Xanthophylls are the typical yellow pigments of leaves. These are oxygenated carotenoids that are synthesized within the plastids. Xanthophylls do not require light for synthesis, so that xanthophylls are present in all young leaves as well as in etiolated leaves.
Xanthophylls in leaves have an important function as accessory pigments, capturing certain wavelengths of sunlight not absorbed by chlorophylls, and thereby increasing overall absorptance of the visible spectrum of sunlight.
There is growing evidence that certain xanthophylls are especially important to plants when exposed to high solar radiation, for preventing damage within the chloroplast. In a process called the xanthophyll cycle, a cellular pool of violaxanthin (a diepoxide) is present in the plastids at dawn, and that compound is changed through an intermediary compound called antheraxanthin (a monoepoxide) into zeaxanthin (epoxide-free) as sunlight intensity increases. At peak solar irradiance, much of the xanthophyll pool therefore exists as zeaxanthin, but this is converted back into violaxanthin for the next day. The purpose of making zeaxanthin is to absorb excessive energy that chlorophyll cannot use, dissipating that unused energy so that the photosynthetic apparatus is not damaged.
In most plants, the yellow coloration is masked by the presence of chlorophyll, and xanthophylls are only revealed when chlorophyll is degraded when the leaf has become senescent (Example: Justicia). These yellow pigments are also observed in leaves with certain nutrient deficiencies (see chlorosis). There are, of course, horticultural forms in which the yellow pigmentation has been selected for leaves and stems, but it is more interesting to find situations in nature where shoots have yellow parts, occurrences that then need to be explained (how and why).