AutumnColorsLitArticle - Davidsonia 14:4 1 11 Autumn...

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111 Davidsonia 14:4 Autumn Colours – Nature’s Canvas is a Silk Parasol The Adaptive Value of Autumn Foliage Abstract The variety and widespread nature of leaf colour change in autumn has led to investigation of the biochemical pathways and compounds responsible. The synthesis of bright red colouration initiated by longer nights prior to leaf abscission in deciduous species points to some adaptive value for this expensive ephemeral trait. It is hypothesized that during the breakdown of the unstable chlorophyll and the dismantling of the nutrient-rich photosynthetic apparatus, red anthocyanins provide a more biochemically parsimonious alternative to the elaborate xanthophyll system. This alternative enables leaves to screen out excess light energy and circumvent photooxidative damage to leaf cells, while allowing photosynthesis to persist at low rates in support of metabolic processes and phloem loading required for nutrient resorption from leaves. Introduction People continue to marvel at the spectacle of familiar green woods and local trees changing into a coat of blazing colours in autumn. Scientific investigations beginning in the 19 th century (see Wheldale 1916) have led to a very good understanding of how this occurs. Research continues today in the fields of physiology, biochemistry, and molecular genetics to further elucidate the complex signalling pathways and mechanisms involved. During the growing season, healthy leaves are green, and appear so due to the high concentrations of chlorophyll within the chloroplasts, organelles which act as microscopic factories converting water and carbon dioxide into carbohydrates and oxygen, using the energy in specific wavelengths of sunlight. Chlorophyll efficiently absorbs red and blue light during this process, but reflects or transmits the green light we observe. In autumn the chlorophyll breaks down and leaves show the yellow and orange colours of carotenoids already present in chloroplasts, but previously invisible because of the overwhelming green of the chlorophyll. In many species, new flavonoid pigments called anthocyanins are synthesized during this period imparting a Robert D. Guy and Jodie Krakowski. Department of Forest Sciences, Faculty of Forestry, University of British Columbia, 2424 Main Mall, Vancouver, BC, Canada, V6T 1Z4. Corresponding author: [email protected]
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112 red colouration, becoming purple as the pH increases (the compounds react with the cellular solution producing a visible colour change). These compounds are stored in vacuoles in the mesophyll and/or epidermal cells of leaves and fruits (Hrazdina et al., 1982). Explaining why leaves develop brilliant hues in the autumn is not so straightforward. One North American aboriginal legend claims that when mighty ancestral hunters slew the celestial Spirit Bear, commemorated in the constellation, his red blood caused the tree leaves to redden in sympathy (Philp 2001). Jack Frost, charged with comprehensively painting every leaf
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AutumnColorsLitArticle - Davidsonia 14:4 1 11 Autumn...

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