http://www.plantphysiol.org/content/155/1/79
The theoretical upper limit for the operational efficiency of plant photosynthesis has been estimated from a detailed stepwise analysis of the biophysical and biochemical subprocesses to be about 4.6% for C3 and 6.0% C4 plants (Zhu et al., 2008, 2010). (These estimates assume a leaf temperature of 30°C and an atmospheric [CO2] of 387 ppm and were calculated relative to the full solar spectrum at the earth’s surface. These efficiencies would be slightly more than double if calculated relative to only the photosynthetically active radiation [i.e. 400–700 nm; Zhu et al., 2008].) While estimates of the theoretical upper limit of photosynthetic efficiency in microalgae have not been conducted as systematically, the same considerations apply as for plants, although microalgae may have a lower respiratory rate that would raise the upper limit for the efficiency of net photosynthesis accordingly (Melis, 2009). Microalgae expressing bicarbonate transporters will mimic the higher efficiency of C4 plants due to suppression of photorespiration, while those that do not actively concentrate CO2 would have upper efficiency limits similar to C3 plants. The highest short-term efficiencies observed for plants in the field, assessed from maximum growth rates, are about 3.5% for C3 and 4.3% for C4 plants, and these drop further to 2.4% and 3.4% when calculated over a full growing season (Monteith, 1977; Piedade et al., 1991; Beale and Long, 1995). For crop plants, these highest full growing season efficiencies are those that define the yield potential (Fischer and Edmeades, 2010) or record yield of the crop and are at least twice greater than photosynthetic efficiencies observed under most commercial farming conditions.