Yellowing of leaves is called chlorosis. The green pigment in plants is chlorophyll. In most plants, chlorophyll synthesis requires light. Gymnosperms, such as pines, are an exception and can form chlorophyll in the dark. For most plants, no new chlorophyll can be made in the dark. The existing chlorophyll has a limited life and eventually degrades. When the chlorophyll degrades, it reveals the yellow cartenoid pigments that were there all along but just masked by the green chlorophyll. The degradation of chlorophyll is also the reason why tree leaves change color in the fall. If you need details of the biochemistry of chlorophyll synthesis, do a google.com search for chlorophyll biosynthesis. References Why do the leaves change color in the fall?
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