The Effect of an 15NH4 15NO3 Fertilization at Flowering on Growth and Nitrogen Fixation of White and Blue Lupins

Abstract

It is well established that legume species differ in intensity and duratio of nitrogen fixation during pod-filling. This was studied on new lines of white and blue lupins by comparing a solely nitrogen fixing and a treatment supplied with 15N-labelled fertizlier at flowering in pot experiments. Nitrogen application at flowering markedly incresed seed and total dry matter and seed and total N yield in white lumpins but remained without effect in blue lupins. simply replacing nitrogen fixation. Nitrogen fixation virtually ceased with the onset of pod-filling in white lupins, causing a massive N-remobilization from vegetative plant parts. In contrast, the blue lupins fixed about 70% of total nitrogen fixation after flowering and displayed growth and net-N-assimilation of the vegetative plant parts. In both species the stems and pods especially supported seed N-filling, indicated by a low% N and a high C/N ratio at ripeness. It is concluded that pod-filling proved to be a critical stage for the seed yield in grain legumes. Exploiting differences in nitrogen fixation in this growth stage, e.g., by breeding, needs more precise knowledge about the regulation mechanisms and source-sink relations.