Differences in drought- and freeze-induced embolisms in deciduous ring-porous plant species in Japan

Deciduous ring-porous species in Japan shed all of their leaves under severe water stress before large vessels in earlywood are embolized, and embolization take place during winter. Water in deciduous ring-porous species is mainly conducted upward via large earlywood vessels of the current year. Water columns in large vessels are vulnerable to drought-induced and freeze stress-induced embolisms. Although a vulnerability curve can be created to estimate the hydraulic capacity of plants, it remains unclear why the loss of conductivity in potted plants of ring-porous species does not reach 100 % under severe drought stress. In this study, two deciduous ring-porous species in Japan (Kalopanax septemlobus and Toxicodendron trichocarpum) were used to explain the species-specific pattern in the water-conducting pathway of the stem. We monitored and visualized the spatial distribution of xylem embolisms in the stem of K. septemlobus saplings under drought stress and freeze stress using compact magnetic resonance imaging and cryo-scanning microscopy. In addition, we evaluated the water ascent in the stems of both species using a dye uptake method. Although embolisms of large vessels were observed under drought stress and in winter, all leaves were dropped to avoid fatal water loss after embolization of some large vessels. In contrast, all large vessels were embolized in winter. Larger-diameter vessels of latewood in T. trichocarpum tended to function in trees growing in the warm temperate zone. Thus, our results suggest that the unclear curve may be derived from a discrepancy between leaf water potential and actual water potential in the xylem under severe drought stress. The frequency of xylem embolisms in deciduous ring-porous species in Japan mainly depends on the number of freeze-thaw cycles.