SEASONAL AND DIURNAL VARIATIONS IN THE OSMOTIC VALUES AND SUCTION TENSION VALUES IN THE AERIAL PORTIONS OF AMBROSIA TRIFIDA

Abstract

The objects of this investigation were to determine the direction and magnitude of the gradients in ossmotic values and suction tension values in the aerial portions of Ambrosia trifida L. and to determine the magnitude of the daily and seasonal variations in these values. Ambrosia trifida, commonly known as the giant ragweed or horseweed, was selected for this investigation because the necessary quantity of plant material of uniform quality was available at a relatively short distance from the laboratory. The theory of the Pull of Transpiration (6, Ii, I2) has been quite generally accepted as the best explanation of the mechanism by which water ascends to the aerial parts of plants. It is not the aim of this paper to discuss or question this theory. To accept this theory it is not necessary to assume, or even expect, that the highest portions of the plant will have the greatest lifting power. However, each portion of the plant must be able to raise sufficient water to its own height and must be able to compete for water with other portions of the same plant, particularly when the water supply is deficient. It might be assumed that the tissues capable of exerting the greatest tensions on the water columns will tend to be the most successful in the competition for water. As yet few data are available concerning the relative osmotic and suction tension values in the various portions of the same planlt. Data are lacking particularly from the standpoint of the effect of a deficiency of water on these relative values. Dixon and Atkins (9) studied the seasonal variations in osmotic value of the expressed sap of the leaves of several species of evergreen shrubs. In general, they found these to have a higher osmotic value in winter than in summer. Similar results on other evergreenls were obtained by Lewis and Tuttle (22), Korstian (20), Gail (I5), and others. Meyer (25) has pointed out that the rise in osmotic values in the leaves of evergreens in the winter is largely due to an increase in soluble sugars. Several attempts have been made to determine whether or not osmotic value gradients exist from the root to the apical portions of the plant. Dixon and Atkins (io) showed that the concentration of sugars in the sap of the