EFFECT OF PHLOEM-TRANSLOCATED MALATE ON NO3- UPTAKE BY ROOTS OF INTACT SOYBEAN PLANTS

In soybean (Glycine max L. Merr. cv Kingsoy), NO3- assimilation in leaves resulted in production and transport of malate to roots (B Touraine, N Grignon, C Grignon [1988] Plant Physiol 88: 605-612). This paper examines the significance of this phenomenon for the control of NO3- uptake by roots. The net NO3- uptake rate by roots of soybean plants was stimulated by the addition of K-malate to the external solution. It was decreased when phloem translocation was interrupted by hypocotyl girdling, and partially restored by malate addition to the medium, whereas glucose was ineffective. Introduction of K-malate into the transpiration stream using a split root system resulted in an enrichment of the phloem sap translocated back to the roots. This treatment resulted in an increase in both NO3- uptake and C excretion rates by roots. These results suggest that NO3- uptake by roots is dependent on the availability of shoot-borne, phloem-translocated malate. Shoot-to-root transport of malate stimulated NO3- uptake, and excretion of HCO3-ions was probably released by malate decarboxylation. NO3-uptake rate increased when the supply of NO3- to the shoot was increased, and decreased when the activity of nitrate reductase in the shoot was inhibited by WO42-. We conclude that in situ, NO3-reduction rate in the shoot may control NO3- uptake rate in the roots via the translocation rate of malate in the phloem.