Which plant type is particularly efficient in hot and dry conditions due to CO2 concentration mechanisms?

Which plant type is particularly efficient in hot and dry conditions due to CO2 concentration mechanisms?

• A. C4 plants.
• B. C3 plants.
• C. CAM plants.
• D. All plants equally.

Answer: (A)

When conditions are hot and dry, concentrating CO2 around Rubisco becomes a major advantage because it lets the Calvin cycle run efficiently even when the stomata are partly closed to save water. In C4 plants, CO2 is first fixed in the mesophyll by the enzyme PEP carboxylase into a four-carbon compound, which is then transported to specialized bundle-sheath cells where CO2 is released for the Calvin cycle. This creates a high local CO2 level around Rubisco, suppressing the oxygenation reaction that leads to wasteful photorespiration under high temperatures. The result is higher photosynthetic efficiency and better water-use efficiency when water is scarce. By contrast, C3 plants lack this CO2-concentrating step, so at higher temperatures photorespiration increases and the carbon fixation pathway becomes less efficient. CAM plants do concentrate CO2 as well, but they do so by opening stomata at night and storing CO2 as malate for daytime use; while water-efficient, this strategy often supports slower growth compared with C4 plants, especially under conditions requiring rapid carbon gain.

When conditions are hot and dry, concentrating CO2 around Rubisco becomes a major advantage because it lets the Calvin cycle run efficiently even when the stomata are partly closed to save water. In C4 plants, CO2 is first fixed in the mesophyll by the enzyme PEP carboxylase into a four-carbon compound, which is then transported to specialized bundle-sheath cells where CO2 is released for the Calvin cycle. This creates a high local CO2 level around Rubisco, suppressing the oxygenation reaction that leads to wasteful photorespiration under high temperatures. The result is higher photosynthetic efficiency and better water-use efficiency when water is scarce.

By contrast, C3 plants lack this CO2-concentrating step, so at higher temperatures photorespiration increases and the carbon fixation pathway becomes less efficient. CAM plants do concentrate CO2 as well, but they do so by opening stomata at night and storing CO2 as malate for daytime use; while water-efficient, this strategy often supports slower growth compared with C4 plants, especially under conditions requiring rapid carbon gain.