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Agronomy QUESTION #2105
Question 1
C4 photosynthesis confers agronomic advantages over C3 photosynthesis under high temperature and high irradiance conditions primarily because:
  • C4 plants have more chlorophyll and larger leaves than C3 plants
  • The C4 CO₂-concentrating mechanism (using PEP carboxylase in mesophyll cells to fix CO₂ as oxaloacetate/malate, then decarboxylating in bundle sheath cells to deliver high CO₂ to Rubisco) suppresses photorespiration and allows near-maximum photosynthesis at high temperatures when Rubisco oxygenase activity would otherwise dominate✔️
  • C4 plants have deeper root systems that access subsoil water inaccessible to C3 plants
  • C4 plants fix nitrogen more efficiently, requiring less protein investment in Rubisco
Correct Answer Explanation
At high temperatures, Rubisco's oxygenase activity (photorespiration) increases relative to carboxylase in C3 plants, wasting 30–50% of fixed carbon. C4 plants pump CO₂ to bundle sheath cells at 10–80× atmospheric concentration, suppressing oxygenase competition and photorespiration. This gives C4 crops (maize, sorghum, sugarcane, millet) higher radiation use efficiency, WUE, and NUE under tropical conditions.