Drought-induced xylem embolisms disrupt plant water transport and can lead to significant losses in hydraulic conductivity.  Until the mid 1990s, it was thought that embolism repair required xylem water potentials to be close to atmospheric pressure.  However, over the last 15 years, evidence has accumulated that plants can repair embolisms under negative pressure, a process sometimes referred to as “novel refilling”.  The underlying mechanisms for this process have remained unknown, and there has been no comprehensive study of this phenomenon across different plant species and environments.  The objective of this study was to document embolism repair under negative pressure in shrubs at eight North American field sites ranging from arid desert environments to humid forest understorys.  Shrub hydraulic traits were characterized by measuring wood density, vessel-, fiber-, and parenchyma-traits, and by constructing xylem vulnerability curves.  Diurnal measurements of branch water potentials, stomatal conductance, and percent loss of stem conductance (PLC) were conducted for 2-4 shrub species per field site (21 species total).  We hypothesized that embolism repair under negative pressure would be restricted to conditions of mild drought-stress and that there would be a trade-off between a plant’s resistance to embolism formation and its ability to repair embolisms.

Results/Conclusions

Records of diurnal patterns of plant water relations are snapshots of one day in a plant’s life.  Not surprisingly therefore, all kinds of patterns of PLC were observed over 24 hour periods for the shrubs included in this study, including increases or decreases in PLC, irregular fluctuations, stable patterns, and peaked patterns with clear evidence of embolism formation during the early day and repair anywhere from afternoon to early morning.  Embolism repair under negative pressure was detected for about half of the species studied, and it was always associated with stomatal conductance, usually nocturnal.  In some cases, embolism repair was observed to occur at branch water potentials below -2 MPa.  Embolism repair was not observed in species with very low vulnerability to embolism formation, such as the chaparral shrub Ceanothus crassifolius, and was undetectable in forest understory shrubs that had very low degrees of native embolisms.  All species that showed embolism repair under negative pressure had fairly abundant paratracheal parenchyma.  The findings suggest that embolism repair under negative pressure is a very common process in shrubs, that it is associated with nocturnal transpiration, and that it involves living cells that are closely associated with vessels.