Question

Given table shows the $C{O_2}$ compensation point and optimum $C{O_2}$ concentration for photosynthesis for ${C_3}$ and ${C_4}$ plants.

| ${C_3}$ plants| ${C_4}$ plants
---|---|---
$C{O_2}$ compensation point| $25 - 100ppm$| A
Optimum $C{O_2}$ concentration| B| $360ppm$

Select the correct values for A and B.
a. $A - 0 - 50ppm$; $B - 300ppm$
b. $A - 0 - 10ppm$; $B - 450ppm$
c. $A - 100 - 150ppm$; $B - 250ppm$
d. $A - 100 - 110ppm$; $B - 290ppm$

Answer 1

The compensation of $C{O_2}$means the point where the plant will not absorb any $C{O_2}$from the surroundings. Therefore the levels of $C{O_2}$that are used in photosynthesis will be the same as the amount of $C{O_2}$released by processes like respiration and photorespiration. The ${C_3}$and${C_4}$ plants have different $C{O_2}$compensation points.

Complete answer:
- ${C_4}$ plants have very low levels of photorespiration and therefore the $C{O_2}$ compensation point is nearly zero or zero.
- At higher carbon dioxide concentration, the rate of photosynthesis is high while at low $C{O_2}$ concentration, there will be high photorespiration. Moreover $C{O_2}$ fixation in ${C_4}$ plants is highly efficient by the PEP carboxylase and hence there is low compensation.
- The optimum carbon dioxide concentration for ${C_4}$ plants is 360 ppm and for ${C_3}$ it is 450 ppm.

Hence, the correct answer is option (B).

Additional information:
Considering the ${C_4}$ plants, the high carbon dioxide level will inhibit the photorespiration. This is because at concentrations which are high, $C{O_2}$ competes better in comparison to ${O_2}$ for Rubisco carboxylase sites and so fixation is at a greater rate. The ecological distribution of ${C_3}$ and ${C_4}$ plants is consistent along with an adaptive role for reassimilation of photorespired carbon dioxide during hot temperatures. One ${C_3}$-${C_4}$ species of genus Flaveria is known to display biochemistry and assimilates some $C{O_2}$ by this pathway. At increasing temperatures, ${C_4}$ plants grow faster which reduces time involved in photosynthesis. However, by this, small plants are obtained with a reduced yield.

Note: The reason for low compensation point value for ${C_4}$ plants is the greater efficiency of carbon dioxide fixation through Phosphoenolpyruvate-carboxylase enzyme whereas the mesophyll cells of ${C_3}$ plants do not have such mechanism of carbon dioxide fixation.