Who Taught Plants Thermodynamics? The Unfulfilled Potential of Plant Water Potential

Abstract

The thermodynamic properties of water have been of interest to plant physiologists since Dixon (1914) first used a pressure chamber to study the 'osmotic tension' of tree sap. With the development of the psychrometer (Spanner 1951) and the modern pressure chamber (Scholander et al. 1965), data have been reported regularly on the chemical potential of water and its components. Apart from the relationship between turgor potential and abscisic acid accumulation (Pierce and Raschke 1980), no unique, fundamental relationships have been established relating the thermodynamic state of water to physiological performance (Ritchie 1981;Turner et al. 1985;Van Volkenburgh and Boyer 1985).From time to time, doubts have been raised about expectations that thermodynamic state variables would lead to explanations of physiological behaviour. Hsiao (1973) and Oertli (1976) both argued strongly that there did not appear t o be any transducers to translate changes in either total water potential ($) or osmotic pressure (a) into physiological responses. For example, in plant tissue suffering severe stress the chemical potential of water is reduced only a few per cent below that of free water. Moreover, the only effect of osmotic pressure appears to be via local concentrations of deleterious compounds in specific compartments within the cell.One justification frequently raised for measuring water potential is that it influences water flow. In fact, however, much of the water flow in the soil-plant system is governed by hydrostatic pressure (P). Water flow in the soil, xylem, phloem, and apoplasm is driven by hydrostatic pressure gradients and not total water potential gradients (Nobel 1974;Passioura 1984). Water potential gradients are probably only of major importance for water flow across membranes between the apoplasm and symplasm. Since equilibrium or near equilibrium of water potential is usually assumed over these short distances and is therefore not measured, characterization of water flow by water potential gradients is of little practical relevance.Not only does hydrostatic pressure seem relevant to the problem of water flow, but it is also the thermodynamic component, in the form of turgor pressure, that recently has received the most attention as the variable influencing the physiology of plants. However, for plants to transduce turgor pressure into a physiological signal, the mechanical potential energy of hydrostatic pressure must be gauged.

The Effect of Magnetized Water on Physiological and Agronomic Traits of Cowpea (Vigna unguiculata L.)

“…Sinclair and Ludlow [15] proposed that RWC was better measur or pl nt's w t r status than thermodynamic state variable (water potential, turgor potential and solute potential). RWC is closely related with cell volume, it may more closely reflect the balance between water supply to the leaf and transpiration rate.…”

QTL for relative water content in field-grown barley and their stability across Mediterranean environments

“…Traits allowing the evaluation of plant water-status, and particularly relative water content (RWC), could give an idea of the level of the water deficit in the plant at a specific time-point. As RWC is related to cell volume, when it is measured on the flag leaf, it may closely reflect the balance between water supply to the leaf and transpiration rate (Sinclair and Ludlow 1985). The potential value of RWC for breeding under droughtstress conditions has been demonstrated by Schonfeld et al (1988) in winter bread wheat.…”

Teasterone 3-Myristate: A New Type of Brassinosteroid Derivative inLilium longiflorumAnthers

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