Many of the yellows and oranges we see in nature are the result of carotenoid compounds. They are what make carrots orange! Carotenoids play a minor role in photosynthesis—they are present throughout the growing season, but are only revealed when chlorophyll breaks down. You are more likely to see brilliant colors when the fall weather has warm, sunny days with cool nights between 32° and 45 °F. Look at a leaf that has been grown in the shade, and you may notice it is a duller color than leaves that grew in the full sun, even on the same plant.
The reds and purples you see in the fall are caused by anthocyanins, which are what you get when sugars combine with compounds called anthocyanidins. Many things affect the exact color produced by anthocyanins, including the pH (acidity or alkalinity) of the cell sap in the leaves. With an acid pH, anthocyanins are often red; with a more alkaline pH they turn purple-to-blue. Because anthocyanins need sugar for their creation, weather that favors photosynthesis (sugar production) is essential. Bright, sunny fall days produce the best colors. Very little photosynthesis occurs on cloudy days, and rain can actually leach the anthocyanins and carotenoids out from the leaves. Different combinations of anthocyanins and carotenoids can result in there being yellow, orange and red leaves all on the same tree at the same time. The exact color a tree produces will vary from year to year. Some people believe they can force a more brilliant red color by adding acid-forming fertilizers to the tree, especially aluminum sulfate. This method may sound like it should work, but there is no scientific evidence to support it.
Photosynthesis | Definition of Photosynthesis by …
Photosynthesis is responsive to certain wave lengths of light energy (see Fig. 7.3, 7.4 & 7.5, Pg. 110-111 in text). The visible spectrum, from Red to Violet light is effective in photosynthesis The most effective colors of light energy are in the Red and Blue/Violet range of the color spectrum. These are the wavelengths of light that are absorbed most strongly by plant pigments, particularly the PRIMARY (and dominant) pigment in photosynthesis, CHLOROPHYLL a.