NITROGENASE ACTIVITY, NODULATION AND SEED PRODUCTION IN VICIA-FABA ASAFFECTED BY METHABENZTHIAZURON

The effects of recommended application rates of the herbicide methaben zthiazuron (MET) (2.5 and 4 kg ha(-1)) on vegetative growth, nodulatio n and N-2 fixation in faba bean plants were studied in the held at Bar celona during 1990 and 1991. Nodulation, specific nitrogenase activity (SNA) of excised nodules, dry weight of plants and seed nitrogen cont ent at harvest were compared in MET-treated, hand-weeded (HW) and cont rol plants. In 1990 the higher MET application (4 kg ha(-1)) caused a decrease in nodulation and acetylene reduction activity (ARA) per plan t during the mid-period of pod-fill, and also in vegetative growth, se ed production and N content of seeds, whereas the lower rate (2.5 kg h a(-1)) did not have a significant effect on any of these factors. Howe ver, in 1991, growth and herbicide effects were different from those i n the previous year. No inhibitory effect of MBT on nodulation, SNA or ARA per plant was observed and vegetative growth, seed production and N content of seeds were not significantly different from those of pla nts from HW plots. In 1991, both 4 kg ha(-1) of MBT and the HW treatme nt caused a stimulatory effect on seed N recovery and yield compared t o control plants. Rates of ARA per plant at late pod-fill were positiv ely related to N recovery by seeds. These differences could have been due to the low rainfall in 1990, which may have permitted MBT to remai n in the soil for a longer period and to inhibit N fixation and plant growth in the Vicia faba crop.