SYMBIOSIS (2009) 48, xx–xx ©2009 Balaban, Philadelphia/Rehovot ISSN 0334-5114 Nitrogen and carbon costs of soybean and lupin root systems during phosphate starvation M.R. Le Roux 1 , S. Khan 2 , and A.J. Valentine 1,3* 1 Plant Metabolomics Group, Department of Biotechnology, University of the Western Cape, Private Bag X17, Belleville 7535, South Africa; 2 Department of Health Sciences, Faculty of Health and Wellness Science, Cape Peninsula University of Technology, P.O. Box 652, Cape Town 8000, South Africa; 3 Plant Biology Division, Samuel Roberts Noble Foundation, P.O. Box 2180, Ardmore, OK 73401, USA, Tel. +1-580-224-6739, Fax. +1-580-224-6692, Email. alexvalentine@mac.com (Received June 1, 2008; Accepted November 17, 2008) Abstract Phosphate (P) starvation is one of the most limiting nutrients to N 2 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 mM P. During P starvation, the lupins showed a lower decline in nodular P concentrations and maintained their biological N 2 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. Keywords: ?? 1. Introduction P limitation is one of the most notable environmental constraints for legumes (Jakobsen, 1985; Israel, 1987; Høgh-Jensen et al., 2002). The high sensitivity of the N 2 fixation process to environmental conditions, may be attributed to the C costs (Mengel, 1994). Legumes relying on N 2 fixation require more P than when N is acquired from soil mineral N (Sa and Israel, 1991; Ribet and Drevon, 1995; Al Niemi et al., 1997, 1998; Tang et al., 2001). The high requirement of P may be linked to its role in nodule carbon and energy metabolism, with at least the plant cell fraction being energy limited under low P supply (Sa and Israel, 1991). * The author to whom correspondence should be sent. Presented at the 15th International Congress on Nitrogen Fixation, January 21–26, 2007, Cape Town, South Africa The effect of P starvation on N 2 fixation and nodule O 2 permeability has been demonstrated for both amide and ureide exporting nodule types (Ribet and Drevon, 1995; Drevon and Hartwig, 1997; Schulze and Drevon, 2001). Most temperate legumes (e.g. lupin, pea, clover) translocate fixed N as amides, notably asparagine and glutamine (Streeter, 1991), whilst tropical legumes (e.g. soybean, cowpea, common bean) export ureides, most commonly allantoin and allantoic acid. Lupins and soybeans also differ in other means, such as being of temperate and tropical origin and having morphologically distinct nodules. However, as noted by Streeter (1991), the two different sets of metabolic capabilities of amino acid and ureide exporting legumes, present a metabolic complexity that is perhaps unsurpassed by some other more typical plant systems. Although the ureide exporters require several more enzymes for ureide biosynthesis, compared to only a few