PARENT SESSION
Poster Session #72: Water Relations II.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Freezing fatigue may increase xylem cavitation vulnerability caused by water stress in two Quercus species.

Rubio, Esther^*,1, Davis, Stephen D.², Bellot, Juan³, Vilagrosa, Alberto¹, ¹ Centro de Estudios Ambientales del Mediterráneo(CEAM), Valencia, Spain² Pepperdine University, Malibu, California³ University of Alicante, Alicante, Spain

ABSTRACT- In summer, we compared the vulnerability to xylem cavitation caused by water stress between two species of evergreen oak, Quercus agrifolia and Quercus berberidifolia of the Santa Monica Mountains of southern California. We compared inland sites, which experience winter freezing events, to coastal sites devoid of winter freeze. The sites were within 5 km of each other. We found that inland species had elevated embolism resulting in 42% loss in hydraulic conductance at a modest xylem pressure of -0.5 MPa whereas coastal species had only 14% embolism at -0.5 MPa. We tested the hypothesis that elevated xylem embolism at inland sites was do to "freezing fatigue" by measuring % embolism at -0.5 MPa on unfrozen stem segments, exposing these same stem segments to a freeze-thaw event, measuring post-freeze embolism at -0.5 MPa (consistently >90% embolism), flushing to remove embolism and again measuring % embolism at -0.5 MPa. The result for both species was an increase in embolism levels of about 40% between pre-freeze and post-freeze conditions, suggesting a significant effect of freezing fatigue. We conclude that high native embolism observed for species of Quercus in our lab and by other investigators may be do to freezing fatigue.

KEY WORDS: Quercus, freezing, xylem cavitation