New evidence for a role of vessel-associated cells and phloem in the rapid xylem refilling of cavitated stems of Laurus nobilis L.

Xylem recovery from embolism was studied in Laurus nobilis L. stems that were induced to cavitate by combining negative xylem pressure potentials (P_X = ?1.1 MPa) with positive air pressures (P_C) applied using a pressure collar. Xylem refilling was measured by recording the percentage loss of hydraulic conductance (PLC) with respect to the maximum 2 min, 20 min and 15 h after pressure release. Sodium orthovanadate (an inhibitor of many ATP‐ases) strongly inhibited xylem refilling while fusicoccin (a stimulator of the plasma membrane H⁺‐ATPase) promoted complete embolism reversal. So, the refilling process was interpreted to result from energy‐dependent mechanisms. Stem girdling induced progressively larger inhibition to refilling the nearer to the embolized stem segment phloem was removed. The starch content of wood parenchyma was estimated as percentages of ray and vasicentric cells with high starch content with respect to the total, before and after stem embolism was induced. A closely linear positive relationship was found to exist between recovery from PLC and starch hydrolysis. This, was especially evident in vasicentric cells. A mechanism for xylem refilling based upon starch to sugar conversion and transport into embolized conduits, assisted by phloem pressure‐driven radial mass flow is proposed.     

Vessel contents during transpiration - embolisms and refilling

A test was made of the previous unexpected observation that embolized vessels were refilled during active transpiration. The contents of individual vessels in petioles of sunflower plants were examined, after snap-freezing at 2-h intervals during a day's transpiration, in the cryo-scanning electron microscope, and assessed for the presence of liquid or gas (embolism) contents. Concurrent measurements were made of irradiance, leaf temperature, transpiration rate, and leaf water potential (by pressure chamber). Up to 40% of the vessels were already embolized by 0900 (transpiration rate ~5 _g_cm-2_s-1, water potential about -300 J/kg), and the proportion declined to a minimum (as low as 4%) at 1500. This was the time of highest transpiration rate (~25 _g_cm-2_s-1) and most negative water potential (-600 to -700 J/kg). Images of vessels with mixed gas and liquid contents showed water being extruded through pits in the walls of the vessels to refill them. The data indicate that: (1) the water columns are weak and break under quite small tensions; (2) embolisms are repaired by refilling the vessels with water on a short time scale (minutes) throughout the day; (3) the vigor of this refilling process is adjusted by the plant on a longer time scale (hours) to the intensity of the water stress; (4) the pressure chamber balance pressure (P) does not measure tension in the vessels; (5) P is also not a measure of water stress (as measured by vessel embolization); and (6) P is a measure of the plant's response to water stress, i.e., a measure of the vigor of the refilling process. The test confirms the previous observations and negates all the assumptions and evidences of the Cohesion Theory. The data are fully consistent with the Compensating Pressure Theory, which predicted the relations demonstrated in this experiment. Using the assumptions of that theory it is easy to outline a simple mechanism by which the refilling of vessels might be achieved by reverse osmosis, and the adjustment in (3) might be achieved by osmoregulation in the starch sheath.     

Vessel contents of leaves after excision - a test of Scholander's assumption

A test was attempted of the assumption that, when a leaf is cut, the xylem still contains water under tension beyond the first vessel cross walls. This assumption enabled Scholander to argue that the balance pressure in his pressure chamber measured the tension in water columns in the vessels before cutting. The numbers of embolized vessels were counted, after rapid freezing of petiole and midrib samples of sunflower leaves, in the cryo-scanning electron microscope. Counts were made on leaves still attached to the plant and at intervals after cutting from the plant (up to 16 min) during a short spring day's transpiration. The lengths of vessels in the leaves, measured by latex particle perfusion, showed that 8% of vessels in the mid-petioles and 0% in the midribs should be opened by cutting. The changing percentages of embolized vessels (E) with time showed that: (1) in intact plants E was close to zero until midday when it rose to ~40%, and then fell progressively to near zero by 1600; (2) in excised leaves there was no detectable change in E immediately after cutting, and, in all but two time courses, no change as large as the 8% of opened vessels within 16 min; (3) but briefly, when E was high (midday), it rose further after cutting to a plateau (_E = 30-40%) in 4 min. From this rate of emptying, the estimated maximum pressure difference between vessels and parenchyma was of the order of 0.05-0.2 MPa (0.5 to 2 bar) at this time. (4) All these changes occurred in the petioles 1 h before they were found in the midribs. The test failed because the expected large pressure difference between vessels and parenchyma was not present. Further, the embolized vessels were refilled at the time of peak transpiration, which would be impossible with any substantial tension in the vessels. Because these results contradict the whole basis of the Cohesion Theory, a second experiment was carried out to test them, and is reported in a companion paper.     

Applications of the compensating pressure theory of water transport

Some predictions of the recently proposed theory of long-distance water transport in plants (the Compensating Pressure Theory) have been verified experimentally in sunflower leaves. The xylem sap cavitates early in the day under quite small water stress, and the compensating pressure P (applied as the tissue pressure of turgid cells) pushes water into embolized vessels, refilling them during active transpiration. The water potential, as measured by the pressure chamber or psychrometer, is not a measure of the pressure in the xylem, but (as predicted by the theory) a measure of the compensating pressure P. As transpiration increases, P is increased to provide more rapid embolism repair. In many leaf petioles this increase in P is achieved by the hydrolysis of starch in the starch sheath to soluble sugars. At night P falls as starch is reformed. A hypothesis is proposed to explain these observations by pressure-driven reverse osmosis of water from the ground parenchyma of the petiole. Similar processes occur in roots and are manifested as root pressure. The theory requires a pump to transfer water from the soil into the root xylem. A mechanism is proposed by which this pump may function, in which the endodermis acts as a one-way valve and a pressure-confining barrier. Rays and xylem parenchyma of wood act like the xylem parenchyma of petioles and roots to repair embolisms in trees. The postulated root pump permits a re-appraisal of the work done by evaporation during transpiration, leading to the proposal that in tall trees there is no hydrostatic gradient to be overcome in lifting water. Some published observations are re-interpreted in terms of the theory: doubt is cast on the validity of measurements of hydraulic conductance of wood; vulnerability curves are found not to measure the cavitation threshold of water in the xylem, but the osmotic pressure of the xylem parenchyma; if measures of xylem pressure and of hydraulic conductance are both suspect, the accepted view of the hydraulic architecture of trees needs drastic revision; observations that xylem feeding insects feed faster as the water potential becomes more negative are in accord with the theory; tyloses, which have been shown to form in vessels especially vulnerable to cavitation, are seen as necessary for the maintenance of P, and to conserve the supplementary refilling water. Far from being a metastable system on the edge of disaster, the water transport system of the xylem is ultrastable: robust and self-sustaining in response to many kinds of stress.     

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought‐induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re‐watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re‐watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re‐watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non‐hydraulic factors influenced stomatal behaviour post drought.     

Axial and radial profiles in conductivities, water storage and native embolism in trunks of young and old-growth ponderosa pine trees

Ponderosa pine has very wide sapwood, and yet the spatial and temporal use of that sapwood for water transport is poorly understood. Moreover, there have been few comparisons of function in tips of old-growth trees in comparison with young trees. In the present study, axial and radial specific conductivity (k_s), leaf specific conductivity (LSC), leaf specific conductance (k_l), native embolism and the compartmentalization of sapwood water storage were characterized in trunks of young and old-growth trees. Trunks of young trees had lower k_s, lower LSC and lower native embolism [corresponding to 5% loss of conductivity (PLC)] than trunks of old-growth trees. However, k_l in young trees was 3.5 times higher than in old-growth trees, supporting the hypothesis that tall trees have a reduced ability to transport water to their leaves. Water storage (capacitance) of young trees was not significantly different than at the base of old-growth trees. Although the top of the old-growth trees had similar k_s, LSC and k_l to the young trees for a given cambial age, they had higher native embolism and lower capacitance. There was no trade-off between k_s and native embolism at any height. In the tree crown, outer sapwood had 35–50% higher k_s than the inner sapwood and 17–25 PLC lower native embolism. At the base of the old trees, there was no significant difference in native embolism between the outer, middle and inner sapwood, showing that refilling of embolisms was complete despite the 130-year difference in wood age among these radial positions. Although during the dry season the inner sapwood tended to be more saturated than the outer sapwood, the outer part of the sapwood contributed up to 60% of the overall stored water. Safer xylem, higher capacitance and higher k_l would appear adaptive in the young trees for regulating their water resource, which is likely to be less reliable than the water availability of older trees with their more developed root system.     

The mechanisms of refilling of xylem conduits and bleeding of tall birch during spring

Seasonal variations in osmolality and components of xylem sap in tall birch trees were determined using several techniques. Xylem sap was extracted from branch and trunk sections of 58 trees using the very rapid gas bubble-based jet-discharge method. The 5-cm long wood pieces were taken at short intervals over the entire tree height. The data show that large biphasic osmolality gradients temporarily exist within the conducting xylem conduits during leaf emergence (up to 272 mosmol x kg(-1) at the apex). These gradients (arising mainly from glucose and fructose) were clearly held within the xylem conduit as demonstrated by (1)H NMR imaging of intact twigs. Refilling experiments with benzene, sucrose infusion, electron and light microscopy, as well as (1)H NMR chemical shift microimaging provided evidence that the xylem of birch represents a compartment confined by solute-reflecting barriers (radial: lipid linings/lipid bodies; axial: presumably air-filled spaces). These features allow transformation of osmolality gradients into osmotic pressure gradients. Refilling of the xylem occurs by a dual mechanism: from the base (by root pressure) and from the top (by hydrostatic pressure generated by xylem-bound osmotic pressure). The generation of osmotic pressure gradients was accompanied by bleeding. Bleeding could be observed at a height of up to 21 m. Bleeding rates measured at a given height decreased exponentially with time. Evidence is presented that the driving force for bleeding is the weight of the static water columns above the bleeding point. The pressure exerted by the water columns and the bleeding volume depend on the water-filling status of (communicating) vessels.     

三个耐旱树种木质部栓塞化的脆弱性及其恢复能力

植物在长期适应赖以生存的自然环境中 ,形成了一套最适宜自身生长发育的生理生态行为 ,采取各种方式来抵御或忍耐水分胁迫的影响。如通过具有深广而茂密的根系格局来保持水分吸收 ,通过气孔调节、角质层障碍作用和小的叶蒸发表面来减少水分散失 ,通过渗透调节和增加组织弹性来保持膨压 ,通过增强原生质耐脱水能力来免受伤害或少受伤害等等。植物遭受干旱危害时 ,首先出现表型反应的多是植物的叶片 ,因此 ,研究植物的耐旱机理多从叶入手 ,对根系类型、分布及根茎比在植物耐旱性方面也有不少报道[1,2 ],而对木质部在干旱适应性反应方面的研究…