The rate of photosynthesis is very sensitive to the level of CO2 inside the chloroplasts, where photosynthesis takes place and CO2 is fixed. Higher chloroplastic CO2 concentrations will generally increase photosynthesis due to increased substrate availability and competitive inhibition of the oxygenation of ribulose bisphosphate (RuBP).
To get from the atmosphere into the chloroplast, CO2 diffuses into the leaf through the stomata, after which it has to travel across internal airspace, cell walls, plasmalemma, cytoplasm, chloroplast envelope and part of the chloroplast stroma before it is fixed in the first step of the Calvin-Benson cycle. The combined conductance to CO2 transfer from substomatal cavities to the site of fixation is termed internal or mesophyll conductance.
By transgenic manipulation of specific components in the pathway of internal CO2 transfer, the RIPE project will try to improve CO2 availability at the site of carboxylation and thereby increase photosynthesis. Since the pathway of water loss via transpiration is partially shared with the pathway of CO2 uptake, proportional increases of mesophyll versus stomatal conductance should also improve leaf instantaneous water use efficiency (rate of CO2 assimilation divided by transpiration rate). If so, improvements in mesophyll conductance may help to improve productivity as well as water use efficiency of food crops.