Question

You are growing C3 and C4 plants in normal (21%) and low (1%) O2 air, under both warm well-lit conditions and cool shady conditions. You find that, in general, the C3 plants have a photosynthetic advantage over C4 plants in:

		warm high light at 1% O2 and cool shade at 21% O2 only
		warm high light at 21% O2 and 1% O2, and cool shade at 21% O2.
		cool shade under 21% and 1% O2, and in warm high light at 1% O2.
		cool shade at 21% O2 and warm high light at 21% O2.

Answer 1

If we are growing C3 and C4 plants in normal (21%) and low (1%) O2 air, under both warm well-lit conditions and cool shady conditions, then we find that, in general, C3 plants have a photosynthetic advantage over C4 plants in cool shade under 21% and 1% O2, and in warm high light at 1% O2. This is because, in cooler environments, C3 plants are more photosynthetically productive and efficient than C4 plants due to the specialized anatomy of C4 plants and the requirement of more energy and resources. According to the Warburg effect, high O2 concentrations in the environment will inhibit the photosynthetic enzyme Rubisco to fix CO2 and initiate photosynthesis. However, oxygen has a high affinity for the Rubisco enzyme under high temperature and light than at low light intensities. So, in the above-mentioned conditions, C3 plants will grow better in cool shade both at 21% and 1% and in warm high light only at low 1% O2 ( at higher O2 concentrations i.e. 21% O2, oxygen reduces C3 plant photosynthetic and water use efficiency leading to photorespiration).