As a part of the A-Z Photographic Glossary of Biological Terms challenge, today it is X for Xanthophyll, as illustrated by this Sassafras (Sassafrass albidum) photographed in the Autumn by the banks of Potomac River in Maryland in the eastern US.
Xanthophylls, from the Greek xanthos (ξανθός, “yellow”) and phyllon (φύλλον, “leaf”), 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 pretty much present in all leaves. The xanthophylls found in the bodies of animals are ultimately derived from plant sources in the diet; for example, the yellow color of chicken egg yolks comes from ingested xanthophylls.
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 (absorptance is the ability of the leaf to absorb radiant energy.)
In the Northern Hemisphere, all the leaves on deciduous trees gradually lose chlorophyll during the growing season, and this loss accelerates just before leaf fall in the Autumn in non-tropical zones. Carotenoid pigments like the xanthophylls that were always present, yet masked by the volume of cholorophyll, are also lost during ageing but some of them are retained after the chlorophyll is lost producing yellow leaf colours.
Depending upon the continent, either the predominant Autumn colour is yellow- Europe and east Asia, or in North America it is red which is the result of the active synthesis of anthocyanin pigments just before the leaves fall; in these leaves the actual shades of red are the consequences of the acquired amounts of anthocyanin, the retention of carotenoids, and some retention of chlorophyll- predominant anthocyanin produce purples, anthocyanin and chlorophyll produce brownish colours, anthocyanins and xanthophylls produce orange hues as in the Sassafras above. In most trees and shrubs the color production is uniform, but in others, like the Red Maple below, we get leaves that are strikingly marked.
For more on why the predominant colours in Europe and east Asia are yellow and oranges, but why in North America they are reds and purples- possibly the evolution of anti-herbivory defences, checkout this recent article.