Pay attention here:
The early perception of neighbor proximity is based on the detection of changes in the spectral composition of solar radiation that are produced by interaction of sunlight with green leaves. Leaves effectively absorb photons in the blue (B, 400–500 nm) and red wavebands (R, 500–600 nm) of the solar spectrum. Absorption in the green (400–500 nm), and particularly in the far-red region (FR, 700-800 nm) is weaker and many photons of these wavelengths are back scattered in the form of diffuse radiation. We are sensitive to the green photons that bounce off the leaves (that is why we see leaves as green), but we cannot see FR radiation because our photoreceptors are blind to wavelengths longer than 700 nm. The plant photoreceptors phytochromes, in contrast, are maximally sensitive in the R and FR regions of the spectrum.
Read that a couple times so it can set in.