Wednesday, November 15, 2006

Wheat Nitrate Assimilation in Elevated CO2 or Low O2 Atmospheres.

Martin Burger1, Arnold Bloom1, and Asaph Cousins2. (1) University of California, Davis, One Shields Avenue, Davis, CA 95616, (2) The Australian National University, Research School of Biological Sciences, Canberra, ACT0200, Australia

Nitrogen concentrations of C3 species usually decline after prolonged exposure to elevated CO2. This may result from a dependence of nitrate (NO3-) assimilation on photorespiration. To investigate the relationship between these two processes, we assessed NO3- assimilation under modified atmospheres using 15N techniques. Wheat (Triticum aestivum cv. Veery) seedlings at a photosynthetic flux density of 1000 μmol m2  sec-1 were exposed to an ambient atmosphere (360 μmol mol-1 CO2; 21% O2), elevated CO2 (720 μmol mol-1 CO2; 21 % O2), or low O2 (360 μmol mol-1 CO2; 2% O2), and provided with nitrogen as 15N-enriched NO3- during a 12 h uptake period from a nutrient solution. Compared to ambient conditions, shoot nitrate assimilation rates decreased by 20% and 30% under elevated CO2 and low O2, respectively. In an alternative approach, we analyzed the d15N signature of NO3- and total N of wheat seedlings exposed to natural abundance levels of 14N and 15N. Given that the enzyme nitrate reductase discriminates strongly against the heavy isotope, the isotopic signature of organic N indicates the extent to which NO3- had become limited. In the shoots, d15N of organic N was lower, and higher for NO3- , in plants grown under elevated CO2 compared to ambient conditions, indicating that NO3- availability at the site of reduction was more limited under ambient than elevated CO2 concentrations. Both methods thus show that conditions diminishing photorespiration also suppress NO3- assimilation.


Handout (.pdf format, 174.0 kb)