is normal man they turn yellow and die when they don't receive enough light if I was u and u have started to flower 1 week ago give the lower sections a good prune cut off and bud sit that does not receive much light this will enable your plant to focus more on the top bud sits and vegin for 4 months that's gonna be one massive beast
I'd also do a little lst try to tie branches out wards to allow more light in the plants and more space for bud growth just get some string and gently tie branches outward so it opens up
I keep 3-4" clean from the bottom. It allows room for my auto-drip system, and it is conducive for good airflow and viewing of the tops of the pots of dead plant matter.