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

Hint: 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.