Yellowing/dying leaves at bottom?

Some of these leaves consist of drying up and dying, some yellowing. My plants are huge now and I expect that some of it is due to lack of light penetrating the canopy, therefore the lower leaves do not have sufficient light; correct? Furthermore, I believe some of this may have been caused due to my ph levels. Although they have consisted within the region of 5.8 - 6.4, I have chosen to flush with a 5.9-6.0 solution of water over the past 3 days. I hope what I have done is right just incase my ph were of inequal levels. I also believe that due to the weather in my country being extremely hot over the past month or so, degree's have risen to as much as 105 degree's farenheit. I know this is terrible, but there is nothing that I am able to do in order to lower the temperature. Luckily, my country is beginning to go back to normal weather so things should only get better! I would like an opinion from several people in order to justify what I should do to fix this issue. I have also decided to move to flowering stage today and it will begin in less than 8 hours. All plants show signs of maturity (internodes not on same level and all female sex shown) so I don't see why I shouldn't start today; right? Lastly, one of my plants has always been lacking in growth overall due to poor care at the beginning (rookie mistakes) but it has come back to its poor quality again. Some leaves show brown spots and some at the TOP are yellowing overall. I would really appreciate some help. Thank you. (Apologies for quality of picture(s)).

AK 47 Pineapple Chunk White Widow White Widow

what you feeding it and is it in just perlite?

No it is grodan rockwool cubes, the extremely small cubes in vast numbers, and general hydroponics.