Question

Statements about the effect of CO2 concentration, temperature and light intensity on the rate of photosynthesis in C3 and C4 plants are given. Based on the statements the correct option is

(i) At high CO2 concentrations, 40°C and saturated light, the photosynthetic rates of C3 and C4 plants are similar. (ii) C4 plants do not reach saturation levels even at 10% of the full intensity of sunlight. (iii) At twice the levels of CO2 concentration (~700 ppm), the net photosynthesis in C3 plants is higher than that of C4 plants. (iv) The net CO2 uptake in a C3 shade plant is equivalent to C4 plant under light saturation condition.

• A. Only ii
• B. i, ii, iii
• C. Only i
• D. i, ii, iii, iv

Answer 1

Answer: (B)

i, ii, iii

Answer 2

Let's analyze each statement:

(i) At high CO2 concentrations, 40°C and saturated light, the photosynthetic rates of C3 and C4 plants are similar.

• C3 plants show a significant increase in photosynthetic rate at high CO2 concentrations due to reduced photorespiration.
• C4 plants have an optimal temperature range of 30-45°C (40°C is optimal). C3 plants have an optimal temperature range of 20-25°C, so 40°C is suboptimal for them.
• However, the substantial benefit of high CO2 for C3 plants can allow their photosynthetic rates to approach or become similar to C4 plants, even at temperatures that are suboptimal for C3 but optimal for C4. Therefore, this statement can be considered correct under certain conditions.

(ii) C4 plants do not reach saturation levels even at 10% of the full intensity of sunlight.

• C4 plants are adapted to high light intensities and have very high light saturation points. They typically do not saturate even at full sunlight. Thus, they certainly would not saturate at just 10% of full sunlight. This statement is correct.

(iii) At twice the levels of CO2 concentration (~700 ppm), the net photosynthesis in C3 plants is higher than that of C4 plants.

• C3 plants are highly responsive to elevated CO2 because it reduces photorespiration and increases carboxylation efficiency. Their photosynthetic rate can increase significantly.
• C4 plants, due to their efficient CO2 concentrating mechanism, are already operating near their maximum photosynthetic rate at ambient CO2 and show little or no increase with further CO2 elevation.
• Therefore, at ~700 ppm CO2, the net photosynthetic rate of C3 plants can indeed surpass that of C4 plants. This statement is correct.

(iv) The net CO2 uptake in a C3 shade plant is equivalent to C4 plant under light saturation condition.

• C3 shade plants are adapted to low light and have lower maximum photosynthetic capacities.
• C4 plants under light saturation exhibit very high rates of CO2 uptake.
• It is incorrect to state that the low CO2 uptake of a C3 shade plant would be equivalent to the high CO2 uptake of a C4 plant under light saturation. This statement is incorrect.

Based on the analysis, statements (i), (ii), and (iii) are correct.