phytochromes are some proteins that can detect light. they can react to red and far-red light. they work like this:
a phytochrom can be in two different isomers. one of them reacts to red light, the other to far red light. phytochrome is synthetised in the form that absorbs red light. when this happens, phytochrome is converted into it’s active form, that brings about many celullar changes. the phytochrome can be brought back into it’s inactive form if it absorbs far red light.

the rest in two hours, Mr and Mrs Smith is on on HBO.

ok, so where was i? oh yeah, short day and long day plants. do you know what they actually are? contrary to what may seem logical to you, they are not plants that require the day to be short/long in order to flower. long day plants require days to be longer than a critical lentgh in order to flower. short day plants require days to be shorter than a critical length in order to flower. but plants do not detect light. they detect the lack of light, they detect the night length- the length of unintrerupted darkness. how they do this depends on the kind of plant. this can be done either by phytochromes or by blue light receptors(zeaxanthin, cryptochrome and phototropin)

hope this helps..

here are some pictures of phytochrome mechanism from Biology, by Campbell and Reece

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hey can some one kindly tell me whats the difference between red light and far red light????
why short day plants do not flower in long days if we consider p660 and p730 interconversion?????