Potential for Using Plant Xylem Sap to Evaluate Inorganic Nutrient Availability in Soil: I. Influence of Inorganic Nutrients Present in the Rhizosphere on Those in the Xylem Sap of Luffa cylindrica Roem.

To examine the availability of nutrients in soil, chemical extraction methods are commonly used. However, such methods require different extractant solutions depending on the types of soil and nutrients being assayed, making it difficult to assay multiple components simultaneously and compare analytical values. We proposed an alternative method, consisting of the collection of xylem sap exuded from plant stems by employing plant roots as a tool for evaluating the levels of available nutrients in soil. We designated this method as the “xylem sap method.” We tested the effectiveness of this method by growing Luffa cylindrica Roem. seedlings for pre-determined periods of time in pots containing culture medium, removing the aerial parts, and collecting the xylem sap from the cut ends. In a sand culture medium, in which all the nutrients provided were considered to be present in an instantly available form, the concentrations of the nutrients in the xylem sap were found to be proportional to those of the nutrients in the culture solution supplied to the medium, confirming that this technique gives a good index of nutrient availability in the rhizosphere. Furthermore, the contents of the nutrients in the xylem sap were proportional to those of the nutrients accumulated in shoots during the growth period. This shows that, at the seedling stage, nutrient uptake by the aerial parts of L. cylindrica during the growth period exactly mirrors the contents in the xylem sap. It was further confirmed that the nutrient contents in the xylem sap were similarly representative of the nutritional status of the plant. However, in a soilbased culture medium, the increase in the total amount of phosphorus and micronutrients in the medium was not reflected in the xylem sap. This was in agreement with existing findings that concentrations of certain specific nutrients are maintained at a fairly steady level in the plant, irrespective of their availability in soluble form in soil. Thus, we concluded that changes in most nutrient concentrations in the xylem sap corresponded to changes in the behavior and availability of nutrients in soil and that the xylem sap method could be used as an accurate indicator of the availability of most nutrients in soil.