Fig. 5.

A conceptual ecophysiological framework for the analysis of nitrogen nutrition in legumes. Potential N supply is the sum of utilization of seed N reserves and N uptake by symbiotic N₂ fixation and root nitrate absorption. The amount of N₂ fixed is the product of nodule dry matter by their efficiency while the amount of nitrate accumulated is the product of root dry matter by their efficiency, which depends on nitrate availability in the soil and on the ability of the plant to retrieve it (as partly determined by rooting depth). N demand is mainly determined by variation in shoot dry matter. The N concentration of the shoot results from the balance between N supply and N demand. The N status can be diagnosed using the N dilution principle. In turn, it modulates photosynthetic ability of the plant. Part of the C assimilates is allocated to roots and nodules, thus allowing potential N supply to be modulated by varying their dry matter and/or their efficiency. N compounds circulating in the phloem are presumably involved in the signalling processes leading to whole plant N feedback regulation of N uptake. Using genetic correlations, it was shown that C costs induced by nodule synthesis are detrimental to their functioning and also to shoot and root growth. Carbon costs induced by root synthesis are presumably detrimental to their functioning at the beginning of the growth cycle only.