NITROGEN ASSIMILATION AND TRANSPORT IN CAROB PLANTS

Most of the nitrate reductase activity (80%) in carob (Ceratonia siliq ua L. cv. Mulata) is localised in the roots. The nitrate concentration in the leaves is relatively low compared to that in the roots, sugges ting that nitrate influx into the leaf may be a major factor limiting the levels of nitrate reductase in the shoot. Transport of nitrate fro m root to shoot appears limited by the entrance of nitrate into the xy lem. In order to study this problem, we determined the nitrate concent rations and nitrate reductase activities along the roots of nitrate-gr own plants, as well as the composition of the xy]em sap and the nitrat e levels in the leaves. Some of the plants were also supplied with nit rate directly through the stem or through the hypocotyl, in order to b ypass the lending of nitrate into the xylem of the roots. The results show that the loading of nitrate into the xylem is a limiting step. Th e cation and anion concentrations of nitrate- and ammonium-fed plants were similar, showing almost no production of organic anions. In both nitrate- and ammonium-fed plants, the transport of nitrogen from root to shoot was in the form of organic nitrogen compounds. The nitrate re ductase activity in the roots was more than sufficient to explain all the efflux of OH- into the root medium of nitrate-fed plants. In carob plants the K-shuttle may thus be operative to a limited extent only, corresponding to between 11 and 27% of the nitrate taken up. Potassium seems to be the cation accompanying stored nitrate in the roots of ca rob seedlings, since they accumulate nearly stoichiometric amounts of K+ and NO3-.