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Environmental and Stress Physiology

Effect of Phloem-Translocated Malate on NO₃^– Uptake by Roots of Intact Soybean Plants ¹

Biochimie et Physiologie Végétales, Institut National de la Recherche Agronomique, Ecole Nationale Supérieure d'Agronomie, Centre National de la Recherche Scientifique URA 573, 34060 Montpellier Cedex 1, France

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 WO₄^2–. 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.

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