The fate of photosynthetic reductants downstream of PSI is one of the most burning question in current photosynthesis research. Apart from cycling back to the PQ pool or reducing O2 in various water-to-water cycles, electrons can follow several routes which vary with physiological conditions: carbon (Calvin-Benson cycle), nitrogen and sulfur reduction, export out of the chloroplast (photorespiration, malate valve), redox regulatory networks (thioredoxins and NTRC).
