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Journal Article

The Osmotic and Vitalistic Interpretations of Exudation

Frank M. Eaton
American Journal of Botany
Vol. 30, No. 9 (Nov., 1943), pp. 663-674
Stable URL: http://www.jstor.org/stable/2437712
Page Count: 12

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Topics: Plants, Exudation, Solutes, Plant roots, Chlorides, Plant cells, Xylem vessels, Osmosis, Sap, Water pressure
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The Osmotic and Vitalistic Interpretations of Exudation
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Abstract

Neither the evidence afforded by the literature, as reviewed in this paper, nor the new experimental data that are presented indicates the necessity of postulating uncertain physical or chemical forces to account for the bleeding and root pressures of plants. These phenomena, in the writer's opinion, can be satisfactorily explained on the basis of the difference between the osmotic pressure developed in the xylem vessels and the sum of the osmotic and capillary forces in the external substrate. Differences between the cryoscopic values found for fresh tissues and the plasmolytic values found after a period of immersion are not necessarily indicative of vital activity. It is well known that living cells under suitable conditions will accumulate solutes. Plasmolytic values represent the water retaining force of cells after the immersion period but they may not represent the initial condition. The application of "rapid high pressure" to tissue masses should yield pure water rather than vacuolar sap, provided the protoplasts are truly semipermeable, the cells are undamaged, and the pressures applied are in excess of the osmotic pressures of the cell contents. Differences between the concentrations of "rapid high-pressure sap" and plasmolyzing solutions are irrelevent both to the questions of active secretion and to the proportion of the cell contents that is vacuolar. Conclusions in favor of active secretion based on differences between the concentrations of mannitol, or other solutes, required to stop exudation and the concentration of the exudates simultaneously issuing from the stumps neglects the ability of roots to accumulate these solutes and the resulting differences in sap concentration between the lower vessels and the stump. Conifers, which customarily do not bleed, were found to exude slowly when their roots were placed in water after having been grown for a time in concentrated solutions. The effect is attributed to a movement of accumulated solutes into the tracheids. Measurements were made at five-minute intervals of the upward and downward movement of sap in the stems of decapitated tomato plants in response to additions to the external solution of mannitol, sucrose, calcium chloride, and sodium chloride. The response in exudation rate appeared to be immediate and new equilibria were indicated between the external and xylem-vessel solutions in 10 to 20 minutes after the additions. The effects of equimolar solutions of the first three solutes were smilar. The hydathode exudate from salt-starved tomato plants had a concentration only 6 per cent as high as the nutrient solution. The secretion of this dilute solution was accounted for by solute accumulation on the part of living cells from the sap stream. The concentration of the sap at the cotyledons was greater than that of the nutrient solution. Exudation occurred from cotton plants when the osmotic concentration of the xylem vessel sap exceeded that of the external solution, but it did not occur when the osmotic differential was in the opposite direction. The plants were grown on substrates with successively higher chloride content. When the culture solutions supporting these plants were replaced with tap water, a curvilinear relationship was found between the rate of exudation and the osmotic differential. This curve when extrapolated appeared to pass through the origin indicating that osmosis was the only force involved in the exudation. Measurements of the osmotic and chloride concentrations in the external solution and in the root sap, the xylem vessels, and leaf sap of the foregoing plants showed: (1) uniform osmotic differentials between the leaf sap and the substrate solution indicating that osmosis satisfactorily accounted for the movement of water into the leaves; (2) a xylemvessel sap, in the plants on the more saline substrates, that was less concentrated than either the external solution or leaf sap indicating that the xylem-vessel sap was continuously under tension; (3) much higher accumulation ratios between leaf cells and their xylem-vessel substrate than between root cells and their exterior substrate; and (4) living root cells in the most saline solution which at the time of the measurements were less concentrated than the external solution.

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