The following is a continuation of the series on fall leaf color in deciduous plants. This post is a continuation on Anthocyanins that cause pink, red, and purple colors in fall leaves.
Kevin Gould of the University of Auckland in New Zealand is at the forefront of research on leaf anthocyanins. Senescing leaves seem to need special protection against bright light exposure because the metabolic pathways for the initial capture of energy don’t lose their efficiency as rapidly as the subsequent processes for processing that energy do. Bright light that reaches senescing tree leaves overloads light-gathering chlorophyll and slows it down (photoinhibition). Anthocyanins can offload some of that excess energy, decreasing photoinhibition, sustaining photosynthesis rates necessary to provide energy for nutrient resorption and other critical processes during senescence.
Anthocyanidins, formed from anthocyanins, are flavanoids: antioxidants that are beneficial to human health and possibly able to help prevent such diseases as cancer, Alzheimer’s disease, and cardiovascular disease. While reading an article on the human health benefits of consuming anthocyanin-rich blueberries, Gould decided to investigate the possibility that the antioxidizing powers of the leaf anthocyanins he was investigating also benefited their source plants. In test tube experiments he found anthocyanins purified from tree leaves were four times more effective at soaking up damaging free radicals than vitamins C and E. He and his colleagues devised a method to induce and observe an oxidative burst of hydrogen peroxide by using a needle to pierce the upper layers of a New Zealand shrub that produced red pimples when pierced by aphids. Gould and his coworkers were able to observe bursts of the powerful oxidant hydrogen peroxide one minute after stabbing leaves with a needle. In red leaf tissues the burst faded quickly, while in green tissues the hydrogen peroxide concentrations soared for at least ten minutes. These results suggest that anthocyanins function as protective antioxidants in plant leaves.
Anthocyanins may protect physiological processes in leaves from cold temperatures. Gould notes that a birch species he encountered in Finland held on to its red leaves year round, despite temperatures that plunged to -40 degrees C. William Hoch of the University of Wisconsin-Madison ranked the intensity of red coloration in autumn of species in nine genera of woody plants either from a cold zone in Canada and the northern U.S. or from a milder maritime climate in Europe. The species that produced the most intense red coloration came exclusively from the North American cold zone.
Linda Chalker-Scott of the University of Washington proposes that anthocyanins help leaves retain water. Anthocyanins dissolve in water, whereas chlorophyll and many other cell pigments do not. Water loaded with any dissolved substance has lower osmotic potential: a decreased tendency for water to flow away. Many plants produce soluble anthocyanins that may help leaves retain water when subjected to osmotic stresses from drought, salt buildup on leaf surfaces, and heat. Loading water with solutes also lowers its freezing point, possibly affording added frost protection to senescing leaves.
The evolutionary theorist W.D. Hamilton and Samuel P. Brown of the University of Montpelier speculated in a recent paper that the healthiest trees might put on the flashiest fall displays of (anthocyanin) red leaves. They further speculated that this leaf signal might give fall-feeding insects, such as aphids, a warning to avoid trees that are healthy and have the best defenses. This is an intriguing possibility for yet another role of anthocyanin in tree protection.
Information from "Why Tree Leaves Turn Color in Autumn" by Jeffrey O. Dawson, Professor of Tree Physiology, Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign. Go to http://web.extension.uiuc.edu/forestry/fall_colors.html for the full article.