Prior to 1898, three factors were considered in the study of color in organisms:17

1. The conspicuousness of the color phenomenon to ensure the survival of animals and plants

2. The relation between color and evolution theories

3. The importance of colors in comparative physiology

As can be observed, multiple functions were assigned to pigments, and studies regarding them were greatly impelled. Four kinds of compounds are considered by virtue of their ability to impart color in living organisms: chlorophylls, carotenoids, anthocyanins, and phycobilins. Other pigments are less important as colorant sub-stances.14 The next sections describe other functions of pigmenting substances, in addition to their property of imparting color. Their nutraceutical properties will be emphasized in Chapter 10.

1. Tetrapyrrole Derivatives

Among the linear tetrapyrrols, phytochrome bilins are very important for the survival of green plants.2 They mainly function at the biochemical and transcriptional level (mRNA species), regulating the activity of many enzymes involved in essential processes (Table 6.4). It has been established that the biological properties of phy-tochrome result from the conversion of Pr to Pfr; indeed, photoconversion goes through several intermediates. Plant life cycle is regulated by phytochrome. The vegetative growth of fully de-etiolated green seedlings is strongly regulated by phytochrome (phytochrome is highly expressed in young, expanding cells recently

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