You said the reason plants do not do well in humid conditions was:
"Humidity is the amount of water vapor in the atmosphere – when it is high, the air absorbs and retains moisture, leaving little for the plants."
Actually, don't plants get their water from the soil through the roots, not the leaves? In normal humidity conditions, doesn't water evaporating from the leaves create the low pressure in the plant "veins" that sucks up water and nutrients from the soil? If the air is humid, less water evaporates from the leaves and fewer nutrients get absorbed. There are quite a few articles on the web that seem to support this view (even one on marijuana growing!), and it is well known that human water retention is worse in humid conditions because sweating is reduced.
Your conclusions and remedies may be correct, but that one sentence troubled me. Perhaps some one more knowledgeable about botany could provide a better explanation.
Don, speaking as one who lives in a very humid climate in summer at least, the plants that do well here are adapted to absorb moisture from the air as well as the soil. Tillandsias (ball moss and Spanish moss are examples) get virtually all their water and nutrients from the air. That's why you see them growing not only on the trees, but on fences and power lines as well. Some plants like bromeliads and orchids that thrive in our sauna-like summers here absorb most of their water and nutrients through the leaves, and aerial roots in the case of orchids. That's why orchid growers spend their lives "misting" their diva plants with weak fertilizer solutions. (yep, I do that, too) I recently told my daughter living in Utah - super hot and dry in summer - that she should not bother trying to grow orchids there without a greenhouse. The humidity's just too low.
Plants that are adapted to drier climates really do struggle here - eg. a lot of the Mediterranean herbs like lavender and sage are almost impossible to keep going over summer. I plant them every year in the fall, they do fine until about now, then they slowly die. Your leaf vs. root absorption explanation is a good one for their decline. Hmm, maybe if I try foliar feeding my lavender in summer it might survive better . . . ? That theory also partially explains why most tomatoes don't go through summer here either. Mostly, though, it's that the night temps are too high, they don't set fruit, and don't get a nightly rest from the heat. I do have two cherry tomato plants still going right now, think I'll try some foliar feeding at night to see what happens to them, too!
Bottom line is, plants adapt and evolve to survive in their surroundings. You can't make a blanket statement that "All Plants" do any one thing because they're all different.
Different plants and different climates require different thinking on gardening.
Are there tomatoe plants which have been bred for your climate?
Up here, often,-- we plant varieties which have been bred to set fruit in cold.
My cold hardy ones this year are Glacier which originated in Sweden.
The other is Red Siberian.
Yes, I've tried them - even the supposedly 'heat tolerant' hybrids have pooped out for me once the nights don't get below 70 or so.
For some reason the cherry tomatoes seem to be more tolerant than the large-fruited ones, too. But . . we just 'make do' with buying tomatoes from up north in the summer. Such a hardship. (grin)
From what I've read, tomatoes require night temps to drop at least into the 60's to set fruit, too. So even if the plants survive, the blossoms drop. I'll re-plant in September, and have tomatoes all winter, though. Can't complain about that!
I got spoiled early on, being originally from Vancouver BC. No issues with either cold or heat tolerance there.