Boron Deficiency, Heat Stress, or Something Else?

My 3 week old Alien OG is having some problems. The middle "finger" of one of the leaves is twisting and developing a brown streak. None of the other leaves show this problem and the other "fingers" on that leaf look perfectly healthy... the problem is entirely isolated to this one "finger". The leaf is a newer one, but it's not at the top of the plant (about 3-4 inches from the top of the plant). I moved the lamp up a bit in case it's heat stress. Could this be Boron deficiency? Any help is greatly appreciated!

Pertinent Info:
5 gal DWC
Nutes are Pure Blend Pro Grow, Cal-Mag, Superthrive, Aquashield
pH currently at 5.9 (Cycles between 5.5-6.0, never been above 6.0)
ppm currently at 520
400w MH

Changed the reservoir just in case. The brown streak hasn't progressed at all in the last 10 hours.

Good call on the water change...

I see what looks like mag deficiency on the leave under the one pictured. And the leaves seem to be curling up at the edges.

Are you using tap water or R/O water?

The issues seem to be minor at this stage though...

smithmark914 said:

Good call on the water change...

I see what looks like mag deficiency on the leave under the one pictured. And the leaves seem to be curling up at the edges.

Are you using tap water or R/O water?

The issues seem to be minor at this stage though...

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disagree, from what i can see there is no M def

-possibly zinc because of the curling & streak, wait for further responses though.

i think it was just heat/light stress...MH's kick off the heat when you dont have them behind glass or in a cooltube

massah said:

i think it was just heat/light stress...MH's kick off the heat when you dont have them behind glass or in a cooltube

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With you on this one, kinda. That aint heat stress from general heat, that leaf has been touching something very hot (bulb), probably about 4-7 days ago and now showing.