- Initially it was believed that CO2 fixation takes place only by Calvin cycle.
- But in 1954, in addition to Calvin cycle, an alternate pathway to CO2 fixation in photosynthesis was discovered by Kortschak et al. who reported the formation of C4 dicarboxylic acid as primary product of photosynthesis in sugarcane.
- D Hatch and C.R.Slack (1966) proposed an alternative pathway of CO2 fixation which is now known as Hatch and Slack pathway or C4-dicarboxylic acid pathway or C4-cycle or ß-carboxylation cycle.
- This cycle is known as C4 cycle because first stable product of this cycle is a four carbon compound known as oxaloacetic acid (OAA)
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