Capillarity and sap ascent in a resurrection plant: does theory fit the facts?


The kinetics of rehydration of stem and leaf tissue in a resurrection plant (Myrothamnus flabellifolia) has been the subject of two recent papers in New Phytologist (Schneider et al., 2000; Wagner et al., 2000; see also Canny, 2000). The rate of rehydration of excised stems was fitted to a model for capillary rise in capillaries with leaky walls. The theory predicted that the stems rehydrated though capillaries 2 µm in diameter. However, a review of published data and theory suggests major internal inconsistencies. A more plausible explanation assumes that the advancing wetting front entraps air bubbles (emboli). The capillary force driving the advance of the wetting front is approximately balanced by the pressure of the entrapped emboli. Hence the rate of advance of the wetting front is probably limited by the rate at which the emboli dissolve.

Plants seemingly defy physics every day: they transport water through conduits in stems where it is retained under negative pressure (typically −0.2 to −2 MPa and in some cases down to −10 MPa; Wei et al., 1999). Water is literally being torn apart and is held together by intermolecular bonds (hydrogen bonds). How do plants perform a feat unachievable even by modern engineering technology?

Water is transported through millions of long narrow conduits (seeComstock & Sperry (2000) for details of recent reviews). The conduits have to be big enough for efficient water transport (about 10–100 µm diameter), but sealed at the sides and ends so they are airtight to keep air at 0.1 MPa pressure from displacing the water at much lower pressure. This is achieved by a system of check valves along the wall of the conduit that permit water transport but not airflow. Most of the surface of a conduit is impermeable to gas and water but punctuated by pits (the check valves) where water passes through a fine cellulose filter with a porosity of a few nm. The conduits are much shorter than the plant so that water must pass from the root to leaf via a daisy-chain of thousands to millions of conduits per cm3 of stem, where each conduit is guarded by thousands of pits. This provides the safety that permits the ‘impossible’ system to work: if one conduit is damaged it will immediately fill with air (embolize), but air cannot pass through the wet pit pores because surface tension of the air–water interface (meniscus) prevents the passage of air until a critical pressure difference develops across the meniscus (Tyree et al., 1994).

Failures at the pits are common. Normally 5–50% of the conduits in plants are embolized, but the plants can still function normally. In extreme cases, however, all the conduits become air filled (e.g. when wild grape vines in New England are freeze-dried by the cold winters or when woody desert shrubs are heat desiccated). How do these conduits refill when wet temperate climate conditions return? If the pits remain dry and unblocked then water rising from the roots can simply displace the air through the dry pit pores, but if the pit filters become wet before all the air is expelled then the check valves regain their normal function and will seal emboli in the conduits blocking water transport. How do plants refill conduits with water and expel all emboli? Also, can the >5% of normally embolized conduits refill while surrounding conduits are filled with water under negative pressure (Holbrook & Zwieniecki, 1999; Tyree et al., 1999)? This has become the subject of considerable interest in recent years.

The two papers from the University of Würzburg concern rehydration from the extreme in Myrothamnus flabellifolia (Schneider et al., 2000; Wagner et al., 2000), a woody shrub from the Namibian deserts. Myrothamnus is a resurrection plant, which when dry appears dead, but after watering is ‘resurrected’ into an obviously living, functional plant. An analogous situation occurs in wild grape stems freeze dried by a long winter. In spring, root pressure pushes a water column up the grape stem and displaces air in conduits through dry open pits above the advancing meniscus (Sperry et al., 1987). But the advance of the meniscus refills only some of the conduits. Full recovery of hydraulic conductivity requires dissolution of emboli entrapped in some of the conduits.

Even without root pressure, an excised dry stem can absorb water by capillarity and the advancing meniscus can displace air through the conduits, and this is the mechanism proposed for Myrothamnus (Schneider et al., 2000). The equations governing capillary rise in leaky pipes were derived to learn how fast root pressure rehydrates intact plants, how fast capillary forces rehydrate cut branches, and how fast water perfused under pressure (to simulate root pressure) rehydrates cut branches. On cursory examination the data on cut branches and theory appear to agree. We are told that water ascends through conduits just 2 µm in diameter. In a glass tube, capillarity lifts water against gravity and, as the water column rises, its weight slows the refilling process until a stable final height is reached, where the weight of the water below the meniscus is balanced by the upward force of capillarity. In real xylem vessels, the walls are leaky, hence water is moving radially through the walls while the column is lifted against gravity. The radial flow initially rehydrates surrounding tissue and is needed to replace water as it evaporates from rewetted leaves, hence a quasi-steady state is reached where water rises to a quasi-stable height. In this case the upward force of capillarity is balanced by the sum of two effects (i.e. the pressure difference required to maintain a quasi-steady rate of radial flow out of the tube plus an additional pressure drop to lift the column against gravity). The quasi-stable height reached in a leaky pipe is less than in a pipe with impermeable walls. But we have to reject the author's explanation because there are a number of striking incongruities in the data presented.

First, Myrothamnus stems have two sizes of conduits, both of which are too large to fit the theory. Of the stem cross section, 20% is occupied by vessels of circular cross section with a mean diameter of 36 µm, and 67% is occupied with rhomboidal conduits (tracheids) 3–6 µm wide and 10–16 µm long in cross section. The remaining 13% of the stem is living ray cells. Using Eqns 9 and 10 from Schneider et al. (2000), theory predicts that large conduits fill by capillarity much faster than smaller ones. Conduits of 36 µm diameter should fill to half the resting height in 60 s and be near resting height in c. 1000 s, whereas a 2-µm conduit requires 4 h (240 times longer) to reach half height (24 cm) and 48 h to complete. The theory fits the rehydration kinetics only for 2-µm diameter conduits. Theory predicts the flow rate, when the stem is half full, to be 4 × 10−17 m3 s−1 per conduit – about half the flow passes out radially to rehydrate other tissues and the other half goes to push the column upward. The flow rate into the base of a 3-mm diameter stem was computed from potometer experiments (Fig. 1b, closed circles, in Schneider et al. (2000)) and is equal to 5.5 × 10−11 m3 s−1. Hence, theory would predict the existence of 1.4 million 2-µm conduits (= 5.5 × 10−11/4 × 10−17) and this is too many to fit into a 3-mm diameter stem. High resolution NMR imaging (Fig. 1, Wagner et al., 2000) shows that only half the cross section is capable of water conduction. Vessel and tracheid walls are 1–1.5 µm thick, hence if the hypothetical 2-µm conduits are cylinders and arranged in a square array they would each occupy 9 µm2, so 1.4 million of them would occupy a total of 12.6 mm2 of cross section – that is, 350% of the available space. If we assume that the responsible conduits are the tracheids, each would occupy 58 µm2, so 1.4 million would occupy 1150% of the available space.

Second, the theory has insurmountable inconsistencies, which should have been anticipated because in the second paper the authors describe the electron microscopy of the conduits (Wagner et al., 2000). The walls are lined with a thick layer of lipid and the ‘intervessel … pits of dry branches were completely filled with lipid’ (Wagner et al., 2000). Since water conduction as well as expulsion of air by an advancing front of liquid would have to pass through these pits between adjacent conduits, capillarity would not expel air but would compress and entrap emboli. The entrapped air would only disappear as it slowly dissolved into the water and the air diffused out of the stem. The theory for this kind of removal of entrapped emboli by dissolution and diffusion has already been worked out (Yang & Tyree, 1992) and verified experimentally for initially nonconductive, horizontal, maple-stem segments 4–8 mm in diameter (Tyree & Yang, 1992) and for thin sections of conifer stems bathed in water (Lewis et al., 1994). Although the theory for the time course of bubble collapse has not been applied to the precise conditions in the Myrothamnus experiments, the theory is consistent with the time scales observed (Schneider et al., 2000). The theory of capillary rise (Schneider et al., 2000) assumes the conduits are unobstructed, hence the theory was inappropriate because the theory fails to account for Henry's law for gas solubility of entrapped emboli and Fick's law, which would predict how fast the entrapped emboli would dissolve in 3–6 mm diameter Myrothamnus stems.

The mysterious lipid lining disappears in rehydrated branches, but the possible function of the lipid is still open for question. It may serve a protective role during dehydration but it clearly impedes the rehydration process. The lipid does, however, complicate the application of Fick's law to predict the rate at which bubbles would dissolve in Myrothamnus stems since it might reduce the diffusion coefficient for air out of the embolized conduits. The lipid also complicates the prediction of the correct contact angle between the meniscus and the conduit walls. The contact angle is needed to predict the capillary forces and hence the pressure of the entrapped emboli. The contact angles calculated (Schneider et al., 2000) are valid only on the assumptions that the conduits are 2 µm and that entrapped air bubbles are not limiting the rate of rise of the wetting front. Both these assumptions are clearly wrong, hence we cannot predict the pressure of the entrapped bubbles.