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 (Phoseolus 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 N-15 labelled by applying (NH4NO3)-N-15 prior to imposing the darkening treatment. Leaf darkening at vegetative growth induced N remobilization as welt 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, white the % N in the whole plant and in various plant parts remained conserved. Directly after leaf labetting, but also at the end of the vegetative growth period, substantial amounts of N-15 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. (c) 2006 Elsevier GmbH. All rights reserved.