Q30 - Wheat Nitrogen Assimilation Under Elevated CO2

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Mineral Nutrition & Nutrient Uptake Calvin Cycle, Photorespiration & C4/CAM Biochemistry

Wheat is grown on more land area than any other food crop, and it receives about 20% of the world’s nitrogen fertilizer.

In the well-drained soils usually used to grow wheat, NO3- is a major N-fertilizer form. If rising atmospheric CO2 levels significantly affect NO3- photoassimilation by wheat, alternative nitrogen fertilizers should be prepared.

Bloom et al. (2012) grew wheat seedlings in controlled environment chambers where CO2 was controlled at ambient or elevated levels (360 or 700 umol/mol), and the plants received either 0.2 mM NH4+ or 0.2 mM NO3- as the N source.

Figure 1. Biomass and leaf area of wheat seedlings grown for 14 days. Red = NH4+ at 360 umol/mol CO2, Blue = NH4+ at 700, Yellow = NO3- at 360, Cyan = NO3- at 700.

Figure 2. Bar chart split into Shoot (top) and Root (bottom). Shows Total N, Protein, NO3- concentration, NO3- reductase activity, and NO2- reductase activity under the same four treatments. In shoots, elevated CO2 increases NO3- concentration and affects enzyme activities. In roots, most parameters are unchanged between CO2 treatments. Bars with different letters differ significantly. Figure 2. Nitrogen concentration, protein, NO3- levels, and enzyme activities in shoots (top) and roots (bottom). Same color scheme as Figure 1.

Q30.1. Whether the form in which N was supplied affects shoot and stem growth depends on ambient CO2 levels.

Q30.2. The plants receiving NH4+ were more responsive to CO2 enrichment than those receiving NO3-.

Q30.3. Under the two CO2 regimes, the N absorption per unit plant mass remained unchanged.

Q30.4. The fate of N, after it was absorbed, was similar under ambient and elevated CO2 as demonstrated by the balance of inorganic and organic N.

Q30.5. In vitro shoot activities of NO3- reductase and NO2- reductase changed compared to roots in the different CO2 regimes.

Q30.6. The results suggest that rising atmospheric CO2 will favor taxa that either prefer NH4+ as a nitrogen source, or assimilate NO3- primarily in their roots.

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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF