My tomato seedling are purple on the underside of the leaves. What does this mean, and what should I do about it?

A great diagnostic site:

http://plantdiagnostics.umd.edu/level2.cfm?categoryID=59

Go down the page to purple coloration. Sounds like this might be the problem. Good to know, I had this happen years ago. Hope it helps.

Try checking this months OG magazine, or no it was last month's, anyway it has a tomato diagnostic guide too. Purple leaves mean the plant is too cold.

Ellen

Purpley plant leaves generally indicates a phosphorus deficiency which could be from a soil that is too cold, too wet, too dry, for the bacteria that feed the plants to be busy, or maybe, possibly a general phosphorus deficiency in the soil.

Thanks for all your replies. I think it may be too cold out on my porch, which is only partially heated. I'll add some liquid bone meal and see if that helps the phosphorus deficiency.

I'm Z4-my tomatoes are still at the cotyledon stage-LOL. Seriously.
My guess-too cold and like Mr.Kimm said,the plants can't take up the phosphorus when it is cold...not sure about adding that liquid P... if it is too cold, the plants will not absorb it anyway...can you bring them in out of the cold for awhile?
I gardened on the seacoast of NH once upon a time and tomatoes went out mid to end of May.

I've got some rather purple looking tomato plants too. It's been quite cool in the room I've had them in.

My question is, will they outgrow this when it warms up? They're very healthy looking plants other than the purplish undersides.

Topofthehill - to answer your question, my porch is warmer now, and I added a heating pad under my tomato seedlings, plus I put some liquid bonemeal on them and the purple is now almost gone. Yea!