It is the evaporation that pulls more water up from the roots. A plant that is transpiring faster than it is pulling up water from its roots will appear wilted. This does not mean it needs to be watered; in fact, watering it more doesn’t help, it just wastes water. What happens, and I’m struggling to remember my botany here, is that the wilting closes the stomata in the leaves so that the plant doesn’t continue to lose water. It’s a plant’s water conservation trick. If you go out in the evening and the plant that was wilted in the heat of the day is no longer wilted, than it doesn’t need to be watered.
Since over watering can cause other problems in plants like tomatoes and peppers, and since some of you are in drought stricken areas, it’s worth reminding yourself of a little plant physiology. And if you like your peppers hot, remember, stressing the plant a little makes the peppers hotter, while over watering makes them mild.
I have to wonder if this applies in Desert areas during 105 degree plus days, hot dry winds most afternoons and no more moisture at night than in the daytime even though it does cool down to 80 degrees or so. : ) I realize some desert areas have more humidity than the Mojave Desert does (we get 11-15% or so), so knowing your own area is important.
I think it also has to do with our soil. Sandy, won’t hold water, or that pesky layer of caliche. Either way, our soil needs continuous, generous supplies of compost, and it doesn’t hold moisture very well. I experiment to see how long I can go between waterings. Most of my plants will do ok on water every 36 hours, but I lost a few on that schedule. I lost valerian, borage, parsley, and some of the comfrey.