EFFECTS OF THE SOURCE OF INORGANIC NITROGEN ON C AND N INTERACTION INMAIZE CALLUS-TISSUE - PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITY, CYTOSOLIC PH AND N-15 AMINO-ACIDS

The effect of N-source on the interaction between carbon and nitrogen metabolism was evaluated by measuring phosphoenolpyruvate carboxylase (PEPcase; EC 4.1.1.31) activity in callus tissue of maize (Zea mays L. cv. Prisma) sub-cultured under different N-nutrition conditions: nitr ate, ammonium or combinations of both. By comparison with the conditio n where both salts were supplied (control), nitrate as the sole N-sour ce led to an increase in PEPcase activity. Ammonium alone gave a drast ic decrease of tissue growth. Extracts from calli grown on equivalent media supplied with N-15-nitrate or N-15-ammonium were analysed by N-1 5-NMR. The labelling of amino acids in the NMR spectra showed that whe n (NO3-)-N-15 was the unique N-source, N-15 mainly accumulated in N de lta Gln, Glu and Ala. With (NH4+)-N-15 only the N delta Gln and gamma- aminobutyric acid were labelled. The addition of both gave rise to lab elled Gln, Asn, Glu, Asp, Ala, Val and gamma-aminobutyric acid indepen dently of the origin of the label. In vivo P-31-NMR allowed the cytopl asmic and vacuolar pH to be measured. The cytoplasmic pH showed an inc rease of approximately 0.3 units when nitrate was the sole source of n itrogen and a corresponding decrease when ammonium was added alone. Va cuolar pH decreased in both treatments. These results are discussed on the basis of the effect of the N-source on carbon metabolism. A hypot hesis of PEPcase activation as due to the increase of cytoplasmic pH u pon nitrate uptake is proposed.