The McCree Curve

The ideal average photosynthetic response of plants.

The McCree Curve Explained

The McCree Curve quantifies the average photosynthesis response of plants to light from 350nm to 750nm, and shows the average sensitivity of plants to light (the plant sensitivity curve) in this region of the spectrum. The McCree curve can be used to compare the spectral output of a plant/grow-light with the average sensitivity curve for plants.

The McCree Curve
In the 1970s, Dr. Keith. J. McCree (1927-2014), who was a professor at Texas A&M University in the Soils and Crop Sciences department and a physicist by education, published a seminal paper entitled "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants".

Dr. Keith. J. McCree
 Dr. Keith. J. McCree

To quote from the abstract of the paper[1]: "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 750 nm. The following factors were varied: species, variety, age of leaf, growth conditions (field or growth chamber), test conditions such as temperature, CO2 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 440 nm, with a shoulder at 670 nm. The average height of the blue peak was 70% of that of the red peak."[3]

From his study data, Dr McCree was able to create a generalized plant light absorption curve (an average curve for all the plants he measured), which is known as the McCree curve. On the left below, you can see the original McCree curves (for all 22 species of plants) as published in Dr. McCree's paper [1], and on the right is a more colourful version of the averaged McCree Curve.

Original version of the McCree curve and colour version of hte McCree curve

For More information about plant light sensitivity and the McCree Curve, see see What Light do Plants Need?

The original data from which the McCree Curve was derived

[1] "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants" - McCree, Keith J. Agricultural and Forest Meteorology 9: 191-216.