Utilization of Net Photosynthate for Nitrogen Fixation and Protein Production in an Annual Legume

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Abstract
The economy of C and N in nodulated [Rhizobium] cowpea (Vigna unguiculata [L.] Walp.) was described in terms of fixation of CO2 and N2, respiratory losses of C and the production of dry matter and protein. Net daytime gain of C by the shoot (net photosynthesis) rose to a maximum at flowering and then declined sharply due to abscission of leaves. Maximum N fixation occurred 10 days prior to maximum net photosynthesis. Shedding of nodules reduced fixation to zero by mid-fruiting. Fifty per cent of the plant''s N and 37% of its net photosynthate were assimilated before flowering; 39% of plant N was incorporated into seed dry matter. Respiration of nodules and roots utilized 24% of the C from net photosynthate assimilated over the growth cycle; night respiration of shoots, 20%; dry matter production in seeds, 17%; and dry matter production in other plant parts, 39%. The proportion of net photosynthate translocated to the nodulated root decreased from 41-14% during growth. Developing fruits were major competitors for translocate. Nodules consumed 9% of the C from the plant''s total net photosynthate, 43% of which was respired, 6% made into dry matter and 51% returned to the shoot with N fixation products. For every 1 g N fixed, net photosynthate equivalent to 6.8 g carbohydrate was consumed by nodules, 25.7 g carbohydrate by the nodulated root. Translocate was used most efficiently for N fixation in late vegetative growth when nodules were most active and their carbohydrate supply still adequate. During vegetative growth and early flowering (0-78 days after sowing) cowpea consumed 17.2 g net photosynthate (as carbohydrate) for every gram of protein synthesized in its shoot. The comparable conversion in seed production was 32.5 g net photosynthate/g seed protein or 6.6 g/g seed dry matter.