Rubisco from two ecotypes of Plantago lanceolata L. that are native to sites differing in atmospheric CO ₂ levels

Rubisco, the most prevalent protein on the planet, initiates the conversion of CO ₂ into carbohydrates during photosynthesis. Responses of this process to atmospheric CO ₂ fluctuations daily, seasonally, and over millennia is still poorly understood. We have hypothesized that higher plants maintain carbon-to-nitrogen homeostasis by adjusting the balance of magnesium and manganese in chloroplasts to alter their relative carbon fixation and nitrogen assimilation rates. The following study examined the influence of magnesium and manganese on rubisco carboxylation and oxygenation in protein purified from two ecotypes of Plantago lanceolata : one adapted to the high atmospheric CO ₂ that occurs near a natural CO ₂ spring and the other adapted to more typical CO ₂ atmospheres that occur nearby. The plastid DNA coding for the large unit of rubisco were similar in both ecotypes. The kinetics of rubiscos from the two ecotypes differed more when they were associated with manganese than magnesium. Specificity for CO ₂ over O ₂ ( S _c/o ) for rubiscos from both ecotypes were higher when the enzymes were bound to magnesium than manganese. This disparity may account for the adaptation of this species to different CO ₂ environments.