Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark

The convergent quantum yield hypothesis (CQY) assumes that thermodynamics and natural selection jointly limit variation in the maximum energetic efficiency of photosynthesis in low light under otherwise specified conditions (e.g. temperature and CO2 concentration). A literature survey of photosynthetic quantum yield (phi) studies in terrestrial plants from C-3, C-4, and CAM photosynthetic types was conducted to test the CQY hypothesis. Broad variation in phi values from C-3 plants could partially be explained by accounting for whether the measuring conditions were permissive or restrictive for photorespiration. Assimilatory quotients (AQ), calculated from the CO2 phi:O-2 phi ratios, indicated that 49% and 29% of absorbed light energy was allocated to carbon fixation and photorespiration in C-3 plants, respectively. The unexplained remainder (22%) may represent diversion to various other energy-demanding processes (e.g. starch synthesis, nitrogen assimilation). Individual and cumulative effects of these other processes on photosynthetic efficiency are poorly quantified. In C-4 plants, little variation in phi values was observed, consistent with the fact that C-4 plants exhibit little photorespiration. As before, AQ values indicate that 22% of absorbed light energy cannot be accounted for by carbon fixation in C-4 plants. Among all three photosynthetic types, the phi of photosynthesis in CAM plants is the least studied, appears to be highly variable, and may present the greatest challenge to the CQY hypothesis. The high amount of energy diverted to processes other than carbon fixation in C-3 and C-4 plants and the poor characterization of photosynthetic efficiency in CAM plants are significant deficiencies in our otherwise robust understanding of the energetics of terrestrial photoautotrophy.