XYLEM HYDRAULIC CHARACTERISTICS, WATER RELATIONS AND WOOD ANATOMY OF THE RESURRECTION PLANT MYROTHAMNUS-FLABELLIFOLIUS WELW

Myrothamnus flabellifolius Welw. is a desiccation-tolerant ('resurrect ion') plant with a woody stem. Xylem Vessels are narrow (14 mu m mean diameter) and perforation plates are reticulate. This leads to specifi c and leaf specific hydraulic conductivities that are amongst the lowe st recorded for angiosperms (k(s) 0.87 kg m(-1) MPa-1 s(-1); k(1) 3.28 x 10(-5) kg m(-1) MPa-1 s(-1), stem diameter 3 mm). Hydraulic conduct ivities decrease with increasing pressure gradient. Transpiration rate s in well watered plants were moderate to low, generating xylem water potentials of -1 to -2 MPa. Acoustic emissions indicated extensive cav itation events that were initiated at xylem water potentials of -2 to -3 MPa. The desiccation-tolerant nature of the tissue permits this spe cies to survive this interruption of the water supply. On rewatering t he roots pressures that were developed were low (24 kPa). However capi llary forces were demonstrated to be adequate to account for the refil ling of xylem Vessels and re-establishment of hydraulic continuity eve n when water was under a tension of -8 kPa. During dehydration and reh ydration cycles stems showed considerable shrinking and swelling. Unus ual knob-like structures of unknown chemical composition were observed on the outer surface of xylem Vessels. These may be related to the ab ility of the stem to withstand the mechanical stresses associated with this shrinkage and swelling. (C) 1998 Annals of Botany Company.