Bottom Leaves Shriveling

I am growing two autoflower plants under one grow light, a Bigg1000. Both plants are flowering, one is further along than the other. The plants have been fine except within the last few weeks the leaves on the bottom branches have been shriveling from the bottom up. I am attaching photos. I have read that at the end of a plant's life leaves begin to yellow and fall off. The leaves on my plants are not yellowing but merely shriveling. On one plant, the bottom 10 inches or so of branches have shriveled. On the other the bottom 8 inches or so. Is what I am experiencing normal? I find it odd that both plants are doing about the same thing. My guess is that the canopy has gotten so large that it is overshadowing the lower branches, and those lower branches are not getting enough light. Are my plants dying? Can anyone help me diagnose the problem if in fact it is a problem and not something normal in the flowering stage? What, if anything should I be doing? This is my first grow. Thank you.