Why do C4 crops often outperform C3 crops under high temperature and low CO2 conditions?

• A. C3 fixes CO2 directly via Rubisco in mesophyll
• B. C4 concentrates CO2 in bundle-sheath cells, reducing photorespiration
• C. C4 has lower water use efficiency
• D. C4 requires less nitrogen

Answer: (B)

The key idea is that C4 plants avoid wasteful photorespiration by concentrating CO2 right where Rubisco works. At high temperatures and low ambient CO2, Rubisco in C3 plants is more likely to fix O2 instead of CO2, which triggers photorespiration and wastes energy and carbon. C4 plants fix CO2 first in mesophyll cells using PEP carboxylase to form a four-carbon compound, then shuttle it to bundle-sheath cells where CO2 is released at a high concentration for carboxylation by Rubisco. This CO2-rich microenvironment greatly suppresses the oxygenase activity of Rubisco, so photosynthesis stays efficient even when the air is hot and CO2 is scarce. That advantage translates into higher carbon gain and better performance for C4 crops under those conditions. While water-use efficiency and nitrogen requirements are relevant in other contexts, the decisive factor here is the CO2 concentrating mechanism that minimizes photorespiration.