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Alfalfa medicago sativa root nodule carbon di oxide fixation



Alfalfa medicago sativa root nodule carbon di oxide fixation



Plant Physiology 72(2): 469-473



In vivo CO2 fixation activity and in vitro phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity were demonstrated in effective and ineffective nodules of afalfalfa (M. sativa L.) and in the nodules of 4 other legume species. Phosphoenolpyruvate carboxylase activity was greatly reduced in nodules from both host and bacterially conditioned ineffective alfalfa nodules as compared to effective alfalfa nodules. Forage harvest and nitrate application reduced both in vivo and in vitro CO2 fixation activity. By day 11, forage harvest resulted in a 42% decline in in vitro nodule phosphoenolpyruvate carboxylase activity while treatment with either 40 or 80 Kg N/ha reduced activity by 65%. In vitro specific activity of phosphoenolpyruvate carboxylase and glutamate synthase were positively correlated with each other and both were positively correlated with acetylene reduction activity. The distribution of radioactivity in the nodules of control plants (unharvested, 0 kg N/ha) averaged 73% into the organic acid and 27% into the amino acid fraction. in nodules from harvested plants treated with nitrate, near equal distribution of radioactivity was observed in the organic acid (52%) and amino acid (48%) fractions by day 8. Recovery to control distribution occurred only in those nodules whose in vitro phosphoenolpyruvate carboxylase activity recovered. CO2 fixation is correlated with N fixation in alfalfa nodules. The maximum rate of CO2 fixation for attached and detached alfalfa nodules at low CO2 concentrations (0.13-0.38% CO2)were 18.3 and 4.9 nmol/h per .cntdot. mg-1 dry weight, respectively. Nodule CO2 fixation was estimated to provide 25% of the C required for assimilation of symbiotically fixed N in alfalfa.

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