Absorption and Xylem Transport of 33 P-Labeled Phosphorus in Nodulated Soybean Plants

Phosphorus (P) is an essential macro-element for plants, and understanding the characteristics of the absorption and transport of P in crops is essential. The low availability of P restricts the growth, nitrogen fixation, and yield of soybean plants. In this research, the radioisotope 33 P was supplied to the culture solution to trace the absorption and transport of P in nodulated soybean plants monitored using an imaging plate. The absorption rate of 33 P was almost the same under the light and dark conditions. The absorption rate of 33 P in the decapitated roots was near to that of the intact plants under light. These results indicate that the P absorption is not affected by evapotranspiration over a short time period. Conversely, the 33 P transport from the roots to the shoot was significantly lower under dark conditions than it was under light conditions, although some 33 P reached the top of the shoots under both the light and dark conditions. The transport of P to the shoots depends on the transpiration supplemented by the root pressure. The multiplication value of the 33 P concentration in the xylem sap and transpiration rate was almost equivalent to the transport rate of 33 P in the intact shoots. This value may be adaptable and used to estimate the transport rate of P for the diagnosis.