Osmotic & specific effects of excess salts on beans

Abstract

In analyzing the cause of growth depression of plants raised under saline conditions, a distinction can be made between effects that are specific and those that are non-specific, with respect to the different salt species present in the root environment (1). Non-specific effects are related to the total concentration of salts, regardless of species. They prevail among the factors that determine the partial molar free energy of water, from the gradients of which depend the magnitude of the driving force, as well as the direction pertinent to the translocation of water in both soils and plants. Known as osmotic effects, nonspecific effects of salts are of a physico-chemical nature where they refer to the nutrient or soil-solution. They are barely distinguishable from physiological effects where they influence the state of water within the plants, as it may have bearing on cell processes. Both types of osmotic effects are associated to the extent that plants are capable of adjusting their internal osmotic pressure (OP) relative to changes brought about in the external OP (6, 8). Specific effects are usually due to the relative concentrations of the various ion species with respect to one another, less frequently to the absolute level of any one element. Their influence is essentially of a physiological nature. Changes in the osmotic concentration within plant cells brought about by varying relative ion concentrations in the growth medium kept at a constant OP should also be recognized as specific effects. Osmotic effects are frequently confounded with specific effects because high total-salt levels normally occur in association with ionic conditions that are unbalanced with respect to plant nutrition. Other specific effects, like trace element toxicities, are usually unrelated to osmotic effects (27). In many solution-culture studies purporting to separate the impact on plant growth due to osmotic effects from that due to specific effects, single salts (8, 9), mixtures of various salts (15), or organic compounds (10), have been used to control the OP of the solutions. Isosmotic solutions containing NaCl, Na,SO4, or CaCl2, caused equal depressions in growth of kidney beans (9), and in water uptake by corn roots (10). Both water uptake and growth were linearly related to the concentrations of any of the three salts in solution. To the extent that isosmotic concentrations of different salts cause equal growth depressions, such as in the examples cited, osmotic effects are predominant. Less likely, though sometimes not impossible, is the alternative explanation that the specific effects of the different salts on the test plant are nearly similar. Any variation in plant response to isosmotic concentrations of different salts indicates the additional significance of specific effects. For instance, MgCl2 and MgSO4 depressed the growth of kidney beans significantly more than did NaCl, Na,SO4, or CaCl9, all salts being compared in isosmotic concentrations. No proportionality existed between growth and Mg-salt concentration (9). With guayule as a test plant, neither was there a proportionality between growth and concentration of any one of the five salts mentioned, nor did the different salts when present in isosmotic concentrations cause the extent of growth depression to be the same. In fact, it appeared that MgCl was most, and CaCl least, toxic (24). Attempts to distinguish between osmotic and specific effects on plant growth require the aid of a compound that, while lowering the partial molar free energy of water, at the same time itself is as inert as possible with respect to the metabolism of the plant species tested. The latter requirement has not always been sufficiently considered. Substances like polyvinylpyrrolidone (26) and polyvinyl alcohol (5) in concentrations equivalent to 1 atm OP have been observed to be toxic to dwarf red kidney beans (11). Other substances, like sucrose, are either actively taken up or, like mannitol, subject to quick microbiological degradation. The usefulness of the compounds mentioned for osmotic studies of any extended length of time is dubious. A search for an osmoticum of the desired qualities has resulted in the selection of Carbowax polyethylene glycol, of a molecular weight of about 20,000, marketed by the Union Carbide Chemicals Co. (11). This compound henceforth will be referred to as C20M. Carbowax polyethylene glycols of lower molecular weights have been applied for various purposes, but no observations have been reported with regard to any physiological toxicity to plants (16). C20M contains considerable amounts of aluminum and magnesium (11). These impurities would have to be removed prior to applying C20M to most plants. Our paper will present details on the application of dialyzed C20M to distinguish I Received February 23, 1961.