Autumn Leaf Chemistry: More Than Just Eye Candy

 

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2 thoughts on “Autumn Leaf Chemistry: More Than Just Eye Candy

  1. This reference
    (http://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/leaves/2002_11_leaf_article.pdf) gives more details on our red sunscreen:
    “When surroundings are bright and cold, photosynthetic efficiency often
    declines. The phenomenon, known as photoinhibition, has been attributed in part to impairment in one of the functional elements of photosynthesis. Normally, two units consisting of pigments,
    proteins and electron-transfer molecules—known as photosystems I and II—absorb light energy. Photoinhibition apparently involves a block in photosystem II. Unchecked, this impairment
    can permanently damage chloroplasts, cells and tissues.
    Investigators can observe photoinhibition because when photosynthetic tissues receive a pulse of intense light, they immediately emit a pulse of visible light—that is, they fluoresce. Detailed analysis of this flash reveals much about photosynthetic function. New techniques to measure this fluorescence have helped investigators test the efficiency of the light reaction of photosynthesis under different conditions and to detect photoinhibition. A variety of factors can contribute to photoinhibition: intense sunlight; low temperature; acclimation of leaves to extreme shade with a subsequent exposure to high light; and inadequate phosphorus, which is important in the production of two energy-rich compounds crucial for photosynthesis— adenosine triphosphate (ATP) and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH). When chloroplasts are overwhelmed with energy, the excess causes chemical and, ultimately, physical damage.
    Plants have evolved several strategies to prevent photoinhibitory damage from intense light, in particular the interconversion of certain xanthophyll pigments as a way of quenching the overload of energy. Anthocyanins also efficiently protect against photoinhibition because they soak up radiant energy at wavelengths poorly absorbed by other accessory pigments, such as in the green waveband at around 530 nanometers.
    In intact tissues, the range of absorbance also extends into shorter (blue) wavelengths, overlapping with the absorbance of chlorophyll, particularly with chlorophyll b, one of the two major forms of the pigment. In addition, anthocyanins are very stable compounds in the mildly acidic environment of the cell vacuoles that contain them. The hardy anthocyanins can thus shield the more delicate chlorophyll molecules housed in the chloroplasts.”

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