yellowing of leaves

Hi ,Would like some advice please. This is my first grow and on day 49 from seed. Around day 40 one of my Fast n Vast auto’s and Critical Mass auto’s suddenly started yellowing heavily at the bottom (both had started flowering) and dropped a tonne of leaves. They are grown in plagron light mix...

I'm currently running GSC auto by fastbuds. It's my first micro grow. I prepared well before I started and even though I have to say that these plants seem to me to be very forgiving, I have had no trouble whatsoever in getting them to their 6th flowering week. They do grow a lot slower then...