Mechanism of freeze-induced embolism in Fagus sylvatica L.

The mechanism of freeze stress-induced embolism in Fagus sylvatica L. branc hes was analyzed under controlled conditions. Excised branches were exposed to successive freeze-thaw cycles in temperature controlled chambers. Therm ocouples were placed on the bark to detect sap freezing exotherms. The degr ee of xylem embolism was estimated after each cycle by the loss of hydrauli c conductivity. After one freeze-thaw cycle the degree of embolism was foun d to decrease with xylem specific hydraulic conductivity, small apical shoo ts being more susceptible to embolism. Exotherms revealed that distal shoot s were freezing first and exuded sap as a result of water expansion. The lo wer water content in apical shoots upon freezing probably induced higher sa p tensions which promoted air bubble expansion and vessel cavitation prefer entially near the apices. When the decrease in water content was experiment ally prevented, embolism developed to a lesser extent. The higher vulnerabi lity of shoot apices may protect the rest of the blanch from winter damage.