Plants drying from the bottom up. Why?

[PaloAltoMark]

Hi Folks:

I have some sugar snap peas growing hydroponically in autopots in my greenhouse. All was going well - good growth, flowering etc. and then the leaves started turning brown and drying out starting at the bottom of the plant and then moving upward.

I'm not sure if this is nutrient burn, heat stress (the ambient temp has been in the 70's so it's hotter in the greenhouse), a nutrient deficiency or some other cause.

Any help would be appreciated.