Short answer: I don't know and could not find actual numbers for specific species. maybe other people who live in hot-summer climates (like Texas) can suggest some genera to avoid, or to try.
http://www.degreedays.net/ (another degree-day calculator)
http://whyfiles.org/2010/what-...
I would be really interested if you find that "stress temperature" for different species, or even a "low temperature" at which little development occurs. I couldn't. So far my best Google search term was "plant heat stress".
One source admitted that even the lower growth thresholds of most plants are not known (coldest temp at which a plant speices grows or matures)
http://www.oardc.ohio-state.ed...
This source describes the lower and upper limuts well, but doesn't give actual tempertaures for any plant but corn:
http://ndawn.ndsu.nodak.edu/he...
"... above a threshold or base temperature below which little growth occurs. This lower threshold temperature varies with plant species. Scientists have determined the lower base temperature for corn is 50 °F (10 °C). In some plants there is also an upper threshold temperature, above which conditions are stressful to the plants. The upper limit for corn is 86 °F (30 °C). "
Here's a PDf that goes into some detail:
http://ucanr.edu/sites/MarinMG...
"Heat stress is often defined as a period in which temperatures are hot enough for a sufficient period of time to cause
irreversible damage to plant function or development. Plants can be damaged by either high day or high night temperatures and by either high air or soil temperatures."
My guess is that the upper "stress threshold" is particulalry dependent upon water availability to the plant, humidity, wind, clouds, and degree of shade or exposure.
Since the high-temperature effect is "stress", it might not be tracked very well by the daily
average temperature accumulated heat units. The average of the day's lowest temperature and highest temperature might never go near the stress threshold if nights cool down even briefly. And yet the temeprature might exceed the stress point for several hours.
Guessing even further, maybe a better statistic for tracking plant's stress level due to high temperatures would be "degree hours" or 'degree minutes" - the total number of hours or minutes when the average temperature
for that minute exceeded the threshold.
Yet another guess: the temperature that stresses the plant would vary, depending on the age of the plant and perhaps the size of the root system relative to the foliage..
