by Sammy Stein

Created on: September 25, 2009

Leaves are the chemical factories of the leaf, converting sunlight energy through a series of cycles and chemical reactions, combined with atoms from water and carbon dioxide molecules, to re-arrange them into simple sugars. These are the basic building blocks for any number of fats, lipids, poteins and other products the plant needs for growth and physiological processes.

To do this using the energy stored in light, the leaf uses a pigment- the main one is chlorophyll (which is green) to capture the energy (molecules in the chlorphyll are excited by the energy, so the energy is captured by the pigment) but this reacts with all colors of the spectrum apart from green light (which it reflects - hance the green color we see). To also make use of other parts of the light spectrum, plants have other pigments - including flavanoids and caratenoids- which are red, yellow, orange and any shade between depending on the color of the leaf. Usually, chlorphyll is present in such quantities that it masks the other colors so we seen the leaves as green (apart from some seaweeds and other aquatics which may be red, brown or blue depending on what part of the light spectrum they use).

Normally, there is so much more green pigment than the others that leaves appear green. Different plants have differently shaded greens according to the density of chlorphyll and where they live. In fall, the leaves withdraw this chloropyhll and so we are able to see the other pigments - the red , orange and yellow colors because these other pigments use different parts of the light spectrum (and reflect different parts, giving them their particular color). By withdrawing chlorophyll the plant saves energy as chlorophyll takes a lot of energy to produce (and also is involved with iron molecules which act as a catalyst and may be rare in the soil so it makes sense for the plant to retain them plus the pigment). We can also see, with the chlorphyll gone, the waste products of the plant which are stored in the leaves.

Together with the other pigments, these give the leaves their autumn hues and tones. An abscission layer forms at the petiole or stem (depending if the leaf is sessile or has a petoile) and the leaf drops off, creating a carpet of gorgeous, but useless to the plant, leaves. This fall color is beautiful and we enjoy it but for the deciduous plant, it is simply a way of saving energy, ridding itself of waste and preparing for the long cold winter ahead.

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