It looks to me like to me like the start of a potassium deficiency. Here is an example I pulled off the web.

Potassium deficiency. Could be ph, could be because potassium is depleted or root issues how old are they?

It is most likely potassium (k) deficiency. Happens often when folks mix their own nutes in soils, or don’t watch their ph. If making your own soil, use Subcool’s recipes to start. It’s a good program for beginners and pros alike. When feeding use good tried and true nutrient programs. Most will have balanced levels of nutes that work well for most strains.
Always start at half strengths, and observe plants. Move up carefully, and keep good notes on all your grows/strains. Spend lots of time observing, and take a few notes everyday.
After 35 years I have made almost every mistake you can, and have over 20 years of notes. Still learning with every grow.
You are not a real grower till you’ve killed a few,(i’ve Killed my share).
Also keep some of Jorge’s books around, lots of good pics.
Good luck
RR1

Sorry, also if you feed your plants too much Epsom salts (magnesium sulfate) you can cause problems like that. Saltsof all types can burn your plants.
I personally like to use cal/mag with my feeding, and foliar feed once a week with 1tsp of Epson salts to 1 1/2 gal of ph 6 water.
And your girls will like a bath now and then.
RR1