Following up on yesterday’s blog about yellow and orange leaves, which are the product of the protective pigments left in the leaf after chlorophyll synthesis has ceased in the fall…
The red color isn’t due to pigments unmasked in the leaf as chlorophyll synthesis wanes in the fall. Instead, red light is reflected when anthocyanin levels increase in senescing leaves, i.e., those leaves about to drop off the tree.
In response to fall temperature and daylight conditions, the stem of each leaf produces a corky membrane at its junction to the branch. This obstruction develops gradually but prevents any more nutrients from moving into or out of the leaf. Chlorophyll synthesis declines with the interruption in the flow of nutrients into the leaf. In addition, any sugar produced by the leaves on occasional warm, sunny days of fall remains in the leaf, raising its concentration there. Under these conditions (warm days, cold nights), leaves synthesize anthocyanin molecules that bind with this sugar, and this combination produces the red color.
Anthocyanins seem to be particularly good at absorbing all wavelengths of light except red, and some researchers believe that these molecules thereby act as protective sunscreen for delicate leaf photosynthetic machinery. Newly emerging leaves in the spring are also often red-tinged and high in anthocyanin.
Eventually, the leaf stem separates from the branch at the corky barrier and drops to the ground, unneeded now that conditions for photosynthesis and growth are poor.
If leaf color is just a product of pigments, why does fall color seem better some years than others? Most likely this is due to optimal weather conditions in spring and fall. Wet and warm spring and summer weather promote leaf growth and increased synthesis of all of the leaf pigments that absorb light. Cold nights, coupled with high intensity of sunlight on fall days, promote destruction of chlorophyll and increased anthocyanin production. Dry weather increases the concentration of sugar in the leaves, as water is evaporated from them in the fall. More sugar + more anthocyanin in the leaves = more intense color.
Thus, the best fall color is a result of better-than-average spring and summer leaf growth and dry, cool fall weather with occasional bright, sunny days.