Growth and Biochemical Composition of Bean Plants as Conditioned by Soil Moisture Tension and Salt Concentration

Section: Resultsmentioning

confidence: 99%

“…There is abundant experimental evidence (5,24) that the salt effect is basically an osmotic one. But the idea that salinity limits water uptake has been questioned because of the (lemonstrate(l (3, 4, 8 made by plants grown on saline media.…”

Section: Resultsmentioning

confidence: 99%

Expansion of Bean Leaves and its Suppression by Salinity

Nieman

1965

“…Some workers have repeatedly stated that water is equally available over this range (5,18,23,24) while considerable data have accumulated which indicate that physiological processes in plants are altered as a result of decreasing soil moisture content even before the onset of wilting (1,2,4,6,7,12,16,21,25). As indicated by KRAMER (15) some of this disagreement exists because of differences in soil moisture tensionsoil moisture content relations for different textural grades of soil.…”

mentioning

confidence: 96%

Availability of Soil Moisture for Active Absorption in Drying Soil

Army

Kozlowski

1951

There is considerable disagreement as to whether or not water is equally available to plants in drying soil over the entire range from field capacity to wilting percentage. Some workers have repeatedly stated that water is equally available over this range (5,18,23,24) while considerable data have accumulated which indicate that physiological processes in plants are altered as a result of decreasing soil moisture content even before the onset of wilting (1,2,4,6,7,12,16,21,25). As indicated by KRAMER (15) some of this disagreement exists because of differences in soil moisture tensionsoil moisture content relations for different textural grades of soil. In certain coarse-textured soils most of the water in the range from wilting percentage to field capacity is held with a tension of less than one atmosphere and probably most of it is readily available. In some fine-textured soils, however, less than half of the available water may be held with a force of less than one atmosphere, and water is withheld from plants with appreciably greater energy over the lower part of the available range than over the upper part. Near the wilting percentage a small decrease in moisture content effects an enormous increase in tension.Data also are available which indicate that active absorption and root pressure specifically are depressed by low soil moisture contents above the wilting percentage. LITVINOV and GEBHARDT (17) have reported a qualitative correlation between drying of soil and cessation of exudation from stumps of detopped plants. KRAMER (14) noted that removal of tops of unwilted herbaceous plants was not always followed by expected exudation. However, exudation usually began soon after the soil was watered. Working with coleus, sunflower and tomato, Kramer found that about 45%o of the soil moisture between the moisture equivalent and the wilting percentage was unavailable to detopped plants. He concluded that the active absorption mechanism does not absorb against a diffusion pressure deficit greater than one or two atmospheres. The proportion of moisture in the "available range" over which exudation does not take place was essentially similar for clay, sand, and sandy loam. The uniformity of the results obtained indicated that the soil moisture content which limits exudation is determined predominantly by soil characteristics and not by plant characteristics. Using sunflower plants and a uniform, fertile sandy loam, McDERMOTT (19) found that within a range of moisture content just above the wilting per-

“…The osmotic pressure of the soil water due to the soluble salts of the soil (which are diffusible through the plant membranes) is envisaged as opposing the entry of water into the plant via the root system. Wadleigh (25,24) has used the "total soil moisture stress," defined as the sum of the tension and the osmotic pressure of the soil water, as a measure of the diffusion pressure deficit which must be exceeded in the epidermal cells of plant roots before water may be absorbed by them. Expressed thus, the concept is unexceptionable.…”

Section: )mentioning

confidence: 99%

The Osmotic Cell, Solute Diffusibility, and the Plant Water Economy.

Philip

1958

134

The question which prompts the present work is: "How can the concept of the plant cell as an osmometer with semi-permeable walls be justified, when solutes may enter the plant cell vacuole?" Textbooks of plant physiology and physical chemistry treat osmosis in detail only for equilibrium conditions and non-diffusible solutes. To extend the quantitative treatment to the case of a diffusible solute we must abandon the equilibrium approach and study the problem dynamically. We introduce the dynamic approach by applying it to the classical cell osmometer; then we proceed to the case of a diffusible solute.DYNAMICS OF THE CLASSICAL CELL OSMOMETER: We use the following symbols:V, cell volume (cm3). VO, cell volume at zero turgor pressure. v = V/Vo -1, relative departure of cell volume from VO. A, external surface area of cell (cm2). AO, value of A at zero turgor pressure. P, osmotic pressure of cell contents (in the sense of Meyer (14), i.e., as a concentration-dependent property of the solution, independent of the hydrostatic system) (atm). T, turgor pressure (atm). [Broyer and others (4,21,22,27) have shown the general equivalence of T and "wall pressure," and that it is a mistaken over-emphasis of an infinitesimal second-order effect to dwell on the distinction between them (6)]. Kw, permeability of cell wall to water (cm sec-1 atm-1). t, time (sec).We suppose that, initially, T = 0, and P = PO, and that the solutes in the system are non-diffusible. If the cell is now placed in free water, water will enter the cell until the (increasing) turgor pressure becomes equal to the (decreasing) osmotic pressure. The dynamics of this process is described exactly by the equation.(1) subject to the initial condition t = 0, V = VOQuite generally, K,, A, P and T are functions of V. We therefore simplify the analysis by taking K,A as constant and equal to K,A0.If we take P proportional to solute concentration (as it is to a first approximation)If we assume that change of cell volume is proportional to change in turgor pressure, we have for theThe elastic modulus, E, corresponds to the "coefficient of enlargement" of Broyer (5). As Broyer states, e is not strictly constant for perfectly elastic cell walls, except for infinitesimal volume changes. Since the assumption that the wall obeys Hooke's law is, in any case, an approximation (9), it seems permissible to adopt e as constant in the present analysis.We recognize that, in real plants, marked deviations from the linear T (v) relation to be used here may occur. In this sense the present work may be considered a first approximation. Essentially similar results would follow from a more precise, though more elaborate analysis along the same lines in which non-linearity in T(v) was taken into account.The existence of a unique T(v) function implies, however, that the cell volume changes are elastic. Obviously, the plastic, irreversible, deformations associated with cell elongation are beyond the scope of the present treatment, though it may prove possible to include these in a sim...