Posted: 05/08/2012 at 14:17
Just to expand a little on the above. Leaves curling downwards at the edges is most often a sign of some sort of contamination by herbicide. Leaves curling upwards is much more common and there can be a number of causes.
Excessive heat can cause it. My toms in the ground are covered with curled leaves because we're into about our third week of high-30sC. It's just the plants' stressed reaction to the heat.
Excessive watering can cause it, particularly with container plants. Pale foliage is another symptom.
Pests - aphids, white fly, etc - can cause it.
Disease can also be a cause. Is there any evidence of disease - specks, spots, whatever - on the foliage? And is the affected foliage old foliage? New? Or are the symptoms all over the plant?