Cannabis is "designed" to grow in full sun, all day every day.
To me it looks like water damage and it is also quite normal for the first/lowest leaves to die off early.
Tying around the leaf stem probably doesn't help either. Tie to branches only and never tight, this can lead to death by reducing nutrient/water flow in the branch/stem. Always tie a loop with plenty of room for the branch/stem.
Hope this helps, Organoman.