CAN GENOTYPES OF SOYBEAN (GLYCINE-MAX) SELECTED FOR NITRATE TOLERANCEPROVIDE GOOD MODELS FOR STUDYING THE MECHANISM OF NITRATE INHIBITION OF NITROGENASE ACTIVITY

In soybeans (Glycine mar L. Merr.), high levels of soil nitrate inhibi t N-2 fixation, and nitrate-tolerant symbioses have been identified wi thin a chemically mutagenized line of cv. Bragg denoted nts382 and wit hin the line K466, a genotype representative of a number of Korean soy bean cultivars. The genotypes nts382 and K466 were examined to see if they could be used as a model system for studying the mechanism respon sible for the short-term (i.e. 3-day) inhibition of specific nitrogena se activity, especially the mechanism behind the greater O-2 limitatio n of nodule metabolism that is characteristic of nitrate inhibition of N-2 fixation in soybean. In nts382, total nitrogenase activity (TNA = H-2 production in Ar:O-2) was inhibited to a lesser degree (48% of co ntrol) relative to Bragg (30% of control), and the nitrate-treated sym bioses showed less of an O-2 limitation of nodule metabolism in nts382 than in Bragg. However, the relative proportion of O-2 limitation to the total nitrate inhibition was similar (40 and 41%) in nts382 and Br agg, respectively. Therefore, the nts382 symbioses may be useful in el ucidating the general mechanism for down-regulation of nitrogenase act ivity in soybean, but would not be a useful model system for studying the control of O-2-limited metabolism following nitrate exposure. The effects of nitrate on TNA and on the degree of O-2 limitation of nodul e metabolism were the same in K466 and a reference cultivar Maple Arro w. Consequently, the tolerance of K466 to nitrate reported previously was attributed to the ability of this symbiosis to maintain nodule bio mass in the presence of nitrate, not to any ability to maintain specif ic nitrogenase activity in the presence of nitrate.