The stem of a tomato plant is the crucial part to protect from low temps. It is where chemical transfer takes place and is where low temperatures can literally gum up the works. I wrote this about 20 years ago and it is worth repeating. This is on my website under the seed starting link.
120°F = Severe heat, but if plenty of water is available, the plants are fine. This temp is way above levels at which pollination can take place. Plants with heavy fruit set may show stress. Nutrient transfer imbalances occur because the plant is busy moving water into leaves instead of moving nutrients into fruit.
92°F = This is the temp at which pollen starts clumping and blossoms begin to drop.
70°F to 92°F = This is the goldilocks zone. Tomatoes grow prolifically, flowers set readily, plants need maximum fertility in the soil. The high end of this range is optimum for spread of several foliage diseases.
65°F to 72°F = the best temperature to grow seedlings. Temperature can be used to slow down growth. 60°F will cause growth to be reduced about 1/4 compared to 70°F.
50°F to 65°F = this is the beginning of cold stress. Tomato plants in this range grow slowly, often produce anthcyanins (turn purple), and become pale green from loss of chlorophyll function.
32°F to 50°F = This is the range where normal tomato plants show severe cold stress. Leaves shrivel, turn yellow, wilt, stems lose turgor, roots stop absorbing water. Rubisco is deactivated by free radicals with byproducts accumulating which causes the leaves to die.
28°F to 32°F = This is the maximum range most tomatoes can withstand without freezing. Note that if frost forms on the leaves, then the leaves will freeze and die. The plant may live and can form new leaves, but the stunting effects take quite a bit of time to overcome.The time a plant can stand at this temperature is very short, in the range of about 6 hours in a 7 day period. If the temperature remains below 50 deg F on average and if the temperature dips below freezing a couple of times, the plants will deteriorate rapidly.
22°F to 28°F = This is the range that a few select varieties can withstand for brief periods of time but stipulating that frost on the leaves will still kill them.
15°F to 22°F = This is the range that a few Russian cultivars are reported to survive, again only if frost does not form. The reports I have read indicate that this tolerance is only for a limited time period, in other words, repeated low temps for 3 days or more will still kill the plants.
0°F to 15°F = A few Russian cultivars are able to handle temps this low for brief periods of time. This is the low end of the range that wild tomato species S. Habrochaites, S. Chilense, and S. Lycopersicoides can withstand.
As the temperature goes below 60°F, tomato plants enter a state where normal photosynthesis ceases. Sugar accumulates in the leaves, rubisco - a crucial chemical in the plant- begins to be deactivated by free radicles. This process causes the leaves to become dysfunctional in such a way that they can not recover. One very special trick that greenhouse growers MUST know is that if plants are exposed to overnight lows below 45°F then the greenhouse must be let rise to a high temp near 100°F the next day. If this is done, then the plants totally reverse all effects of being too cold the night before.
