Which condition diminishes carbon fixation rate within C4 photosynthetic pathways in plants?

💡 Explanation

When PEP carboxylase activity is reduced in C4 plants, the initial carbon fixation to form oxaloacetate is impaired because it compromises the mechanism to concentrate CO2. Therefore overall carbon fixation diminishes, rather than effects from bundle sheath changes, meso. diffusion, or RuBisCO inefficiencies which play different roles.

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