Effects of applied nitrogen upon aspects of nitrogen metabolism and transport in developing nodulated winged bean plants

Nodulated winged bean Psophocarpus tetragonolobus (L.) DC., cv. UPS 122] were grown under constant environmental conditions and supplied with mineral nutrient solution in which nitrogen was absent or was present as nitrate (12 mg N week^-1 plant^-1). Nitrate treatment dramatically promoted plant growth, increased fruit weight 1.6 fold, was necessary for tuberisation and enhanced nodulation. The in vitro accumulation of ¹⁴C into asparagine and aspartate components of excised nodules supplied with exogenous ¹⁴CO₂ and ¹⁴C]-D-glucose was greater for nitrate-treated plants, whilst accumulation into ureides was reduced by nitrate treatment. Levels of amino acids in xylem sap were greater for plants supplied with a complete nutrient solution, than those grown without applied nitrate, particularly for asparagine, glutamine and proline. Xylem ureide levels were greater for plants grown in the absence of supplementary nitrate. Nitrogen accumulated in leaf, stem and petiole, and root nodule tissues for utilisation during fruit development; peak nitrogen levels and time of anthesis were retarded for plants grown without applied nitrate. The shoot ureide content increased during fruiting, coincident with decreases in the total nitrogen content, indicating that ureide pools are not utilised during the early reproductive phase. However ureide reserves, particularly allantoin, were utilised during the later stages of pod fill. Enzyme activity which metabolised asparagine was found throughout the plant and was identified as K⁺-dependent asparaginase (EC 3.5.1.1) and an aminotransferase. Apart from temporal differences in developmental profiles of enzyme activity, the activity of these enzymes and of allantoinase (EC 3.5.2.5) in developing tissues were similar for both treatments. The main differences were greater asparaginase and asparagine:pyruvate aminotransferase activities in root tissues and fruit of nitrate-supplied plants; allantoinase activity in the primary roots of plants grown without nitrate decreased during development, whilst activity in developing tubers (nitrate-supplied plants) increased.