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What causes greening in potato?

Exposure of potato tubers to light either in the field, in storage, on the store shelf, or at home, will induce the formation of a green pigmentation on the surface of the potato. This is called "greening" and indicates the formation of chlorophyll. This pigment is completely safe and is found in all plants, lettuce, spinach etc. It is primarily found in leaves and is responsible for a plant's ability to make food, photosynthesis.

Greening is strongly affected by three light factors: quality, duration, and intensity. Chlorophyll is green because it reflects green light while absorbing red-yellow and blue light. Chlorophyll formation is most efficient under red-yellow light. Under green light, there is practically no potato greening and there is little under blue or ultra-violet lights. Day-light fluorescent lights are quite capable of inducing greening, more so than incandescent light. The longer and more intense the light, the more greening will occur.

A fourth key factor is temperature during light exposure. This is important because greening is an enzymatic response and enzyme activity is increased with increasing temperature. There is no greening when temperature is less than refrigeration temperature, and is most rapid room temperature.

Source : http://www.panhandle.unl.edu/potato/index.htm