odd localized intense leaf under curl, causes?

experimenting with
50-50 perlite/peatmoss mix.
water every 3 days.
RH %50-60.
temp 65-80F, but 80F for maybe 2 hr every day or so.
decent air flow in my grow, 400w hps + 196w cfl bay


let "super curl claw" = SCC

The SCC seems localized to a few branches so i didn't think it was due to nutrient excess.
they were transplanted april 1st.
the past couple weeks I occasionally put off watering a day or two and plants suffered but always bounced back in like 24 hours.
Also the SCC happened over a couple days.


I would like to know what specific thing is causing this ----->WP_20130427_006.jpg

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