Really need to see a photo. It could be anything from a disease or pests or even perfectly normal. New growth is much more prone to mid-day wilting but it disappears as the growth matures. The color could be a need for nitrogen.

But without much more info this is all just a guess. Without seeing at least a pic of the plant and knowing more details about how you are growing the plants, the variety names, if they are in the ground or in containers, size of the containers, age and size of the plant, how and what you feed etc. it really isn't possible to answer.

Dave

Dave, thank you very much for reply. Here are links to pictures of my tomatoes. Please, maybe you know what is cause of wilting.
http://i1093.photobucket.com/albums/i432/Winatasha/home056.jpg
http://i1093.photobucket.com/albums/i432/Winatasha/home049.jpg
http://i1093.photobucket.com/albums/i432/Winatasha/home047.jpg

Here is a link that might be useful:

If the color in the pics is accurate and that really is yellow at the base of the leaves (where it joins the stems) - I can't tell for sure from the angle - then it is herbicide damage from drift or from contaminated mulch. Sure seems to be a lot of that going around this year as there have been many similar posts about it on this forum already this spring.

Any herbicides sprayed in the area recently? You? Neighbors? County spraying for weeds? If will drift for 1/2 a mile. If it was the mulch then I would have expected it to show up sooner so drift is the most likely cause.

Some plants will grow out of it but some won't. If they were mine I would just give them a mild 1/2 strength dose of fertilizer (at the roots only level) and continue to monitor them.

Hope this helps.

Dave