I've been growing my cherry tomato plant from seed for around 2 weeks now and it's now a very tiny plant. I've noticed that its leaves open and close overtime and of course, that's it absorbing sunlight. But yesterday, I noticed that it wasn't opening for the whole day. I'm a little concerned about it.
It only has 2 leaves. And it expands during daylight and closes at night which it stops doing now. It is in a medium sized pot next to my window and the temperature is at around 20-30.
I didn't realize you were talking about a pre-transplant seedling when I answered (so, I don't think what I said after this section would match your issue; if it hasn't been growing well, I'd probably give it some monopotassium phosphate in the soil, instead, to help the plant on its way to maturity, to help with cold tolerance, to help the roots, and to help it absorb water; monopotassium phosphate is safe for small plants, in my experience, but if you have high levels of phosphorus or potassium, which I doubt, then it may hurt—but don't do a foliar spray; don't add nitrogen until it's older and significantly bigger; don't overwater). I agree that the plant is probably too cold (phosphorus does help plants to grow better when it's cold, however). I've had small plants behave similarly in cold conditions, but monopotassium phosphate perks them up. If the undersides of the leaves are purple, that's an additional sign of phosphorus deficiency (and that happens more when it's cold, in my experience). You may try a heat mat, too, but it may have already depleted its phosphorus reserves (which a heat mat may not correct).
The link I gave says not to apply monopotassium phosphate to plants that are under stress, but my experience shows that it's helpful for young plants under cold-stress, or that are under stress because of phosphorus and/or potassium deficiency.
It's possible that extra phosphorus could have helped my outdoor tomatoes that I mentioned in the next section, too.
It's difficult to tell for certain if I know your problem without a photo, but it sounds like your plant is stressed, and not acclimatized to your growing conditions. That's surprising for a cherry tomato (but not all cherries are hardy horses).
If this is your issue, at its current stage, the plant will probably not recover, especially if left in its current conditions, and maybe not otherwise, either. The fruits you get, if you get fruit, might end up seedless (that was my experience). Perhaps the soil is too clay-like for its tastes (maybe it's used to containers and potting soil), or the air is too dry. I had similar issues with Sophie's Choice and Cherokee Tiger Black Pear last year (the vast majority of my 101 varieties didn't have those issues). It's not an utterly unheard of problem.
On the other hand, it may have copper deficiency (which it probably could recover from, if that's the only problem), which can cause leaves to behave something as you describe, too. If you do organic gardening, copper deficiency is more likely than if you don't, I've read. Copper (probably copper sulfate) is what makes some synthetic fertilizers blue. Copper can help with water absorption, and it can have anti-fungal properties. If the plant's leaves are only showing their undersides, that may have to do with copper deficiency, maybe too much light or something.
