Nitrogen and carbon costs of soybean and lupin root systems during phosphate starvation

Abstract

Phosphate (P) starvation is one of the most limiting nutrients to N2 fixation in legumes. Soybeans and lupins present different climatic origins, nodule morphologies and metabolic complexities, which may have various adaptive responses to short-term P starvation. Lupins and soybeans were cultivated hydroponically for 3 weeks. Short-term P starvation was induced for 14 days by switching the P-supply to 2 μM P. During P starvation, the lupins showed a lower decline in nodular P concentrations and maintained their biological N2 fixation (BNF), in contrast to the soybeans. The lupins also maintained their photosynthetic rates and the nodular construction and growth respiration costs under P starvation, whilst soybeans showed a decrease in photosynthetic rates and an increase in nodular construction and growth respiration costs under P starvation. There was a also a shift towards more organic acid synthesis, relative to amino acid synthesis in lupin nodules than soybean nodules under P starvation. The lupins had higher amino acid concentrations in their nodules, whilst the soybean nodules maintained their ureide levels at the expense of a decline in amino acids. These results indicate that lupins may to be better adapted to maintaining BNF during short-term P starvation than the soybeans.