It sounds like your plants have been sunburnt. You can't take a plant that has been germinated indoors and expose it to UV light. A plant grown outside is exposed to UV light from the moment it sticks its' head out of the soil. Not so indoors. That is why you have to harden off plants grown indoors. You harden them off by gradually exposing them to more and more sunlight, starting with a couple of hours in the shade. Those that have been badly sunburnt may not make it, but those that only have a little damage should come back. Just keep them out of the sun and use your grow lights. Harden them off when they get bigger

Thanks. I am very surprised that seedlings could get burnt in May in MA! I was worried that they weren't getting enough light! Thanks for your response.

Here is a photo:

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As I've already stated in your other post, you need to put the plants in larger containers. They are probably root bound and need room for the roots to spread out.

Is that a yogurt container? Any how, it looks to be plenty big enough judging by the size of the seedling. I doubt that being root bound has caused the leaves to look like that. I agree that it is sunburned, and possible wind burnt from the open window.

That container is just fine for that size plant. Anyhow if it were root bound the leaves wouldn't look like that. The leaves would be yellow and the plant would fail to thrive.

Yours look to be normal growth. And yes, they do look to be sunburnt or wind burnt, but not damaged beyond repair. UV light is the same in May as it is in July. Imagine taking a person who has never been exposed to UV light and putting him in the sun. Even in May, That person would have a dandy sunburn. The same with plants.

As I said before--put them under the lights and harden them off before they go outside permanently.

The leaves would be yellow and the plant would fail to thrive.

The top leaves look yellow to me!

Yes, this is a yoghurt conainer (with drainage holes), but the plant is not root-bound. The cucumber that I mentioned is in in a larger container (3 by 3 inches - a real one designed for plants!), and is also not root bound. I am hoping you are right that it is sun/wind damage as these I can control (at least for now) - I much prefer this verdict over hearing that I have a fungus or bacterial infection. So I'll moderate the light exposure & be careful to harden-off gently. Thanks for all the comments!