funny thing in the garden. any ideas?

okay, this all began 2 weeks ago this past saturday. it happened on a night that got down to about forty degrees, and when i, and my girls awoke, the top half of one plant was not tracking light. Not drooping, but doing that thing where they are not standing up to the sunshine. we are now up to six plants doing the same thing.
there is no hemp canker.
they are not overwatered.
and keep in mind, just the top half of the plant is doing it. the bottom half is still tracking light.
i am treating for fungus gnats, but do not believe the symptoms fit fungus gnats.

also, no discoloration, but that first plant that did it, has not grown any taller since then, when her peers have gained 12 - 16 inches in that time.

any ideas???
 
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