Organic acid accumulation may inhibit N₂ fixation in phosphorus-stressed lupin nodules

Nodulated lupins (Lupinus angustifolius cv. Wonga) were hydroponically grown under conditions of low phosphate (LP) or adequate phosphate (HP) to assess the effect of phosphoenolpyruvate carboxylase (PEPC)-derived organic acids on nitrogen assimilation in LP nodules. LP conditions are linked to altered organic acid metabolism, by the engagement of PEP metabolism via PEPC. In LP nodules, the enhanced organic acid synthesis may reduce the available organic carbon for nitrogen assimilation. The diversion of carbon between the organic acid- and amino acid pools was assessed through key nodular enzymes and ¹⁴CO₂ metabolism. Under LP conditions, increased rates of organic acid synthesis via PEPC and malate dehydrogenase (MDH), coincided with reduced nitrogen assimilation via aspartate aminotransferase (AAT), aspartate synthetase (AS) and glutamine synthetase (GS)/glutamate synthase (GOGAT) activities. There was a preferential metabolism of nodular ¹⁴CO₂ into organic acids and particularly into malate. High malate levels were associated with reduced N₂ fixation and synthesis of amino acids. These results indicate that phosphorus deficiency can enhance malate synthesis in nodules, but that excessive malate accumulation may inhibit N₂ fixation and nitrogen assimilation.