“Why Leaves Turn Red”—and Cranberries, Too?
Since joining USDA-ARS and Horticulture, UW-Madison last year, Jyostna Mura has been researching, absorbing, and sharing published work on cranberries, and in adjacent fields that might relate to cranberries. Many times in the last 18 months, she’s offered me a paper I hadn’t seen before. Though this one (Why Leaves Turn Red by Lee & Gould) is from the field of forestry, and it tells some of the backstory and some of the physiology of anthocyanins in leaves. This paper has helped me understand what kind of questions to ask as we deepen our research into fruit color and cold tolerance. Lee and Gould’s subtitle tells the story: “pigments called anthocyanins probably protect leaves from light damage by direct shielding and by scavenging free radicals.”
Colorful fall leaves have been studied for more than 200 years, but until the late 1990s, researchers had only been able to scratch the surface. What we learned in elementary school, “leaves change colors because green chlorophyll breaks down, unmasking the base color of the leaf—but the base color isn’t very important,” turns out to be only a small part of the true story. The authors did a study of the prevalence of several types of pigments (rhodoxanthin, betalain, and many more), and found anthocyanin is the most widespread red pigment—it occurs in about 70% of the plants in their research forest.
Anthocyanins have received lots of research attention—but lots of it came up with wrong answers. For example in the 1830s it was “discovered” that anthocyanin was produced as a byproduct of chlorophyll breaking down—oops! Lots of other wrong trees were barked up: “we see anthocyanins in fruit and in flowers—so their main function is to attract pollinators and seed dispersers.” Because color is easily tracked in genetics experiments, geneticists after Mendel used anthocyanins as a marker without understanding them deeply; and molecular biologists measured anthocyanins as a means of tracking which genes were activated by light exposure.
So “what do anthocyanins do? Why do leaves produce anthocyanins?”
Anthocyanins are water-soluble bright red, mauve, crimson, purple, and blue color pigments found in many plants’ fruit, flowers, and leaves. Anthocyanins are intensely colored under acidic conditions. Many researchers believe that the accumulation of anthocyanins is a way for plants to soften the negative effects of stress. Plants produce anthocyanins when they are faced with stresses: drought stress, high-temperature stress, high metal, high-light stress, low-temperature stress, as well as herbivore and pathogen attacks. Anthocyanin accumulation is one of the effective photoprotective strategies for plants. But why do plants need protection against light?
In winter, low temperatures can inhibit the electron transfer necessary for photosynthesis. It can also inhibit the production and translocation of carbohydrates. Low temperatures do not affect the capture of light energy by leaves, but they do affect the activity of enzymes involved in the photosynthesis cycle. When leaves receive more light energy than can be used by photosynthetic organs, they reduce quantum (photosynthesis) efficiency due to excess energy. Under low temperatures and intense light, the chloroplasts of plants are overexcited, and the excess light energy spurs the plant to form free radicals. Free radicals destabilize otherwise stable molecules, and the result is photo-oxidative damage to the photosynthetic mechanism. Photoinhibition damage occurs when plants receive more light than they can use—so anthocyanins often function as “photoprotective pigments”, reducing the amount of light penetrating the leaf, and preventing damage caused by excessive incident light.
Autumn leaves of the Asian species Cornus kousa (a) and the North‐American Vaccinium corymbosum (b) showing that anthocyanin production is sun‐induced. Shaded parts of leaves remained yellow due to the lack of anthocyanin, whereas sun‐exposed parts turned red. Photos: E. I. Arndt. New Phytologist, DOI: (10.1111/nph.15900)
When plants produce anthocyanins, they store them mainly in vacuoles in the upper layer of the leaf epidermis, but also in the cell wall, chloroplast envelope, and cell nucleus. Anthocyanins form a light-shielding layer on the plant foliar surface, actively filtering a portion of the light energy by absorbing radiation between 270um and 280um. This light does not reach the photosynthetic mechanism to spur free radical formation. In addition to this “sunscreen” effect, anthocyanins also scavenge free radicals. Free radicals may be produced by an excess of light in other wavelengths, and these free radicals can be safely stabilized by the anthocyanin so damage is not done to the delicate photosynthetic mechanism.
So plants produce anthocyanins specifically when light intensity is high, but temperatures are low enough to slow photosynthesis. They produce anthocyanins to protect themselves against the damaging effects of collecting more energy than they can use. After centuries of study, it is exciting to develop this understanding of our favorite shade of red. There may be more mechanisms at work, and there may be more uses for anthocyanin still to be uncovered—but this helps us to understand the cranberry’s strategy as we move into and through harvest.
David W. Lee and Kevin S. Gould
Why Leaves Turn Red
D. J. KYLE, I. OHAD*, AND C. J. ARNTZEN
Membrane protein damage and repair: Selective loss of a quinoneprotein function in chloroplast membranes
Eva-Mari Aro, Ivar Virgin, Bertil Andersson
Photoinhibition of Photosystem II. Inactivation, protein damage and turnover
Chilling injury in chilling-sensitive plants: A review
Lukatin A; Brazaityte A; Bobinas C, Duchovskis P
Image credit: New Phytologist, Volume: 224, Issue: 4, Pages: 1464-1471, First published: 09 May 2019, DOI: (10.1111/nph.15900)
Palma, C.F.F., Castro-Alves, V., Morales, L.O., Rosenqvist, E., Ottosen, C.O. and Strid, Å., 2021. Spectral composition of light affects sensitivity to UV-B and photoinhibition in cucumber. Frontiers in plant science, 11, p.2016. https://www.frontiersin.org/articles/10.3389/fpls.2020.610011/full
Gould, K.S., 2004. Nature’s Swiss army knife: the diverse protective roles of anthocyanins in leaves. Journal of Biomedicine and Biotechnology, 2004(5), p.314. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1082902/
Yu, Z.C., Lin, W., Zheng, X.T., Chow, W.S., Luo, Y.N., Cai, M.L. and Peng, C.L., 2021. The relationship between anthocyanin accumulation and photoprotection in young leaves of two dominant tree species in subtropical forests in different seasons. Photosynthesis Research, 149(1), pp.41-55. https://link.springer.com/article/10.1007/s11120-020-00781-4 Lee, D.W. and Gould, K.S., 2002. Why leaves turn red: pigments called anthocyanins probably protect leaves from light damage by direct shielding and by scavenging free radicals. American Scientist, 90(6), pp.524-531. https://harvardforest.fas.harvard.edu/sites/default/files/leaves/2002_11_leaf_article.pdfThis article was posted in Cranberry and tagged Allison Jonjak, anthocyanin, anthocyanins, Cranberries, Jyostna Devi Mura.