Relation between Root Respiration and Absorption.

The rate of absorption of water by roots of transpiring plants is in general closely dependent upon the rate of transpiration, and a change in the rate of transpiration is soon followed by a corresponding change in the rate of absorption (3). Since the rate of absorption lags behind and apparently is dependent on the rate of transpiration it is usually considered that under conditions such that transpiration exceeds absorption the roots play a passive role in the absorption of water. It has been reported by certain workers, however, that a close relation exists between root respiration and water absorption. NEWTON (6) observed that the rate of root respiration was increased when the concentration of the culture solution in which the roots were immersed was increased. He suggested that this was caused by the greater expenditure of energy required to absorb a given volume of water from a more concentrated solution. HENDERSON (1) reported that there was a direct relation between respiration and water absorption in the roots of transpiring corn seedlings and concluded that the absorption of water was brought about by the expenditure of energy released in respiration. LOWENECK (5), on the other hand, found no relation between root respiration and rate of transpiration of Zea maize or Sonchus oleraceus. In view of these differences in opinion it seemed desirable to make a further study of the problem, using large plants. Tomato seeds of the Marglobe variety were germinated in moistened quartz sand and the seedlings were transferred to solution cultures. The plants were grown in an unshaded greenhouse until approximately two months old. The most vigorous plants were then selected and sealed at the level of the crown into 250 ml. potometers. Four potometers were placed in a constant-temperature water bath and were connected to a common source of aerated tap water. The oxygen content of the water in the potometers was determined by the Winkler method, determinations being made before and after each hour exposure of the plants to differing environmental conditions. Since all four potometers were connected to the same source of aerated water, the four root systems were under similar conditions, as indicated by the fact that the determinations of oxygen content at the beginning of each experimental period were always in close agreement. Nearly 100 determinations of the absorption of oxygen and water were made under conditions producing widely differing rates of transpiration and hence of water absorption. The results are presented in table I.