Wheat (Triticum aestivum L. cv Minaret) was grown long-term under CO<sub>2</sub> partial pressures of 36 and 70 or 100 Pa with various N applications (4 to 23 g m-2 N), to test hypotheses of N re-allocation: 1) a decrease in N from leaves to other organs, 2) a relative decrease in N from Rubisco to other photosynthetic components. Elevated CO<sub>2</sub> did not affect phenology, main stem leaf appearance, the pattern of N allocation throughout the plant, or the fraction of crop N in grain at harvest, but 1) stimulated biomass and yield by 5 to 20% over the N range used, and 2) caused a faster loss of N and components from flag leaves during grain-fill. Responses of photosynthesis to varying pCi were fitted, and rates of maximal carboxylation and non-photorespiratory respiration estimated. The former, was proportional to Rubisco content, and light-saturated photosynthetic rate at 70 Pa CO<sub>2</sub> was proportional to A TP-synthase. Potential photosynthetic rates at 70 Pa CO<sub>2</sub> were calculated, compared with observed, and used to estimate excess investment in Rubisco. The excess was greater in high N treatments than low, declining as leaves senesced. The fraction of Rubisco estimated to be in excess, was strongly dependent on leaf N content, increasing from - 5% in leaves with 1 g N m-2 to -40% in leaves with 2 g N m-2. Growth at elevated CO<sub>2</sub> usually decreased the excess somewhat, but only as a consequence of a general decrease in leaf N, given that relationships of components to leaf N content were independent of CO<sub>2</sub> and N treatment, demonstrating that no direct CO<sub>2</sub> effect on N allocation within leaves had occurred. It is concluded that there is scope for improving the N-use efficiency of C<sub>3</sub> crop plants in elevated CO<sub>2</sub> conditions, by genetic manipulation to decrease the amount of Rubisco.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:343812 |
| Date | January 2000 |
| Creators | Theobald, Julian C. |
| Publisher | Open University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://oro.open.ac.uk/58084/ |
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