A Study of the Absorption and Utilization of Phosphate by Young Barley Plants

Abstract

The effects of the phosphate status of young barley plants and of respiratory inhibitors on the distribution of phosphate absorbed during experimental periods lasting 24 hours has been studied. The pretreatment of plants with 10 p.p.m. P increases the upward movement of phosphate when the plants are subsequently treated with0·001 p.p.m. P. No such effect occurs when 10 p.p.m. P are supplied. Sodium azide, 2:4-dinitrophenol, and diethyldithiocarbamate reduce the absorption of phosphate when the concentration of the external solution is 10 p.p.m. When, however,0·p.p.m. P is provided, contrasting results are obtained, the most striking feature being that a considerable increase in the content of shoots may be induced. The extent of this effect is variable. A series of experiments with 10 ⁻³ M. azide indicates that the extent to which shoot content is increased depends on the extent to which phosphate is retained in the roots in the absence of the inhibitor. These results suggest that the small amount of absorbed phosphate found in the shoots of plants supplied with low concentrations of phosphate is due to the metabolic retention of phosphate in the roots. It appears that two apparently distinct active processes in roots are subject to inhibition by the three inhibitors employed, namely metabolic retention and the transfer of ions across the root to the stele. The relative extent to which these processes are inhibited depends on the condition of the plants; metabolic retention may be reduced without comparable effects on absorption. It is considered that these results are incompatible with the hypothesis that absorption is directly mediated by the electron transfer in respiration. An alternative mechanism is discussed.