Water rise kinetics in refilling xylem after desiccation in a resurrectionplant

The acropetal water refilling kinetics of the dry xylem of branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia were determ ined with high temporal resolution by observation of light refraction at th e advancing water front and the associated recurving of the folded leaves. To study the effect of gravity on water rise, data were acquired for cut up right, horizontal and inverted branches. Water rise kinetics were also dete rmined with hydrostatic and osmotic pressure as well as at elevated tempera tures (up to 100 degreesC) under laboratory conditions and compared with th ose obtained with intact (rooted) and cut branches under field conditions. Experiments in which water climbed under its capillary pressure alone, show ed that the axial flow occurred only in a very few conducting elements at a much higher rate than in many of the other ones. The onset of transpiratio n of the unfolded and green leaves did not affect the rise kinetics in the 'prominent' conducting elements. Application of pressure apparently increas ed the number of elements making a major contribution to axial xylem flow. Analysis of these data in terms of capillary-pressure-driven water ascent i n leaky capillaries demonstrated that root pressure, not capillary pressure , is the dominant force for rehydration of rooted, dry plants. The main rea sons for the failure of capillary forces in xylem refilling were the small, rate-limiting effective radii of the conducting elements for axial water a scent (c. 1 mum compared with radii of the vessels and tracheids of c. 18 m um and 3 mum, respectively) and the very poor wetting of the dry walls. The contact (wetting) angles were of the order of 80 degrees and decreased on root or externally applied hydrostatic pressure. This supported our previou s assumption that the inner walls of the dry conducting elements are covere d with a lipid layer that is removed or disintegrates upon wetting. Consist ent with this, potassium chloride and, particularly, sugars exerted an osmo tic pressure effect on axial water climbing (reflection coefficients > zero , but small). Although the osmotically active solutes apparently suppressed radial water spread through the tissue to the leaf cells, they reduced the axial water ascent rather than accelerating it as predicted by the theory of capillary-driven water rise in leaky capillaries. Killing cells by heat treatment and removal of the bark, phelloderm, cortex and phloem also resul ted in a reduction of the axial rise rate and final height. These observati ons demonstrated that radial water movement driven by the developing osmoti c and turgor pressure in the living cells was important for the removal of the lipid layer from the walls of those conducting elements that were prima rily not involved in water rise. There is some evidence from field measurem ents of the axial temperature gradients along rooted branches that interfac ial (Marangoni) streaming facilitated lipid removal (under formation of ves icle-like structures and lipid bodies) upon wetting.