Does UVA light stimulate UVR8 receptors?

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When you emit UVA photons near the 400 nm boundary (where UVA light ends and visible light begins), these UVA photons have a longer wavelength, so they contain less energy. On the other hand, when you emit UVA photons near the 315 nm boundary (where UVB light ends and UVA light begins), these photons have a shorter wavelength and therefore contain more energy.

This means that not all UVA light contains the same amount of energy.

For example:

335 nm UVA is part of the UVA range (315-400 nm), and the energy at 335 nm is higher than that of longer wavelength UVA photons (e.g., 395 nm).
Energy comparison:

335 nm UVA: Around 3.70 eV
395 nm UVA: Around 3.14 eV

The UVA/UVB boundary is at 315 nm, and photons emitted at this boundary (both UVA and UVB) have nearly the same energy.

Energy at 315 nm:

315 nm UVA: Around 3.94 eV
315 nm UVB: Around 3.94 eV

So, if you use a UVA light that emits photons with longer wavelengths, like 395 nm UVA, it will not stimulate UVR8 receptors as effectively as a photon with a shorter wavelength, like 335 nm UVA, because shorter wavelengths (like UVB) carry more energy and are more efficient in activating these receptors.

This is why UVB light, with its shorter wavelengths, is often preferred for stimulating the UVR8 receptors in plants. A shorter wavelength UVB photon is hundreds of times more energetic than a longer wavelength UVA photon.

One final note: For both a 400 nm UVA photon and a 400 nm visible light photon, the energy is approximately 4.97 × 10⁻¹⁹ joules. So, the distinction between UVA and visible light at 400 nm comes from the type of light used, not from the energy of the photons, which is nearly the same at this wavelength.