XYLEM DYSFUNCTION CAUSED BY WATER-STRESS AND FREEZING IN 2 SPECIES OFCOOCCURRING CHAPARRAL SHRUBS

Water transport from the roots to leaves in chaparral shrubs of Califo rnia occurs through xylem vessels and tracheids, The formation of gas bubbles in xylem can block water transport (gas embolism), leading to shoot dieback, Two environmental factors that cause gas embolism forma tion in xylem conduits are drought and freezing air temperatures, We c ompared the differential vulnerabilities of Rhus laurina and Ceanothus megacarpus, co-dominant shrub species in the coastal regions of the S anta Monica Mountains of southern California, to both water stress-ind uced and freezing-induced embolism of their xylem, Rhus laurina has re latively large xylem vessel diameters, a deep root system, and a large basal burl from which it vigorously resprouts after wildfire or freez ing injury, In contrast, Ceanothus megacarpus has small-diameter vesse ls, a shallow root system, no basal burl and is a non-sprouter after s hoot removal by wildfire, We found that R, laurina became 50% embolize d at a water stress of -3 MPa and 100% embolized by a freeze-thaw cycl e at all hydration levels, In contrast, C, megacarpus became 50% embol ized at a water stress of -9 MPa and 100% embolized by freeze-thaw eve nts only at water potentials lower than -3 MPa, Reducing thaw rates fr om 0.8 degrees C min(-1) to 0.08 degrees C min(-1) (the normal thaw ra te measured in situ) had no effect on embolism formation in R, laurina but significantly reduced embolism occurrence in well-hydrated C, meg acarpus (embolism reduced from 74 to 35%), These results were consiste nt with the theory of gas bubble formation and dissolution in xylem sa p, They also agree with field observations of differential shoot dieba ck in these two species after a natural freeze-thaw event in the Santa Monica Mountains.