Why Leaves Turn Red
1. Until recently, a popular misconception concerning the functions of anthocyanins was the belief that by increasing the rate of metabolism in leaves, these pigment molecules are able to warm the leaves as well as protect photosynthetic structures from intense sunlight. In addition, it was also thought that anthocyanins are capable of protecting plant tissues against Ultraviolet (UV) radiation, based on observations that UV radiation has the ability to stimulate the synthesis of anthocyanin. However, in actuality these pigments are not well suited for functioning as protection against UV radiation, because they do not strongly absorb wavelengths (UV-B) that are most damaging to the plant tissues. Also, these pigments are produced internally and are thus in a difficult place to effectively provide protection to leaves from UV.
2. One way bright light may cause damage in plants is through photoinhibition-
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Photoinhibition refers to a phenomenon in plants marked by a sharp decrease in photosynthetic capacity when exposed to bright light. There is a general consensus that the primary target of photoinhibition is photosystem II. In the presence of intense light, chloroplasts can be overwhelmed with energy which can result in damage to the plant both chemically and physically. However, anthocyanins can reduce photoinhibition by absorbing the wavelengths of light that are otherwise scantily absorbed by other pigments. Furthermore, while anythocyanins reside in the slightly acidic vacuoles of the cell, they remain very stable. Therefore, these resilient compounds can also protect the more fragile chlorophyll molecules within the chloroplasts.
Besides serving as protection against photoinhibition, there is also evidence that anthocyanins inhibit superoxide production. When the leaves of plants absorb excess sunlight, the production of reactive oxygen species and free radicals is increased. Since free radicals are molecules with unpaired electrons, most are very reactive and unstable. When a point is reached where the naturally occurring antioxidants can no longer slake these free radicals, they can become destructive, obliterating the nearby biological equipment and may even result in the death of the cell.
By absorbing green light anthocyanins appear to have the ability to hinder damage by photooxidation. Their absorption of green light serves as a barrier, blocking the underlying chloroplasts from the wavelengths of light that it cannot utilize in energy production. In addition, the light energy absorbed by anthocyanins does not get transferred to the chloroplasts. Rather the energy has one of two possible fates: it is maintained in the same vacuole that the anthocyanin is contained in or most times it is slowly dissipated and released as heat.
3. The colorful aspects of anthocyanins have been accentuated to serve as warning signals against predators such as insects and microbes. The red colors of the leaves alert predators and discourage them from consuming the leaves. However, this deterrence of herbivores during the autumn season would only be effective and provide a selective advantage to the plants if they could be protected in the following seasons as well. An experiment has shown that the autumn colors may serve as warning signals against autumn colonizing pests such as aphids. Therefore, the aphids do not lay their eggs on the trees containing the anthocyanins and thereby prevent the leaves from pest invasion the subsequent year, when the eggs would have hatched had the eggs been laid.