What Is the McCree Curve? Understanding How Plants Use Light

Plants rely on light to grow, of course—but not all light is equal. The McCree Curve is one of the foundational scientific findings that helps us understand which parts of the visible light spectrum plants use most efficiently. At AquaHorti, understanding this curve helps us build better PAR/DLI sensors and help growers make smarter lighting decisions.

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1. Origins: Who Was McCree & What Did He Do

• The “McCree Curve” comes from the work of plant physiologist Dr. Keith J. McCree.
• In his 1972 paper “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants,” he measured how 22 different crop species responded to individual wavelengths of light.
• Specifically, he looked at the quantum yield of photosynthesis—that is, how many molecules of CO₂ are fixed per photon of light at each wavelength.

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2. What the Curve Shows

• The curve is plotted over the 400‑700 nm range (visible light that plants use) — this is the PAR range.
• It shows that plants absorb and use blue (~430‑450nm) and red (~640‑680nm) light most efficiently—those are the peaks in the curve.
• Green light (500‑600nm) is used less efficiently, though not useless; light in those wavelengths penetrates deeper into canopies and lower leaves.
• Outside the PAR range (below ~400nm, above ~700nm), photosynthesis quantum yield drops off sharply.

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3. Why It Matters for Growers & Lighting Design

• When choosing grow lights or designing lighting schedules, lights that emit strongly in the blue and red peaks of the McCree Curve will generally support better photosynthesis per watt.
• Knowing the curve helps in evaluating PAR meters / sensors: a good sensor should have a response curve close to McCree’s, meaning it measures “true usable light,” not just apparent brightness.
• Using lights with spectrum mismatched to the McCree Curve can lead to wasted energy and suboptimal growth.

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4. Limitations & New Extensions

• McCree’s original work was with daylight and single‑wavelength sources; in modern LEDs and certain horticultural applications (e.g. far‑red light, UV), there is research suggesting extended spectrums (beyond 700nm) may also play roles.
• Some plants respond differently depending on light history (shade, canopy structure, etc.), so McCree Curve is a useful guide—not a rigid rule.

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5. Takeaway

• The McCree Curve is the scientifically‑measured efficiency of photosynthesis across wavelengths in the PAR range (400‑700nm).
• It offers guidance for light choice, sensor design, and understanding plant growth.
• At AquaHorti, we use this curve to make our PAR meters more accurate, and help you to choose lighting and schedule setups that give plants the light they can use.