You have a problem with one stem as far as I can see... and, in your diary, you mention several of them falling over because of loose soil. I, too, am trying to grow outdoors and have had two fall over for the same reason... all I did was to push some soil up around the stem a little bit and I also had to "stake" one because the stem was pretty weak. I don't think this is heat stress at all... for one thing, the temps you report in your diary are actually pretty low! And, again, this has been MY problem. They need temps up around 75-80F - growth is dramatically slowed in the vegetative state at 65-dgrees... Another problem you're having is all the rain you report! Have you ever tested the PH of the rain water? I live in Maine and always thought we had acid rain because that's what we're told... but when I tested my rain water, the PH was 7.4 which is much too high for cannabis... Since my plants are in pots (like yours), I make sure I move them under something when it rains so they will not get rain water on them... and to deal with the cooler nighttime temperatures, I bring them indoors (although if you're growing something else indoors, beware you may be bringing in bugs). I have a separate tent they go into - the lights give them warmth AND increased "sunlight". The change has been remarkable.
So here's what I suggest... pull up some of your soil around the stem - not much, but enough to support it better - start giving them some grow nutes at about 1/2 strength (they're needing some NPK right now) - give them as much light as you can (and sunlight is WAY better than artificial, but artificial is WAY better than nothing), don't give them anything until the soil is really dry, and finally try to get them warmer. I noticed in your diary you're putting a plastic bottle over them - this might be a bit too humid for them which runs the risk of fungus (although there's no sign of fungus now so don't worry about that). It would probably be good if you could erect a hoop tent over them to protect them from harsh wind, pelting rain and cooler temperatures...
Hang on... don't give up... they WANT to survive - and they're WEEDS!!!!
Good luck!