The tender leaves of some plants and trees are brown in colour. Why?

Brown, purple or wine-red coloured new flushes of plant growth are due to a group of pigments called anthocyanins. Several plant species of the family Myrtaceae(e.g. Eucalyptus), especially in the tropics, exhibit such pigmentation pattern. The primary role of anthocyanins and similar compounds in new leaves is to protect the developing photosynthesis machinery from the damaging effects of ultraviolet spectrum of sunlight. In addition, anthocyanins guard the photosynthetic pigment, chlorophyll, from intense light, which makes chlorophyll get over loaded leading to loss of efficiency (termed `photo-inhibition'). In temperate regions, older leaves change colour during the fall just before falling off. These old leaves are also subject to photo-inhibition as much as young foliage. They use pigmentation as a means to perform efficient photosynthesis until they fall off so that the tree can have reserve for the winter.

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Anthocyanins are also known to act as `anti-oxidants' protecting cellular components such as membranes, DNA and protein from the damaging effects of highly reactive oxygen species. Bursts of a common oxidizing agent, hydrogen peroxide, are usually seen at sites of insect damage. In anthocyanin containing leaves these bursts are short-lived causing minimal damage. It is possible that plants use anthocyanins to minimise damage to their young growth.In addition, in most plant species the odd-coloured foliage is not preferred by herbivores and ants, thus offering protection to new outgrowth. Certain precursors of anthocyanins also offer protection against some disease-causing fungi.