Nitrogen def / lack of light?

[BHill]

Good evening fellas,



Hoping for a little help/advice here if possible.

I have 6 plants, Feminised Critical Kush. Barnys Farm. They reside in a 1.2 x 1.2 x 1.8 tent under a 600w hps.

I switched to 12/12 4 and a bit weeks ago, 31 to be exact. Plants have been ok up until the last 4-5 days when a number of leaves started to yellow. I assumed it is a nitrogen deficiency, they get less in flower yes?, my friend reckons it is lack of light as its only happening in spots that lack light i.e against the walls of tent etc. 2 of the larger plants in particular seem to be affected most, these 2 are very close to the intake which he again said could be the issue. Below are pics of the leaves on the two plants. Is it a nitrogen def/lack of light? I can only guess from going off what I read, I haven't really had any issues in the previous few grows so I am not sure what it is.


I am a bit stumped so thought I would seek some help on here. Any help would be greatly appreciated.


Photo

Media - Canna Terra Professional Plus

Nutes - Canna Terra line

Temps lights on - 26/28

Temp lights off - 20/22

Humidity lights on - 45/50%

Humidity lights off - 55/65%