Would C ₄ rice produce more biomass than C ₃ rice?

Evidence suggests that C ₄ plants produce greater amounts of biomass per unit of intercepted photosynthetically active radiation. This is due in large part to two factors. First, C ₄ plants have a greater quantum yield than C ₃ plants at 30°C (the C ₄ advantage diminishes at lower temperatures and as atmospheric CO ₂ partial pressures rise). Second, C ₄ plants have greater rates of CO ₂ assimilation per unit leaf nitrogen (this benefit diminishes as leaf area index and/or canopy nitrogen content increases). The protein cost of C ₄ enzymes per unit chlorophyll is calculated and found to be similar to that of C ₃ photosynthesis. However, the rate of CO ₂ assimilation per unit nitrogen in C ₄ plants is greater than that of C ₃ plants because high CO ₂ partial pressure in the bundle sheath cells enables Rubisco to operate near its maximum catalytic rate and suppresses photorespiration. Rice leaf anatomy is examined with respect to locating the C ₄ metabolism. Chloroplasts in bundle sheath cells represent only a minute fraction of those present in the rice leaf. In addition, whereas mesophyll cells are immediately adjacent to bundle sheath cells in terrestrial C ₄ leaves, there are numerous mesophyll cells between adjacent veins in rice, which would diminish the efficiency of the C ₄ cycle. To engineer the C ₄ pathway into rice is therefore a formidable challenge.