in vitro plants need light? - (Jan/02/2011 )

Hallo,
just a quick question: why do in vitro plants (or callus etc.) need light?

I suppose this is becuase there are some light sensitive reactions occuring in plants ? But wich reactions? Any examples?
(I know that photosynthesis is down, so thats not the reason..)

I never had a lot of basic plant science so I wonder what reactions need light?

Different type of plants needs different amount of light.Light is an important environmental factors. Generally, Red light shortens the time necessary for producing the plants. Blue & whit dark light were found to be promotive.

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