Effects of Water Stress on the Respiratory and Nitrogen-fixing Activity of Soybean Root Nodules

Abstract

The aim of this work was to discover whether sugar moved into the cortex and apex of roots of Pisum sativum via the symplasm or the free space. The method was to observe whether flooding the free space of the root with sugar solutions inhibited the movement of ¹⁴C from the stele into the surrounding cells. Seedlings were arranged in a compartmented box so that the free space of the apical 23 mm of the roots could be flooded with sugar, and [₁₄C]glucose could be applied locally to regions of the steles, 36–46 mm from the apices, that had been stripped of cortex. After incubation, the apical 6 mm of the roots were excised, and the regions 6–23 mm from the apex were separated into stele and cortex. The movement of ¹⁴C into this cortex and apex was not reduced by flooding the free space with any of the following: 1.5 mM, 50 mM sucrose; 0.3 mM, 3.0 mM, 100 mM glucose; 3.0 mM fructose. When [¹⁴C]glucosyl sucrose was applied to the steles, the asymmetrical labelling was retained in the sucrose isolated from the apical 6 mm and the cortex. In comparable experiments seedlings were allowed to photosynthesize in ¹⁴CO₂- The presence of 1.5 mM sucrose or 3.0 mM glucose in the free space of the apical 46 mm of the roots of these seedlings did not reduce the movement of label into the cortex and apex. These results are held to be consistent with the symplasmic transport of sugar from the stele into the surrounding cells.