpiration, photosynthesis, and stem water potential were characterized nondestructively on intact barnyard grass plants during an acute drought event. Plants were rewatered and returned to growth conditions for 10 d, during which time recovery of stem xylem embolism and transpiration were monitored. Leaf xylem embolism and declines in leaf gas exchange occurred mostly between -1.0 MPa and -2.0 MPa, whereas stem xylem embolism occurred mostly between -3.0 MPa and -4.0 MPa. In all measured plants, which included embolism levels up to 88%, stem xylem embolism reversed completely within 24 h after rewatering, and this refilling supported recovery of transpiration and growth after plants were returned to growth conditions. This study provides direct evidence of complete and functional stem xylem refilling. These results present a clear need to elucidate underlying mechanisms and the adaptive significance of this phenomenon as well as its prevalence in nature.