Because the base of the stems are being shaded out by the top canopy.

It costs the plant energy to maintain leaves. If the leaves don't get enough light to produce more energy than they take to maintain, the plant will shed them.

Looks like this is indoors under a grow light, so they just aren't getting enough reflected light underneath the canopy to make lower leaves worth it. When you move them outside and they start getting hit with light from every direction, the lower leaves will grow back. Usually.

Thanks Edymnion, you have been more help than you realize in numerous posts. I figured that was part of the reason. New design next year for my grow room!

Perhaps even a cardboard "wall" lined with the matte side of aluminum foil facing the plants to encase the light in and redirect it to the bases of the plants?

I wouldn't worry a whole lot about some of the bottom leaves falling off as long as the taller plants are not affecting the shorter ones. But it does look like you are battling the common problem of overcrowding. You either need more room/lights or less plants. Most of us go with more room/lights and more plants. hahaha
Bruce

"200 plants, and 3 shop lights... Challenge accepted"

Bruce: hahaha, that made me laugh out loud :)

I suggest you use whatever you can to raise the shorter plants so their tops are at the same height as the taller ones. I have used vaseline jars, coffee cups, bricks, small tupperware containers, and empty seedling pots to do this. If the plants aren't all about the same distance from the lights, you get a viscious cycle where the ones closer to the light grow faster than the ones further away. Another thing I do is move all the tallest plants to one end of the growing area with the shortest ones at the other end so I can hang the light fixture at an angle.

Thank you for all your help yet again. Plants much happier and look better after all of your advice. It's simple & logical really once you hear it LOL Challenge Accepted :)