CONDUIT DIAMETER AND DROUGHT-INDUCED EMBOLISM IN SALVIA-MELLIFERA GREENE (LABIATAE)

The relationship between conduit (vessel and tracheid) diameter and wa ter-stress-induced air embolism was examined using a double staining t echnique. Comparisons were made between irrigated control plants at wa ter potentials of -1.3 MPa and water stressed plants at about -8 MPa. Water stress was induced either by natural drought conditions or by la boratory drying of shoots from previously irrigated shrubs. Stem segme nts were perfused with 0.1% basic fuchsin to mark the initially conduc tive conduits, and, following high pressure perfusion of 10 mM citric acid to remove embolisms, with 0.1% alcian blue to mark the initially embolized conduits. Hydraulic conductance per pressure gradient (k(h)) was measured before and after embolisms were removed. Diameters of no n-embolized and embolized conduits were then measured microscopically in transverse stem sections. In irrigated controls there was little em bolism and mean diameters were not significantly different for emboliz ed vs. non-embolized conduits. For both artificially dehydrated and na turally droughted plants there was a 91% drop in k(h) due to embolism, and the mean diameter of embolized conduits was about 30 mu m vs. 21 mu m for non-embolized conduits. With increasing conduit diameter ther e was an increased probability of embolism. Wider conduits may have la rger pores in their pit membranes, thus increasing their vulnerability to water-stress-induced embolism. Alternatively, wider conduits may m erely have more pits, thus increasing their statistical chances of hav ing a particularly large pore in an air-exposed pit membrane. Narrow v essels and tracheids provide an interwoven auxiliary transport system that appears to be of importance to transport when many of the wider, more efficient conduits become embolized.