Light Ratios

Today's topic is all about light: the spectrum, the wavelengths, and what happens when light hits leaves. Most of the following info is paraphrased from Keara A. Franklin's Tansley Review on Shade Avoidance (New Phytologist 2008).

Most basic science textbooks have a bit about the light spectrum, with a picture that looks something like this:

Red light is right next to infrared (heat). Wavelength is measured from the crest (highest point of each wave) across the trough (lowest point after the peak) to the next crest and for visible light is expressed in nanometers (nm). A nanometer is a billionth of a meter. The visible spectrum of light ranges approximately 400-700 nm. Red light falls at the 625-740 nm end of that. For plant study purposes, red light is considered 660-670 nm and far-red light is 725-735 nm.

Plants don't absorb all wavelengths of visible light equally. That's why they appear green to us humans--they reflect off green spectrum light while preferentially absorbing reds and blues. A shaded plant will receive much less of these desirable wavelengths because their opportunistic neighbor will get to them first. Thus, plants have phytochromes to detect the relative difference in amounts of red and far-red light. Interestingly, plants have another way of knowing the difference between shading by a neighboring plant and the shade of, say, a building. They can also detect higher levels of the hormone ethylene around a plant. In any case, if a cell on a plant leaf gets hit with lots of far-red light and just a little red light, the phytochromes within it change their form. And that's how it all begins.

Image above: the light spectrum, from the World Wide Web