Hardening off - Accomodation and Adaptation

Going to use indoors by a south facing window as an example.  Plant gets min. sunlight but is plenty warm because your house is warm.  It is early spring, night temps only get down to 50.  You want to put your plants outside.  Of course you have to harden them off / introduce them to the new conditions slowly.  We've talked about methods.  We've talked about why.  But I dont think there is a conversation about what it is the plants are doing while we are hardening them off.

Temperature - .If you suddenly change the temp from 75 degrees to 50 degrees, you will have sick plants.  But if you introduce them gradually to lower temperature, they adapt.  What is changing in the plant to let them deal with the lower temps?

Sunlight - If you go from min. window light to full direct sunlight, chances are you will get some burnt leaves.  But if you do things like give them a little each day by moving them in and out or putting them in a partial shaded area, well then something changes and the plants can manage.  What do you suppose changes?

Wind - This one is probably a no brainer.  Stems get thicker the same way we grow scars.  Moving the stem back and forth makes tiny little things break and rebuild, making the stem thicker. 

My Guess is that there is something about natural sunlight that stimulates the growth of what ever it is that protects the plants from natural sunlight.  Hard to imagine the same is true of cold, so I wonder if what ever it is that protects them from lower temp is not triggered by the sunlight rather than the cold.

Thoughts?  Other guesses?

Guesses:
 
Temperature - Maybe the expansion and contraction of the plant cells. When indoors, the high and low differences are relatively close where outside the temperature can be 90 degrees (even if not 90 degrees outside, the plant cells are sitting in direct sunlight, cooking per-say) and then down to 50 degrees at night. The cells would be expanding and contracting to a vastly larger percentage than it was indoors. The softer cells (leaf cells) would likely take the bigger hit since they are less densely packed and the cell walls are not as rigid as say the stem cell walls. Also, the sheer temperature differential shock. Have you ever seen liquid nitrogen poured onto a counter top? It boils. Even though the counter top is only room temperature. Theory of relativity.
 
Sunlight - Same requirement of acclimation as temperature, kind of like getting used to the dark like stepping into a closed room with the lights off at night, or getting used to the light when you step outside from a dark room at noon without glasses on. Even once your pupils dilate or contract (taking out that physical argument) you still need a few minutes to get used to the light change.This in combination with heat could maybe ruin cell walls. Not to mention that the direct sunlight itself would be a cause of heat. Percentage of UV may also be a factor.
 
These are just guesses. Even if I am correct, there are probably more factors such as chemical reactions being different with the different environments. Have fun!

     Hardening to low temps might have something to do with pathogen resistance. Cool temps slow growth, and a plant that isn't growing will be more susceptible to bacterial or fungal attack simply because it doesn't the energy needed to fight disease. Getting used to cool temps might involve stepped-up transcription of R genes (plant immune system).
     Hardening to sunlight involves developing protection to UV. Plants produce phytopigments like anthocyanin to absorb UV before it can damage sensitive tissues.
     Along with stem lignification to provide mechanical strength, plants also need to adapt to the drying effects of wind, intense sunlight and humidity swings. This involves stepping up cutin production in leaves and stems. This creates a waxy, waterproof coating on tissues that helps prevent water from escaping. Also plants need to get used to closing stomata (breathing pores in leaves) in response to dry conditions. 

Looks like UV-B causes plants to start producing sunscreen. Those same UV-B rays get filtered by glass, shade cloth, etc. I am guessing.

http://phys.org/news/2011-04-experts-reveal-dont-sunburn.html

Am wondering if this explains why I can go from HPS lighting to outdoors without hardening off for light.  Not wind either because have lots of fans on them.  Maybe they built their own sun screen to protect from my lights.  Now temp, that is the opposite cause in the HPS area it is about 90 degrees right now.

Metal halide bulbs put out the most UV-B, but apparently high-pressure sodium lights do put some out. Interesting!

suchen, there is 1000 watt HPS on one side and 1000 watt MH on the other.  The original idea is both would be on light tracks so the light would overlap everywhere in that area.  I ran out of money, so the way they are set up there is an area in between that certainly gets both, but the ones I experimented with came out of the HPS side.  Now that you said what you did, going to try it with a few from the halide side.

Anything that goes from flourescent right into full sunlight turns yellow / whitish on the first day.  A couple flat fried.  Love experimenting but hate loosing plants.

That would be a great experiment, but I know how you feel about losing plants. Just grow more! I grow under cheap-as-can-be fluorescent and thankfully got some shade cloth this year to protect them as they hardened off. All they really had to adjust to was temperature.

I've had to put a bunch of my plants in our conservatory due to lack of space. It's sort of shielded, wood, lots of windows and plastic roof, it's the closest I'm currently to a green house (still waiting for ours). I didn't really harden them off to the cold, once we started getting temps over 6 degrees celsius *(42ish F) during night I left them in there, it's a degree or two warmer in the conservatory than outside. I think the smallest seedlings, the ones with their first true leaves are a bit shocked and are either not growing or just doing it very slowly. The larger plants seem to do a lot better. My tomatoes and cucumbers are doing fantastic though. Not too bothered by the chili seedlings, what are 8 plants compared to the 50 I still have inside. But I guess this makes it my experiment of sorts.

Reading through a few posts of hardening off have a question that I cant find a direct answer to. Plants have been inside under a light 16hrs on and then off. Was doing some bottom watering outside to avoid making a mess, started with my larger plants and then left them outside in the sun. Full sun to slight over cast over the 30 or so minutes they were outside. Two of the 8 plants out there had their leaves wilted and weakened. 
 
So I know I see some shade to start but it seems like those are for when they are outside all day or longer periods of time. I thought it said sun 1 hour to start and increase. Trying to get some of these plants some actual sunlight. So should I be getting them partial sunlight (under a tree) for an hour a day and increase, then move to full sun an hour a day and increase? 
 
Maybe my grow light isn't as bright as I think and it isnt putting out enough heat to get these bad boys even slightly acclimated. 
 
Thoughts?

I used a shade cloth this year to transition my plants outside. Shade greatly helps the hardening off process. Your New Mexican sun might be too brutal for the plants like my Floridian sun. I have them under the shade cloth 100% of the time.

suchen said:

I used a shade cloth this year to transition my plants outside. Shade greatly helps the hardening off process. Your New Mexican sun might be too brutal for the plants like my Floridian sun. I have them under the shade cloth 100% of the time.

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Your probably right. I had to move my garden over about 15 feet this year (kids trampoline kicked me out!) and the plants aren't doing as good this year as they have in years past. And the sun hasn't even come yet. I guess I will have to make a little shade box for them.