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The McCree curve is a quantum efficiency lighting curve used in botany and should be used only as an initial foundation for understanding photosynthesis rates by wavelength. It is far more accurate than charts for chlorophyll dissolved in a solvent or charts for green algae, and it is common for these charts to get mixed up.

  
Assimilation chamber for measuring photosynthesis
 

It was developed in the early 1970s by Keith McCree, a Ph.D physicist that was a professor of Soil and Crop Sciences at Texas A&M University. He tested 22 different crop plant types for photosynthesis rates with a PPFD of 18-150 µmol/m2/sec, in monochromatic light at 25nm intervals from 350nm to 750nm, and using the single leaf model. The McCree curve is only valid for these conditions. Monitoring CO2 uptake was used to measure photosynthesis rates.

The measurements were made to provide a basis for discussion of the definition of 'photosynthetically active radiation'. The action spectrum, absorptance and spectral quantum yield of CO2 uptake were measured, for leaves of 22 species of crop plant, over the wavelength range 350 to 750nm. The following factors were varied: species, variety, age of leaf, growth conditions (field or growth chamber), test conditions such as temperature, COs concentration, flux of monochromatic radiation, flux of supplementary white radiation, orientation of leaf (adaxial or abaxial surface exposed), For all species and conditions the quantum yield curve had 2 broad maxima, centered at 620 and 440nm, with a shoulder at 670nm. The average height of the blue peak was 70% of that of the red peak. K. J. McCree, The action spectrum, absorptance and quantum yield of photosynthesis in crop plants.

Key Takeaways and limitations

  
Weighting factor for photosynthesis (source)
 

• The McCree curve illustrates that all of 400-700nm is useful for photosynthesis including green light, and not just red and blue.
• The McCree curve should not be used for very high lighting levels. It is only valid at about 15-150 umol/m2/sec of monochromatic light.
• The McCree curve only looks at the single leaf model of plant growth, not the whole plant model. It also does not take in to account multi-wavelength lights including mixing in far red to try to increase photosynthesis efficiency (see the Emerson enhancement effect for more).
• McCree also tested the underside (abaxial) of leaves, and found that they were also performing photosynthesis. In many cases the underside of a leaf will have a lower chlorophyll density, and may reflect more green light than the topside (adaxial) of a leaf, which may lower green light photosynthesis. The work of McCree demonstrated that both sides of a leaf can be used efficiently for photosynthesis. Dicotyledons may reflect more green light on the abaxial (underside) of a leaf, while monocotyledons will have the same green reflectance on both sides of a leaf.
• YPF (YPFD) or 'yield photon flux (density)' is PPFD that has been weighed to the McCree curve. It is fortunately rarely used in botany but you do sometimes see it. There are special PAR sensors that give measurements in YPFD instead of PPFD, as well as spectroradiometers that can do this.

The McCree curve describes the quantum efficiency of photosynthesis at different wavelengths at low light. The action spectrum describes actual photosynthetic rates and thus is the curve on which to base spectral selections. However, it is still largely unknown how the McCree curve translates to yield, appearance, desired development (e.g., flowering), good nutritional composition, and high levels of extractable products (e.g., aromatics, oils, pharmaceuticals) for a wide variety of crops and light sources. Tessa Pocock. The McCree Curve Demystified