Nitrogen from senescing lower leaves of common bean is re-translocated to nodules and might be involved in a N-feedback regulation of nitrogen fixation.

The objective of the present study was to elucidate whether remobilized N from lower leaves is involved in causing the drop in N(2) fixation during pod-filling in common bean (Phaseolus vulgaris L). Moreover, we addressed the question of whether remobilized N from lower leaves would reach the nodules. Nodulated common bean plants were grown in a growth chamber in quartz sand. During a 2-week period, at vegetative and at reproductive growth, 50% of the leaves (lower part) were either excised or individually darkened, thereby removing the same photosynthetic capacity yet allowing N to be remobilized from the darkened leaves. Moreover, at the vegetative growth period, three lower leaves per plants were (15)N labelled by applying (15)NH(4)NO(3) prior to imposing the darkening treatment. Leaf darkening at vegetative growth induced N remobilization as well as reduced N(2)-fixation rates and growth. Leaf excision at reproductive growth enhanced N(2) fixation. Changes in N(2)-fixation rates were in all cases the result of altered growth rates, while the % N in the whole plant and in various plant parts remained conserved. Directly after leaf labelling, but also at the end of the vegetative growth period, substantial amounts of (15)N from the leaves could be recovered in nodules in the control, and in higher amounts in the leaf-darkening treatment. It is proposed that nitrogen from leaves circulates within the plant via nodules, and that the strength or composition of this circular flow may be the signal for a putative N-feedback effect.