The leaves on tomato plants should be straight with a slight downward curve. While tightly curled leaves on a tomato plant are a sign of stress on the plant, they do not mean that the plant is going to die.
The leaves on tomatoes curl upward and inward on themselves when they are receiving too much water. Overwatering causes this reaction in tomato plant leaves and it is common during wet springs or in areas where the soil is poorly drained.
Downward curling leaves are a sign of herbicide injury. Tomatoes are very vulnerable to growth regulator herbicides, and as a result, may display curling leaves, weakened stems or seedless fruit.
Reduce the water that the tomato plant receives. Do not water the tomato plant again until the first 2 inches of the soil are dry to the touch. Do not spray growth regulator herbicides on windy days where the breeze may take the herbicides to your tomatoes.