# Diurnal Changes in the Transport Rates of Ureides, Amides, Cations, Anions, and Organic Acids Estimated by Xylem Sap Exudate and the Water Flow Rate of Soybean Plants

## TL;DR

This study examines how soybean plants transport nitrogen compounds and other nutrients through xylem vessels during the day and night.

## Contribution

The study confirms that water flow and nutrient transport rates in soybean plants can be estimated using detached roots and shoots.

## Key findings

- Xylem sap exudation and transpiration rates are higher in light and lower in darkness.
- Nitrogen compound concentrations are high during the day but transport rates are lower at night.
- Night temperatures only affect cation transport, not other xylem constituents.

## Abstract

Ureides; allantoate, allantoin, and amides; asparagine, and glutamine are the N2 fixation products in soybean root nodules, and they are transported through xylem vessels. We estimated the transport rates of xylem constituents by multiplying nutrient concentrations by the water flow rate. Nodulated soybean plants were grown with an N-free solution under either 28 °C day/18 °C night or 28 °C day/28 °C night conditions, and diurnal changes in nutrient concentrations in xylem sap and transpiration rate were determined every 2 h. Under both conditions, xylem sap exudation rate and transpiration rate were high in light, and low, but not zero, in darkness. The sum of the xylem sap exudation rate and transpiration rate from detached shoots was almost the same as the water flow rate of intact plants at any time. All the N compounds exhibited a similar pattern: concentrations were high, but transport rates were lower at night. The proportions of N constituents were constant throughout the day and night. The composition and transport rate of xylem sap were not affected by night temperatures, except for cations. The results confirmed that the water flow rate and transport rate of xylem constituents can be estimated using detached roots and detached shoots.