The economy of C and N in nodulated cowpea ( Vigna unguiculata (L.) Walp.) was described in terms of fixation of CO 2 and N 2 , 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 midfruiting. 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 to 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 to 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.
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