An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure |
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Authors: | L C Enns M J Canny M E McCully |
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Institution: | (1) Department of Biology, Carleton University, Ottawa, Ontario;(2) Present address: Division of Plant Industry, Commonwealth Scientific and Industrial Research Organisation, 2601 Canberra, ACT, Australia;(3) Present address: Research School of Biological Sciences, Australian National University, Canberra, A.C.T., Australia;(4) Present address: Botany Department, University of Washington, Seattle, Washington, USA |
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Abstract: | Summary Solute osmotic potentials (x) in the vessels of hydroponically grown maize roots were measured to assess the osmotic-xylem-sap mechanism for generating root pressure (indicated by guttation). Solutes in vessels were measured in situ by X-ray microanalysis of plants frozen intact while guttating. Osmotic potentials outside the roots (o) were changed by adding polyethylene glycol to the nutrient solution. Guttation rate fell when o was decreased, but recovered towards the control value during 3–5 days when o was greater than or equal to –0.3 MPa, but not when o was equal to –0.4 MPa. In roots stressed to o = –0.3 MPa, x, was always more positive than o, and x changed only slightly (ca. 0.05 MPa). Thus the adjustment in the roots which increased root pressure cannot be ascribed to x, contradicting the osmotic-xylem-sap mechanism. An alternative driving force was sought in the osmotic potentials of the vacuoles of the living cells (v), which were analysed by microanalysis and estimated by plasmolysis. v showed larger responses to osmotic stress (0.1 MPa). Some plants were pretreated with abundant KNO3 in the nutrient solution. These plants showed very large adjustments in v (0.4 MPa) but little change in x (0.08 MPa). They guttated by 4 h after o was lowered to –0.4 MPa. It is argued that turgor pressure of the living cells is a likely alternative source of root pressure. Published evidence for high solute concentrations in the xylem sap is critically assessed.Abbreviations o
external water potential
- x
osmotic potential of xylem sap
- v
osmotic potential of vacuolar sap
- EDX
energy dispersive X-ray microanalysis
- CSEM
cryo-scanning electron microscope
- LN2
liquid nitrogen
- PEG
polyethylene glycol |
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Keywords: | Cell turgor Guttation Osmotic potentials Root pressure Xylem sap Zea mays |
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