Why Leaves Turn Red in Autumn

While the blazing red colors of autumn are one of nature’s most beautiful phenomena, until recently the purpose behind this show has been a mystery.  The red pigments, called anthocyanins, are produced in the leaves of many species during autumnal senescence, which is the time when photosynthetic components are dismantled and foliar nutrients, particularly nitrogen (N) and phosphorus (P), are transferred from leaves to perennial portions of the plant for storage.  The recovery of these nutrients, termed resorption, significantly affects plant growth and fruiting the following year, and is therefore important to plant fitness.  

Because senescence can lead to greater vulnerability to damage from bright light, and that anthocyanins have been shown to be effective at shading light in foliage, we hypothesized that anthocyanins in autumn are produced to protect leaves from bright light that may otherwise damage the photosynthetic system.  Protecting the photosynthetic system in autumn is important to the plant because a constant supply of energy is needed to support the many processes of senescence and to drive the translocation stream that carries nutrients out of the leaf.  This idea is termed the resorption protection hypothesis. 

Previous theories have generally viewed the development of anthocyanins in autumn leaves as being coincidental.  For example, a common explanation has been that sugars become trapped in senescing leaves and that these sugars are then converted to anthocyanins.  Other descriptions have suggested that anthocyanins are present throughout the growing season and only become visible in autumn as chlorophyll levels decline.  In fact, anthocyanins are produced midway through the senescence process and foliar sugar levels decline drastically during senescence. 

We tested the resorption protection hypothesis using mutants of species that normally produce anthocyanins during senescence, but were unable to produce these pigments due to the mutation.  Mutants of three species were used: redosier dogwood, Cornus sericea (L.), Elliott’s blueberry, Vaccinium elliottii (Chapmn.) and Sargent Viburnum, Viburnum sargentii (Koehne).  Paper birch, Betula papyrifera (Marsh), was also included in the study to compare the nutrient resorption of a species that does not produce anthocyanins in autumn foliage. 

Mutant and wild-type (non-mutant) plants of these three species, along with wild-type paper birch were subjected to three environmental regimes during senescence: a controlled low-stress treatment consisting of moderate light and temperature, a controlled high-stress treatment where plants were exposed to five days of bright light and near-freezing temperatures before being moved to the low-stress environment, and an outdoor treatment in which the plants were grown outdoors in ambient conditions during the summer and autumn in Madison, WI.  Levels of N were measured in pre-senescent and senesced leaves, and the condition of the photosynthetic systems were determined by measuring photochemical efficiency throughout the autumnal senescence period. 

Within the low-stress environment, no differences were observed in photochemical efficiency and nitrogen recovery during senescence between mutant and wild-type plants.  This demonstrated that the mutants were not inherently weaker than wild-type plants and that when not under bright-light stress, mutant and wild-type plants were equivalent during senescence. 

In contrast to the low-stress environment, mutant plants exposed to the outdoor and high-stress environments displayed significantly lower photochemical efficiencies and reduced nitrogen recovery than did wild-type plants containing anthocyanins.  The lower photochemical efficiencies of mutant plants indicated that the photosynthetic systems of these leaves had become damaged by bright light.  Subsequently, these mutant leaves had a lower capacity to recover N during senescence.  These results indicate that anthocyanins in wild-type plants were protecting the senescing photosynthetic systems by shading the leaves from excess light.  This conclusion is further supported by the fact that the onset of lower photochemical efficiencies of mutant leaves was coincidental with the development of anthocyanins in wild-type plants. 

Paper birch, a species that does not produce anthocyanins during autumn, recovered N as effectively as the anthocyanin producing species in all three senescing environments.  This species also displayed higher photochemical efficiencies during senescence than did the anthocyanin producing species.  This indicated that the photosynthetic system of B. papyrifera experienced lower stress levels than those of the anthocyanin producing species despite the lack of foliar anthocyanins.  It is known that plants have numerous ways of protecting against bright-light stress, and anthocyanins are only one of these mechanisms.  Thus, paper birch was able to protect itself during senescence without using anthocyanins.  

These findings are consistent with other observations of autumnal anthocyanins, such as the large amount of anthocyanins produced in leaves exposed to full sunlight while leaves shaded within the canopy of a plant produce fewer or no anthocyanins during senescence.  Also consistent with the resorption protection hypothesis are observations of higher levels of anthocyanins produced during senescence in plants native to regions where cold temperature are common in autumn.  Cold temperatures reduce the capacity of plants to utilize light energy and therefore compound the effects of bright-light stress in leaves during senescence.  Thus, the shading provided by anthocyanins would be expected to be of greater benefit in cold climates. 

A full report of this research will be published in the November 2003 issue of the Journal Plant Physiology.  This research is part of a larger project supported in part by the Wisconsin Nurseryman’s Association. 

– Dr. William A. Hoch*, Dr. Eric L. Singsaas# and Dr. Brent H. McCown*

 *Department of Horticulture, University of Wisconsin-Madison
# Department of Biology, University of Wisconsin-Stevens Point

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