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Water rise kinetics in refilling xylem after desiccation in a resurrection plant
- H. SCHNEIDER, N. WISTUBA, H.-J. WAGNER, F. THÜRMER, U. ZIMMERMANN
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- Journal:
- The New Phytologist / Volume 148 / Issue 2 / November 2000
- Published online by Cambridge University Press:
- 21 December 2000, pp. 221-238
- Print publication:
- November 2000
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The acropetal water refilling kinetics of the dry xylem of branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia were determined with high temporal resolution by observation of light refraction at the 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 upright, horizontal and inverted branches. Water rise kinetics were also determined with hydrostatic and osmotic pressure as well as at elevated temperatures (up to 100 °C) under laboratory conditions and compared with those obtained with intact (rooted) and cut branches under field conditions. Experiments in which water climbed under its capillary pressure alone, showed 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 transpiration of the unfolded and green leaves did not affect the rise kinetics in the ‘prominent’ conducting elements. Application of pressure apparently increased the number of elements making a major contribution to axial xylem flow. Analysis of these data in terms of capillary-pressure-driven water ascent in leaky capillaries demonstrated that root pressure, not capillary pressure, is the dominant force for rehydration of rooted, dry plants. The main reasons for the failure of capillary forces in xylem refilling were the small, rate-limiting effective radii of the conducting elements for axial water ascent (c. 1 μm compared with radii of the vessels and tracheids of c. 18 μm and 3 μm, respectively) and the very poor wetting of the dry walls. The contact (wetting) angles were of the order of 80 ° and decreased on root or externally applied hydrostatic pressure. This supported our previous assumption that the inner walls of the dry conducting elements are covered with a lipid layer that is removed or disintegrates upon wetting. Consistent with this, potassium chloride and, particularly, sugars exerted an osmotic 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 resulted in a reduction of the axial rise rate and final height. These observations demonstrated that radial water movement driven by the developing osmotic 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 primarily not involved in water rise. There is some evidence from field measurements of the axial temperature gradients along rooted branches that interfacial (Marangoni) streaming facilitated lipid removal (under formation of vesicle-like structures and lipid bodies) upon wetting.
Hydraulic architecture of Monstera acuminata: evolutionary consequences of the hemiepiphytic growth form
- J. LÓPEZ-PORTILLO, F. W. EWERS, G. ANGELES, J. B. FISHER
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- Journal:
- The New Phytologist / Volume 145 / Issue 2 / February 2000
- Published online by Cambridge University Press:
- 01 February 2000, pp. 289-299
- Print publication:
- February 2000
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The hydraulic architecture of the secondary hemiepiphyte Monstera acuminata was examined in native plants from Los Tuxtlas, Veracruz, Mexico, to determine how it compared to better-known growth forms such as trees, shrubs, lianas and primary hemiepiphytes. Monstera acuminata starts its life cycle as a prostrate herb. As it ascends a tree or other vertical support, the stem becomes thicker, produces larger leaves, and may die back from the base upwards until only aerial feeding roots serve to connect the stem to the soil. Unlike the pattern of vessel-size distribution along the stems of woody dicotyledons, M. acuminata has its wider vessels at the top of the stem, decreasing in diameter towards the base. Also peculiar is the fact that Huber values (axis area/distal leaf area) tend to increase exponentially at higher positions within the plant. Based on the hydraulic conductivity (kh) and leaf-specific conductivity (LSC, kh/distal leaf area), the base of the stem potentially acts as a severe hydraulic constriction. This constriction is apparently not limiting, as aerial roots are produced further up the stem. The plants have remarkably strong root pressures, up to 225 kPa, which may contribute to the maintenance of functional vessels by refilling them at night or during periods of very high atmospheric humidity, as in foggy weather and rain. In common with dicotyledonous plants, vessel length, vessel diameter, kh, specific conductivity (ks, kh/axis area) and LSCs were all positively correlated with axis diameter. The features of the hydraulic architecture of M. acuminata may be an evolutionary consequence of an anatomical constraint (lack of vascular cambium and therefore of secondary growth) and the special requirements of the hemiepiphytic growth form.
Daily embolism and refilling of xylem vessels in the roots of field-grown maize
- M. E. McCULLY, C. X. HUANG, L. E. C. LING
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- Journal:
- The New Phytologist / Volume 138 / Issue 2 / February 1998
- Published online by Cambridge University Press:
- 01 February 1998, pp. 327-342
- Print publication:
- February 1998
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Embolisms in the vessels of maize axile roots of different types were observed directly after rapid freezing of intact, functioning roots in the field, by cryo-scanning electron microscopy. Quantification of the degree of embolization in each root was made by counting empty and full vessels of both the late and early metaxylem (LMX & EMX), and expressed as percent embolized vessels of the LMX, and %EMX poles containing embolized vessels. Contents of the connecting xylem (CX) at branch root junctions, and of xylem in branch roots were observed also, but not systematically quantified. Records of % embolized vessels were made from dawn to dusk on summer days in Ottawa under moderate irradiance, and in Canberra under high irradiance. Measurements in Canberra were supported by estimates of irradiance, of stomatal conductance, and of chamber balance pressure of bagged and unbagged leaves. Soon after sunrise embolisms appeared in all types of vessel, at balance pressures c. 300–400 kPa, and increased rapidly with increasing irradiance. During the middle of the day % embolized vessels reached a maximum (LMX ≈70% in Ottawa, and ≈80% in Canberra). At all times the EMX vessels were less embolized. The midday maximum was brief in Ottawa, and % embolized vessels fell to a low value during the afternoon. In Canberra the maximum was prolonged into late afternoon. By dusk nearly all vessels were once again filled with sap. The balance pressures measured during vessel refilling in Canberra ranged from 500 kPa to 1200 kPa. At all times of the day sap was seen entering some embolized vessels. Almost all were refilling by mid- to late-afternoon. Such refilling was especially frequent at junctions of branch roots with the axile roots. X-ray microanalysis of the sap entering the vessels, and of the liquid filling or partly filling vessels, showed the concentration of mineral solutes present in the sap was below the threshold of detection (≈12 mM). These results are discussed in relation to current opinions about embolisms and vessel refilling.