J.R. Evans and S. von Caemmerer
Evidence suggests that C4 plants produce greater amounts of biomass per unit of intercepted photosynthetically active radiation. This is due In large part to two factors. First, C4 plants have a greater quantum yield than C3 plants at 30 °C (the C4 advantage diminishes at lower temperatures and as atmospheric C02 partial pressures rise). Second, C4 plants have greater rates of C02 assimilation per unit leaf nitrogen (this benefit diminishes as leaf area index and/or canopy nitrogen content increases). The protein cost of C4 enzymes per unit chlorophyll is calculated and found to be similar to that of C3 photosynthesis. However, the rate of C02 assimilation per unit nitrogen in C4 plants is greater than that of C3 plants because high C02 partial pressure in the bundle sheath cells enables Rubisco to operate near its maximum catalytic rate and suppresses photorespiratlon. Rice leaf anatomy is examined with respect to locating the C4 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 C4 leaves, there are numerous mesophyll cells between adjacent veins in rice, which would diminish the efficiency of the C4 cycle. To engineer the C4 pathway into rice is therefore a formidable challenge.
The C4 photosynthetic pathway has evolved on numerous occasions in diverse plant species. The question is, Would it be useful to genetically engineer the pathway into rice? In this chapter, we wish to examine the evidence that C4 productivity exceeds that of C, species and why this might be so. We will also place the problem of putting C4 metabolism into rice in the context of its leaf anatomy. The complex details confronting the molecular biologists' task are beyond the scope of this chapter.
C02 fixation in C3 plants is catalyzed by the enzyme Rubisco (ribulose-l,5-bisphosphate carboxylase-oxygenase). Rubisco also catalyzes the reaction with 02 resulting in photorespiration. With present-day atmospheric C02 concentrations, around one-third of the RuBP is consumed by the oxygenase pathway at 30 °C. The C4 photosynthetic pathway has evolved to virtually eliminate oxygenase activity by Rubisco. This is achieved by bicarbonate reacting with phosphoeno/pyruvate, PEP, a carboxylation reaction insensitive to 02. The C4 acids formed in the mesophyll cells diffuse into bundle sheath cells where they are decarboxylated. A diffusive resistance restricts C02 leakage out of the bundle sheath such that the C02 partial pressure in the bundle sheath cells is greatly increased, resulting in the suppression of photorespiration. Whereas C4 plants from different species share a common carboxylation pathway, there are three major types of decarboxylation pathways, each liberating CO, into a different bundle sheath organelle (Hatch 1987).
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