Nitrogenase activity and N fixation are stimulated by elevated CO in a tropical N-fixing tree.

Seeds of Gliricidia sepium, a fast-growing woody legume native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in environmentally controlled glasshouses for 67-71 days under ambient CO (35 Pa) and elevated CO (70 Pa) conditions. Seedlings were watered with an N-free, but otherwise complete, nutrient solution such that bacterial N fixation was the only source of N available to the plant. The primary objective of our study was to quantify the effect of CO enrichment on the kinetics of photosynthate transport to nodules and determine its subsequent effect on N fixation. Photosynthetic rates and carbon storage in leaves were higher in elevated CO plants indicating that more carbon was available for transport to nodules. A CO pulse-chase experiment demonstrated that photosynthetically fixed carbon was supplied by leaves to nodules at a faster rate when plants were grown in elevated CO. Greater rates of carbon supply to nodules did not affect nodule mass per plant, but did increase specific nitrogenase activity (SNA) and total nitrogenase activity (TNA) resulting in greater N fixation. In fact, a 23% increase in the rate of carbon supplied to nodules coincided with a 23% increase in SNA for plants grown in elevated CO, suggesting a direct correlation between carbon supply and nitrogenase activity. The improvement in plant N status produced much larger plants when grown in elevated CO. These results suggest that Gliricidia, and possibly other N-fixing trees, may show an early and positive growth response to elevated CO, even in severely N-deficient soils, due to increased nitrogenase activity.