Abstract

Experiments to develop indices of N2 fixation activity for the narrow-leafed lupin (Lupinus angustifolius L.) are reported. In Experiment 1 Unicrop narrow-leafed lupins were inoculated at sowing with effective Rhizobium lupini WU425. The conversion factor relating C2H2 reduction to N2 fixation was not constant throughout growth but increased from 0.9 (50-60 day period) to 6.6 (110-120 days). A nodulation index [(nodule wt/shoot wt) x 100], developed to account for plant size, declined with increasing nitrate supply and with increasing plant age. However, minor shifts in the nodulation index represented large shifts in plant dependence on N2 fixation (p), when plants were 60-100% dependent on N2 fixation. Concentrations of nitrate in extracts of the shoot axes and nodulated roots increased with increasing nitrate supply. Although the presence of nitrate in the rooting medium was evidenced by its presence in xylem exudate, the relationship between nitrate supply and the nitrate contents of xylem exudate was generally poor. Functions were developed to describe the relationships between the nodulation index and p, and between both shoot and root nitrate and p. Other experiments highlighted the synchrony of nitrate supply and its appearance in root tissues and the lack of diurnal fluctuations in nitrate concentrations of plant parts.