Regulation of nodulation and nodule mass in relation to nitrogenase activity and nitrogen demand in Discaria trinervis (Rhamnaceae) seedlings

A time course experiment of nodulation, N-2 fixation and growth was conduct ed on Discaria trinervis seedlings after inoculation with Frankia isolate B CU110501, to study the regulation of nodulation in relation to nitrogenase activity. Nodulation started at 1.3 weeks after inoculation and reached a p lateau at 7 weeks after inoculation. Nodules were mainly restricted to tap roots. Nitrogenase activity was first detected at 3.6 weeks after inoculati on and increased throughout the 9-week experimental period. No H-2 evolutio n could be detected, thus the symbiosis appeared to be phenotypically Hup(). Plant growth increased after onset of nitrogenase activity, except for r oot length growth. Nitrogenase activity expressed on either plant dry matte r basis, nodule dry matter basis, or leaf area basis, did not change signif icantly during the study. N content of all plant parts. increased to a maxi mal value at 7 weeks after inoculation At this time formation of new nodule s stepped and nodule dry mass relative to total plant dry mass did no longe r increase. The results support the idea of a feedback regulation of nodule growth and activity through internal N level.