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ABSTRACT Rubisco, the most prevalent protein on Earth, catalyzes both a reaction that initiates C₃ carbon fixation, and a reaction that initiates photorespiration, which stimulates protein synthesis. Regulation of the balance between these reactions under atmospheric CO₂ fluctuations remains poorly understood. We have hypothesized that vascular plants maintain organic carbon‐to‐nitrogen homeostasis by adjusting the relative activities of magnesium and manganese in chloroplasts to balance carbon fixation and nitrate assimilation rates. The following examined the influence of magnesium and manganese on carboxylation and oxygenation for rubisco purified from two ecotypes of Plantago lanceolataL.: one adapted to the elevated CO₂ atmospheres that occur near a natural CO₂ spring and the other adapted to more typical CO₂ atmospheres that occur nearby. The plastid DNA coding for the large unit of rubisco was similar in both ecotypes. The kinetics of rubiscos from the two ecotypes differed more when associated with manganese than magnesium. Specificity for CO₂over O₂ (S_c/o) for rubisco from both ecotypes was higher when the enzymes were bound to magnesium than manganese. Differences in the responses of rubisco from P. lanceolata to the metals may account for the adaptation of this species to different CO₂ environments.