ANSI / ASABE S640 JUL2017 , Quantities and Units of Electromagnetic Radiation for Plants ( Photosynthetic Organisms ) defines Photosynthetic Photon Efficacy ( PPE ) as :
The photosynthetic photon efficacy ( Kp ) is the photosynthetic photon flux divided by input electric power . The unit is micromoles per second per electric watt ( μmol × s-1 × We-1 ), or micromoles per joule ( μmol × J-1 ).
Ignoring the technical jargon , the key point here is micromoles of photons . Photosynthesis occurs when a photon is absorbed by a photopigment ( primarily chlorophyll A or B ). In accordance with the Stark-Einstein law ( a . k . a . the second law of photochemistry ), one photon initiates one chemical reaction , regardless of the photon ’ s wavelength . We must therefore count the number of photons per second ( measured in micromoles per second — one micromole is 60 billion billion photons ) rather than lumens or radiant watts for horticultural purposes . Referring to Fig . 1 , McCree ( 1972 ) measured the relationship between wavelength and photosynthesis to produce the averaged “ McCree curve .” He also acknowledged the Stark-Einstein law , which accounts for the blue line between 400nm and 700nm . What this means is that we can ignore the spectral power distribution of any light source within the range of 400nm to 700nm . As long as we have a calibrated PAR ( a . k . a . “ quantum ”) sensor — which is basically a radiant wattmeter with the spectral response shown in Fig . 1 — we can measure micromoles of photons per second .
Fig . 1
PHOTONS in effect supercharge the process of photosynthesis , and must — not should , but must — be taken into consideration when defining photosynthetically active radiation .”
In a recent paper , Zhen and Bugbee ( 2020 ) presented an excellent argument in favor of redefining photosynthetically active radiation to include the spectral range of 400nm to 750nm . The title of the paper even includes the phrase , “ Implications for Redefining Photosynthetically Active Radiation .” The authors are unquestionably correct ; far-red photons in effect supercharge the process of photosynthesis , and must — not should , but must — be taken into consideration when defining photosynthetically active radiation . This does not mean , however , that the PAR metric should be redefined . Quoting from the abstract of Zhen and Bugbee ( 2020 ): “ Far-red alone minimally increased photosynthesis … far-red photons are equally efficient at driving canopy photosynthesis when acting synergistically with traditionally defined photosynthetic photons .” In other words , if we assume a spectral range of 400nm to 750nm , we cannot unambiguously measure the photosynthetic photon efficacy of a light source without knowing its spectral power distribution . That is , without knowledge of the entire spectral power distribution within this range , we cannot predict the rate of photosynthesis .