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Definition

noun

(botany)

(1) A plant that utilizes the C₄ carbon fixation pathway in which the CO₂ is first bound to a phosphoenolpyruvate in mesophyll cell resulting in the formation of four-carbon compound (oxaloacetate) that is shuttled to the bundle sheath cell where it will be decarboxylated to liberate the CO₂ to be utilized in the C₃ pathway.

(2) A plant in which the CO₂ is first fixed into a compound containing four carbon atoms before entering the Calvin cycle of photosynthesis.

Supplement

A C₄ plant is better adapted than a C₃ plant in an environment with high daytime temperatures, intense sunlight, drought, or nitrogen or CO₂ limitation. Most C₄ plants have a special leaf anatomy (called Kranz anatomy) in which the vascular bundles are surrounded by bundle sheath cells. Upon fixation of CO₂into a 4-carbon compound in the mesophyll cells, this compound is transported to the bundle sheath cells in which it is decarboxylated and the CO₂is re-fixed via the C₃ pathway. The enzyme involved in this process is PEP carboxylase. In this mechanism, the tendency of rubisco (the first enzyme in the Calvin cycle) to photorespire, or waste energy by using oxygen to break down carbon compounds to CO₂, is minimized.

Examples of C₄ plants include sugarcane, maize, sorghum, amaranth, etc.

Compare: C3 plant, CAM plant.
See also: C4 carbon fixation pathway, Calvin cycle.