NITROGEN ASSIMILATION BY ANNUAL LUPINES (LUPINUS: FABACEAE) IN THE GRASSLANDS OF COASTAL CALIFORNIA (NITROGENASE, NITRATE REDUCTASE, UPTAKE, FIXATION)

Legumes growing in fertile agricultural soils derive a large part of their nitrogen requirement from the soil. Despite the ability of legumes to form dinitrogen fixing symbiotic root nodules, plants such as soybeans take up and reduce large amounts of nitrate. The relative contribution of symbiotic dinitrogen fixation and nitrate reduction to the nitrogen nutrition of native legumes growing in less fertile soils is unknown.; Plants of Lupinus nanus (growing at Jasper Ridge, Valley Grassland community) and of L. bicolor (growing at Bodega Head, Coastal Prairie community) were sampled throughout the winter and spring growing season and assayed for root nodule nitrogenase activity, and for nitrate reductase activity in both leaves and roots. Nitrogenase activity was estimated by the acetylene reduction assay, on excised nodules. Nitrate reductase was assayed by the in vivo method. Soil nitrate and nitrification potential (by aerobic incubation) were measured as was soil water status. Increase in total plant nitrogen was followed.; Laboratory experiments on growth chamber-grown plants were used to assess the effect of environmental factors on nitrogen fixation, nitrate uptake and nitrate reduction.; Annual lupines used both nitrate reduction and nitrogen fixation as nitrogen sources. The contribution of soil nitrate was small (approx. 7%), but came early in the growing season, prior to active nodule formation. As soil nitrate levels decreased, lupine nitrate reduction also decreased and eventually disappeared, even though nitrification activity remained high. Lab experiments indicated that the absence of nitrate reductase activity was due to the inability of lupines to compete with other species for the uptake of soil nitrate, due to a lower affinity uptake system.; Nitrate use by native lupines was opportunistic. When nitrate levels were high and competition low, nitrate reductase was induced. Nodulation was more programmed and was not inhibited by high soil nitrate. This had adaptive value, since the growth of nodules was essential for the continued nitrogen assimilation of lupines later in the season, when nitrate was no longer available.