WATER UPTAKE BY EXCISED ROOT SYSTEMS OF THE TOMATO DUE TO NON-OSMOTIC FORCES

THE CLASSICAL concept of water uptake into cells by means of osmosis is so universally accepted that it is often regarded more as a fact than a convenient theory. Root pressure also is often considered to be entirely osmotic in nature (Sabinin, 1925; Crafts and Broyer, 1938). However, experiments are known which make it appear as if osmosis is inadequate to explain the water uptake of cells. Ursprung and Blum's experiments (1921, 1925) seem to show that water uptake by the cells of the endodermis is possible against an osmotic gradient. Bennett-Clark and Bexon (1940) have experiments which tend to show that the "water absorbing power" of the leaf cells exceeds the osmotic pressure of the vacuole. Heyl (1933) concluded from a study in which he investigated the effect of temperature upon the rate of bleeding and the rate of backward movement of the exudate under influence of plasmolytica, that the mechanism of bleeding cannot be exclusively osmotic in nature. The purpose of this investigation was to find out experimentally whether or not osmosis alone is capable of explaining root pressure. In order to do this, a simple basic experiment was designed by which it was possible to measure (A) the osmotic pressure of the exudate2 and (B) the pressure with which water is taken up by the roots. If these two values are equal in magnitude, root pressure can be explained on the basis of osmotic pressure alone. If the pressure with which water is taken up by the roots (root pressure) is larger than the osmotic pressure of the exudate, then root pressure is due to other causes in addition to osmosis.3 The root pressure was determined as the osmotic pressure of a mannitol solution which exactlv compensated the root pressure. TEcHNIQuE.-Seeds of "San Jose Canner" tomatoes were sown in sand in flats and watered with Hoagland solution. After the seedlings were six to eight weeks old, they were transferred to culture solutions with forced aeration in one of the air-conditioned greenhouses (26.50C. and 70 per cent humidity). For constancy of temperature and other properties of these greenhouses, see Went (1942). In order to avoid "shock" reactions the experiments (with the exception of the one mentioned in fig. 3) were carried out at the same constant conditions as those under which the plants were grown. The in1 Received for publication April 18, 1942. 2 The nomenclature regarding osmotic quantities proposed by Meyer (1938) and Meyer and Anderson (1939) is used throughout. Therefore "osmotic pressure" of the exudate denotes "osmotic value" or "osmotic concentration" of the older literature. 3 It is assumed that the osmotic pressure of the exudate equals, or at least is not higher than, that of the contents of the vessels down in the water-absorbing region of the roots. Evidence for the correctness of this assumption is given later in the paper. dividual plants were held with non-absorbent cotton in paraffined wooden lids, which were placed on onegallon mayonnaise jars which were painted white over a black undercoat and filled with Hoagland solution (Hoagland and Arnon, 1938). In order to obtain plants rich in carbohydrates which would be capable of carrying on metabolism for a considerable time after they were decapitated, the nutrient solution was not renewed, but the water lost by transpiration was replaced by distilled water. In this way low-salt plants which are rich in carbohydrates (Hoagland and Broyer, 1936; Grossenbacher, 1938) were obtained. The pH was adjusted periodically and maintained between 5 and 6, and ferric chloride was added weekly. The Hoagland solution was renewed in a few special experiments which will be mentioned later. In the experiments mentioned in the last section, dealing with the effects of KCN, a nutrient solution suggested by Arnon and Hoagland (1940, p. 477) was employed. Exceptional root growth was obtained in this solution and the pH adjustments were not necessary. Iron citrate was given twice weekly. After the plants had grown from four to seven weeks in the culture solution, they were cut about one to two inches above the uppermost roots. This decapitation was done about noon, because it was found that plants cut earlier in the day often did not bleed at all, even when left for a day or longer. After decapitation the plants were transferred to distilled water. In some experiments distilled water was put in the culture jars a few days before the plants were decapitated (fig. 1). The root system was put in distilled water because it eliminates accumulation of solutes as a complicating factor in the process of exudation; it also increases the rate of exudation over that of root systems kept in nutrient solution. In some experiments (fig. 2) the roots were kept in a full-strength Hoagland solution instead of in water. This did not influence the results, however. It only caused a slower rate of exudation and a higher osmotic pressure of the exudate. During the experiments, as well as during the growing period, forced aeration was used continuously. The stump of the plant was attached to a thickwalled U-tube with narrow bore, by means of a small piece of rubber tubing firmly attached to the stump with string. In this manner a perfectly water-tight connection was obtained which allowed the U-tube to be interchanged conveniently for a horizontal "potometer" tube (for rate determinations). The majority of plants bled profusely, often at a rate of 2 cc. per hour. After a minimum of 2 cc. of the exudate was collected for freezing point determinations, the U-tube was replaced by a horizontal thick-walled tube (inside diameter 0.042 inches). The meniscus moved rapidly outward.