Xylem conduits of a resurrection plant contain a unique lipid lining and refill following a distinct pattern after desiccation

The axial and radial refilling with water of cut dry branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia was studied in bo th acro- and basipetal directions by using H-1-NMR imaging. NMR measurement s showed that the conducting elements were not filled simultaneously. Axial water ascent occurred initially only in a cluster of a very few conducting elements. Refilling of the other conducting elements and of the living cel ls was mainly achieved by radial extraction of water from these initial con ducting elements. With time, xylem elements in a few further regions were a pparently refilled axially. Radial water spread through the tissue occurred almost linearly with time, but much faster in the acropetal than in the ba sipetal direction. Application of hydrostatic pressure (up to 16 kPa) produ ced similar temporal and spatial radial refilling patterns, except that mor e conducting elements were refilled axially during the first phase of water rise. The addition of raffinose to the water considerably reduced axial an d radial spreading rates. The polarity of water climbing was supported by m easurements of the water rise in dry branches using the 'light refraction' (and, sometimes, the 'leaf recurving') method. Basipetal refilling of the x ylem conduit exhibited biphasic kinetics; the final rise height did not exc eed 20-30 cm. Three-cm-long branch pieces also showed a directionality of w ater climbing, ruling out the possibility that changes in the conducting ar ea from the base to the apex of the branches were responsible for this effe ct. The polarity of water ascent was independent of gravity and also did no t change when the ambient temperature was raised to c. 40 degreesC. At exte rnal pressures of 50-100 kPa the polarity disappeared, with basipetal and a cropetal refill times of the xylem conduit of tall branches becoming compar able. Refilling of branches dried horizontally (with a clinostat) or invert ed (in the direction of gravity) showed a pronounced reduction of the acrop etal water rise to or below basipetal water climbing level (which was unaff ected by this treatment). Unlike water, benzene and acetone climbing showed no polarity. In the case of benzene, the rise kinetics (including the fina l heights) were comparable with those measured acropetally for water, where as with acetone the rise height was less. Transmission electron microscopy of dry branches demonstrated that the inner surfaces of the conducting trac heids and vessels were lined with a continuous osmiophilic (lipid) layer, a s postulated by the kinetic analysis and light microscopy studies. The thic kness of the layer varied between 20 and 80 nm. The parenchymal and interve ssel pits as well as numerous tracheid corners contained opaque inclusions, presumably also consisting of lipids. Electron microscopy of rehydrated pl ants showed that the lipid layer was either thinned or had disintegrated an d that numerous vesicle-like structures and lipid bodies were formed (toget her with various intermediate structural elements). These, many other data and the physical-chemical literature imply that several (radial) driving fo rces (such as capillary condensation, Marangoni forces, capillary, osmotic and turgor pressure forces) operate when a few conducting elements become a xially refilled with water. These forces apparently lead to an avalanche-li ke radial refilling of most of the conducting elements and living cells, an d thus to the removal of the 'internal cuticle' and of the hydrophobic incl usions in the pits. The polarity of mater movement presumably results from high resistances in the basipetal direction, which are created by local gradients in the thickn ess of the lipid film as a result of draining under gravity in response to drought. There are striking similarities in morphology and function between the xylem-lining lipid film and the lung surfactant film lining the pulmon ary air spaces of mammals.