title: Nitrogenase activity and N2fixation are stimulated by elevated CO2in a tropical N2-fixing tree.
abstract: Seeds of Gliricidia sepium, a fast-growing woody legume native to seasonal tropical forests of Central America, were inoculated with N2-fixing Rhizobium bacteria and grown in environmentally controlled glasshouses for 67-71 days under ambient CO2(35 Pa) and elevated CO2(70 Pa) conditions. Seedlings were watered with an N-free, but otherwise complete, nutrient solution such that bacterial N2fixation was the only source of N available to the plant. The primary objective of our study was to quantify the effect of CO2enrichment on the kinetics of photosynthate transport to nodules and determine its subsequent effect on N2fixation. Photosynthetic rates and carbon storage in leaves were higher in elevated CO2plants indicating that more carbon was available for transport to nodules. A14CO2pulse-chase experiment demonstrated that photosynthetically fixed carbon was supplied by leaves to nodules at a faster rate when plants were grown in elevated CO2. 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 N2fixation. 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 CO2, suggesting a direct correlation between carbon supply and nitrogenase activity. The improvement in plant N status produced much larger plants when grown in elevated CO2. These results suggest that Gliricidia, and possibly other N2-fixing trees, may show an early and positive growth response to elevated CO2, even in severely N-deficient soils, due to increased nitrogenase activity.
authors.affiliation: Department of Biology, Texas Tech University, Lubbock, TX 79409-3131, USA fax: (806) 742-2963; e-mail: cmdtt@ttacs.ttu.edu, , , , , , US.
authors.forename: D T
authors.initials: DT
authors.lastname: Tissue
authors.affiliation: Department of Biology, George Mason University, Fairfax, VA 22030-4444, USA, , , , , , US.
authors.forename: J Patrick
authors.initials: JP
authors.lastname: Megonigal
authors.affiliation: Department of Biology, West Virginia University, Morgantown, WV 26506-6057, USA, , , , , , US.
authors.forename: Richard B
authors.initials: RB
authors.lastname: Thomas
keywords: Gliricidia sepium; Key words Elevated CO2; N2 fixation; Nitrogenase activity; Tropical tree