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SUMMARY

Nitrogenase activity of the root nodules of field-grown Vicia faba increased during plant growth until 3–4 weeks after the onset of flowering. The subsequent decline in activity was never greater than to 20% of the maximum, until finally plants were killed by frost. Nodules of medium size (100–200 mg fresh weight) showed greatest nitrogenase activity.

After photosynthesis in 14CO2 for 30 min, maximum accumulation of 14C-assimilates in the nodules occurred within 90 min of synthesis. The relatively low levels of radioactivity associated with the organic acid fraction of the nodules, the absence of sucrose which is present in large quantities in the roots and the appearance of radioactivity in the nodule amino acids within 30 min of feeding the shoot 14CO2 show that photosynthates are rapidly metabolized on arrival in the nodules. The distribution of radioactivity among the constituents of the basic fraction confirms the importance of glutamate, glutamine and aspartate as early products of the metabolism of the fixed nitrogen in broad bean nodules receiving ample supplies of new photosynthates.