I work in hydroponic farming. If you live in the US then the produce you find in a grocery store has been grown and harvested in severely suboptimal conditions for human nutrition. Outcomes will vary across hydroponic systems but you will see in a few years that produce grown hydroponically under natural light 1) has higher nutritional density, 2) tastes better, 3) has a longer shelf life. Much of this is due to being harvested when the plant is ready and transported relatively short distances rather being harvested early for multi-week cold chain transport.

Good info thanks

Sophisticated growers routinely get lab tests that provide a breakdown by element of the leaves, fruit, or other parts of the plant. This is mostly to assess plant health, but also gives an indication of the nutritional value to humans.

It's possible to manipulate the element breakdown of the edible parts of the plant over a surprisingly wide range with an appropriate nutrient solution. For example, they grow low-potassium vegetables for people at risk of hyperkalemia:

https://www.mdpi.com/2311-7524/7/4/87/pdf

Of course the lab test doesn't indicate whether the elements are in a form available to humans. I'm not aware of any biochemical mechanisms by which hydroponic vegetables with the same element breakdown would be less nutritious, though I'm not sure how you'd prove none exist.

An obvious question would be selenium; RDA is about 50 micrograms, and the upper "safe limit" is 400 micrograms. https://www.webmd.com/a-to-z-guides/supplement-guide-seleniu...

That's an extremely trace mineral.

Where would the plants store useless (to them) elements and for what purpose?

I don't know the biology, so I can't answer specifically, but surely there are many cases of "useless" and even detrimental elements being stored in living bodies. Just look at heavy metal accumulation in animals for example.

Water in the plant, cells, really anywhere. Same thing happens when we eat or drink something. Various molecules enter our blood, organs, etc.