Diurnal changes in xylem pressure of the hydrated resurrection plant Myrothamnus flabellifolia: evidence for lipid bodies in conducting xylem vessels

Abstract

The resurrection plant Myrothamnus flabellifolia has the ability to recover from repeated prolonged and extreme desiccation cycles. During the dry state the inner walls of the xylem vessels seemed to be covered, at least partly, by a lipid film as shown by Sudan III and Nile Red staining. The lipid film apparently functioned as an ' internal cuticle ' which prevented the adjacent parenchyma ray cells from complete water loss. The hydrophobic nature of the inner xylem walls was supported by the finding that benzene ascended as rapidly as water in the xylem of dry Myrothamnus branches. On watering, numerous lipid bodies were found in the water-conducting vessels, presumably formed from the lipid film and\or from lipids excreted from the adjacent living cells into the vessels. The presence of lipid bodies within the vessels, as well as the hydrophobic properties of the inner xylem walls, could explain the finding that the xylem pressure of hydrated, well watered plants (measured both under laboratory and field conditions with the xylem pressure probe) never dropped below c. k0.3 MPa and that cavitation occurred frequently at low negative xylem pressure values (k0.05 to k0.15 MPa). The xylem pressure of M. flabellifolia responded rapidly and strongly to changes in relative humidity and temperature, but less obviously to changes in irradiance (which varied between 10 and c. 4000 µmol m −# s −" ). The morphological position of the stomata in the leaves could explain the extremely weak and slow response of the xylem pressure of this resurrection plant to illumination changes. Stomata were most abundant in the furrows, and were thus protected from direct sunlight. Simultaneous measurements of the cell turgor pressure in the leaf epidermal cells (made by using the cell turgor pressure probe) revealed that the xylem and the cell turgor pressure dropped in a ratio of 1 : 0.7 on changes in the environmental parameters, indicating a quite close hydraulic connection and, thus, water equilibrium between the xylem and cellular compartments. An increase in irradiance of c. 700 µmol m −# s −" resulted in a turgor pressure decrease from 0.63 to 0.48 MPa. Correspondingly, the cell osmotic pressure increased from 1.03 to 1.22 MPa. From these values and by assuming water equilibrium, the osmotic pressure of the xylem sap was estimated to be 0.25-0.4 MPa. This value seems to be fairly high but may, however, be explained by the reduction of the water volume within the vessels due to the floating lipid bodies.

Water rise kinetics in refilling xylem after desiccation in a resurrection plant

“…A radius of c. 1 µm is not only compatible with the mean radius of the ' pores ' of the perforation plates of the vessels, but also with the dimensions of the opening angle of the corners of the rhomboidal tracheids (Schneider et al, 1999 ;Wagner et al, 2000). As already mentioned, corners of rhomboidal capillaries considerably increase the rate of water spreading (i.e.…”

“…Field and laboratory experiments were carried out under various environmental conditions (Schneider et al, 1999). The ambient irradiance, temperature and rh were measured with a light meter and a hygro-thermometer (LI 189 SA, LI-COR Inc., Lincoln, NE, USA, and testo 610, Testo GmbH, Lenzkirch, Germany, respectively).…”

“…Although several surface properties can affect the contact angle (see the Theoretical Considerations section), we assume that the high values result from the hydrophobic properties of the inner walls of the xylem conduit (Schneider et al, 1999 ;Wagner et al, 2000). Hydrophobic impregnation and covering of the walls considerably limit water rise under its own capillary pressure in comparison with hydrophilic inner capillary walls.…”

“…interfacial) forces. In addition, the inner walls of the empty conducting elements of the xylem of M. flabellifolia are presumably covered with a lipid film (Schneider et al, 1999) influencing wetting and thus water ascent.…”

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

“…Analysis of experimental data and light microscopy (Schneider et al, 1999) support the view that the refilling of the dry xylem of M. flabellifolia is apparently hampered by a hydrophobic (lipid) film on the inner wall of the conducting elements. This film must be removed or thinned before wetting can occur and water can rise to the apex.…”