Young bean plants (Phaseolus vulgaris L. var Saxa) were fed with three different types of inorganic nitrogen, after being grown on nitrogen-free nutrient solution for 8 days. The pattern of "CO2 fixation was investigated in photosynthesizing primary leaf discs of 11-day-old plants (3 days with nitrogen source) and in a pulse-chase experiment in 13-day-old plants (5 days with nitrogen source).Ammonium caused, in contrast to nitrate nutrition, a higher level of 14C incorporation into sugar phosphates but a lower incorporation of label into malate, glycolate, glycerate, aspartate, and alanine. The labeling kinetics of glycine and serine were little changed by the nitrogen source. Ammonium feeding also produced an increase in the ratio of extractable activities of ribulose-1,5-bisphosphate carboxylase to phosphoenolpyruvate carboxylase and an increase in dark respiration and the CO2 compensation concentration. Net photosynthesis was higher in plants assimilating nitrate.The results point to stimulated turnover of the photosynthetic carbon reduction cycle metabolites, reduced phosphoenolpyruvate carboxylation, and altered turnover rates within the photosynthetic carbon oxidation cycle in ammonium-fed plants. Mechanisms of the regulation of primary carbon metabolism are proposed and discussed.The effect of NH4' on photosynthetic carbon assimilation has been repeatedly reported (3, 8, 12, 13, 16, 17, 19-23, 27, 29) a decrease in the concentration of malate and an inhibition of malic enzyme activity (2). However, the supply of N03-to cells of Spinacia oleracea had no apparent effect on the distribution of photosynthetic products compared with N-free nutrition, whereas in NH4t-fed cells, fixation of CO2 into carbohydrates decreased but increased in carboxylic and amino acids (16).The effect of inorganic N sources on the composition of N compounds in the bleeding sap and leaves of the bushbean Phaseolus vulgaris has previously been reported (6, 27). Coupled with the above observations, this has led us to investigate the effect of different N sources on various carboxylating enzymes and on the pattern of "4CO2 fixation in primary leaves of nonnodulated bean plants grown in liquid culture. Our results show that different N sources, fed through the roots of Phaseolus vulgaris, cause changes in photosynthetic carbon metabolism.
MATERIALS AND METHODSPlant Material. Seeds of Plaseolus vulgaris L. var Saxa (strain Vatter) were soaked overnight in tap water, rinsed, and placed on wet tissue paper in darkness for 3 d at 24 ± 2°C. Seedlings were transferred to coarse quartz sand, watered with N-free nutrient (27), and grown under an illumination of 7000 to 9000 lux. After a further 4 d (day 8), the plants were set into plastic tanks filled with 14 L nutrient solution containing a total N concentration of 3.5 mm either as NH4', or NH4NO3 (27). The pH of the nutrient solution was adjusted with either KOH or H3PO4. All subsequent investigations were carried out on primary leaves. "4C02/'2C02 Pulse-Chase ExpeAments. After...