Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?
Since, through C4 cycle, a plant can photosynthesise even in presence of very low concentration of CO2 (upto 10 parts per million), the partial closure of stomata due to xeric conditions would not bring much
effect. Therefore, the plants can adapt to grow at low water content, high temperature and bright light intensities.
This cycle is specially suited to such plants which grow in dry climates of tropics and subtropics. Besides, the photosynthetic rate remains higher due to absence of photorespiration in these plants. It can be visualised that both C4 cycle and photorespiration are the result of evolution or might have been one of the reasons of evolution for the adaptation of plants to different environments. C4 plants are about twice to efficient as C3 plants in converting solar energy into production of dry matter.
The productivity of a plant is measured by the rate at which it photosynthesises. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C4 plants have a mechanism for increasing the concentration of carbon dioxide. In C4 plants, the Calvin cycle occurs in the bundle-sheath cells. The C4 compound (malic acid) from the mesophyll cells is broken down in the bundle-sheath cells. As a result, CO2 is released. The increase in CO2 ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. Thus, C4 plants are highly productive.