Xylem embolisms can form when a plant is under water stress.  There is increasing evidence that such embolisms can be repaired even while xylem sap is under considerable negative pressure. The goal of this study was to document diurnal formation and repair of xylem embolisms in two chaparral shrubs, Malosma laurina and Ceanothus crassifolius, in southern California during winter, spring and summer, and relate the observed patterns to diurnal patterns of stomatal conductance and branch water potentials.  We hypothesized that differences in water relations between the two species would be reflected in their wood anatomies and vulnerabilities to embolism formation.  It was also hypothesized that the plants would exhibit lower stem hydraulic conductivity (higher presence of embolisms) during the day than the night, and that there would be a greater loss of hydraulic conductivity and less embolism repair during the dry summer months than in winter or spring.  Diurnal measurements of plant water relations were conducted in February, April, May, and July of 2009.  A quantitative analysis of stem wood anatomy included measurements of vessel, fiber, and parenchyma traits, including the theoretical implosion resistance of vessels.  Xylem vulnerability curves were constructed using the air injection method. 

Results/Conclusions

Branch water potentials and their diurnal patterns were very similar between the two species in February, but declined much more strongly in Ceanothus than in Malosma towards the summer dry season.  Both species had consistently similar rates and diurnal patterns of stomatal conductance and substantial rates of nocturnal conductance.  Ceanothus was much more resistant to embolism formation and had consistently lower levels of native embolism than Malosma.  Diurnal formation and repair of embolisms was detected only for Malosma in February and April, when it occurred during the evening while stomata were open and water potentials ranged between -0.2 and -0.6 MPa.  These findings add to the growing evidence that embolisms in drought-adapted plants can be repaired while the xylem is under negative pressure and while nocturnal transpiration is occurring.  Malosma had larger and fewer vessels than Ceanothus, and Malosma fibers were larger in diameter and had a low degree of lignification.  This suggests that they may act as water storage tissue, which could dampen diurnal fluctuations in branch water potentials and act as a source of water for embolism repair.  The ability of Malosma to store water in its wood may account for its flowering and fruiting during the summer.