Nitrogen assimilation and growth of wheat under elevated carbon dioxide

Abstract

A tmospheric CO 2 concentrations have increased from about 280 to 370 mol mol Ϫ1 since 1800 (1, 2) and may reach 500-900 mol mol Ϫ1 by the end of the century (3). Several responses of higher plants to such changes were not anticipated (4). For example, a doubling of CO 2 level initially accelerates carbon fixation in C 3 plants by about 30%, yet after days to weeks of exposure to high CO 2 concentrations, depending on species, carbon fixation declines until it stabilizes at a rate that averages 12% above ambient controls (5). This general phenomenon, known as CO 2 acclimation, is correlated with a decline in the activity of Rubisco and other enzymes in the Calvin cycle (6, 7). The change in Calvin cycle enzyme activities is not necessarily selective; rather, it often follows a decline in overall shoot protein and N contents (8). Shoot N contents diminish by an average of 14% with a doubling of CO 2 (9), a difference that exceeds what would be expected if a given amount of N were diluted by additional biomass (8).In wheat, CO 2 acclimation varies with N supply (10, 11). Wheat shoots accumulate free NO 3 Ϫ under elevated CO 2 (12), and shoot protein declines (13) despite little change in total N (12, 14). Here, we present several lines of evidence that elevated CO 2 concentrations inhibit NO 3 Ϫ assimilation in wheat shoots and suggest two physiological mechanisms for this phenomenon. Materials and MethodsWe surface-sterilized wheat (Triticum aestivum cv. Veery 10) seeds for 1 min in 2.6% NaClO, washed them thoroughly with water, and germinated them for several days on thick paper toweling (germination paper) saturated with 1 mM CaSO 4 . Twenty seedlings were transplanted to 19-liter opaque plastic containers filled with an aerated nutrient solution containing 0.2 mM NH 4 NO 3 , 1 mM CaSO 4 , 0.5 mM K 2 HPO 4 , 0.5 mM KH 2 PO 4 , 1 mM MgSO 4 , 0.2 g liter Ϫ1 Fe-NaEDTA, and micronutrients (15). The nutrient solution was replenished every 3 days.The containers were placed in a controlled environment chamber [Conviron (Winnipeg, MB, Canada) PGV-36] equipped with a nondispersive infrared CO 2 analyzer (Horiba, Kyoto, no. APBA-250E) and a Conviron process controller that added CO 2 to maintain the chamber concentration at 360 mol mol Ϫ1 for the photon flux density (PFD) response experiments (i.e., shoot photosynthesis as a function of photosynthetic PFD at plant height) and either 360 or 700 mol mol Ϫ1 for the A-C i response (i.e., shoot photosynthesis as a function of internal CO 2 concentration) experiments. The growth͞N relations experiments were also conducted at 360 or 700 mol CO 2 mol Ϫ1 . The CO 2 added was filtered through a KMnO 4 column to remove contaminating hydrocarbons such as ethylene. A combination of high-pressure sodium, metal halide, and incandescent lamps provided a photosynthetic PFD of 700 mol m Ϫ2 s Ϫ1 at plant height. The light͞dark period was 16 h͞8 h at 25°C and 15°C, respectively.Gas-Exchange Measurements. We transferred a seedling about 14 days old into a measurement system in which a split ...

Oxygen consumption during leaf nitrate assimilation in a C3 and C4 plant: the role of mitochondrial respiration

“…By this measure, NO 3 -assimilation in the shoots of wheat, a C 3 plant, was detectable only under ambient or lower CO 2 concentrations (Figs 2b & 4b; see also Bloom et al 2002). In wheat, nitrate assimilation under ambient or lower CO 2 concentrations may contribute to the smaller differences between net and gross O 2 evolution (DO 2 ) under NO 3 -nutrition (Figs 2c & 4c).…”

“…+ as a nitrogen source (DAQ) provided a measure of shoot NO 3 -assimilation (Myers 1949;Bloom et al 1989;Cen, Turpin & Layzell 2001;Bloom et al 2002). By this measure, NO 3 -assimilation in the shoots of wheat, a C 3 plant, was detectable only under ambient or lower CO 2 concentrations (Figs 2b & 4b; see also Bloom et al 2002).…”

Nitrate assimilation in plant shoots depends on photorespiration

“…Leaves in the shoot cuvette were at their normal orientation; thus the angle of incidence was between 0°and 45°for Arabidopsis and 70°and 80°for wheat. Net gas fluxes from the shoot were monitored with the instrumentation described previously (15,24). In brief, an infrared gas analyzer (Horiba VIA-500R, Kyoto) measured CO 2 fluxes, a custom O 2 analyzer based on heated zirconium oxide ceramic cells measured O 2 fluxes, and relative humidity sensors (Vaisala, Helsinki) measured water vapor fluxes.…”

“…We proposed a relatively simple explanation for these responses: elevated CO 2 concentrations inhibit the assimilation of nitrate (NO 3 Ϫ ) in shoots of C 3 plants ( [14][15][16]. Because NO 3 Ϫ is the prominent form of inorganic nitrogen available to plants from temperate well aerated soils (17), diminished NO 3 Ϫ assimilation dramatically alters the nitrogen balance in C 3 plants (15).…”

“…Because NO 3 Ϫ is the prominent form of inorganic nitrogen available to plants from temperate well aerated soils (17), diminished NO 3 Ϫ assimilation dramatically alters the nitrogen balance in C 3 plants (15). Much of our evidence was based on estimates of shoot NO 3 Ϫ assimilation derived from calculations of the difference in the assimilatory quotient (⌬AQ, ratio of net CO 2 consumption to net O 2 evolution) between plants that received NO 3 Ϫ as their sole nitrogen source and those that received ammonium (NH 4 ϩ ) as their sole source.…”