Some plants will wilt in heat despite having adequate water available to their roots - various species of Ligularia come to mind in this respect. There's nothing to be done about it, if you want to grow this genus in a hot and sticky climate, except avert your eyes from their discomfort. Humidity Humidity is the amount of water vapor in the atmosphere. The main sources of water vapor in the air around us are evaporation from the Earth's surface and transpiration by plants. Plants also respond to changes in humidity.
Low humidity increases transpiration rates, causing plants to draw more water from the soil.
High humidity slows transpiration, which helps to reduce soil water needs, but if the air remains humid at night, the plants ability to cool itself is reduced, just as ours is when our perspiration doesn't evaporate. Many bacteria, fungi and virus spores require moisture to travel and find new host sites. Continual high humidity around plant foliage provides this moisture film and encourages the development of these pathogens. High humidity and low soil moisture encourage powdery mildew - my phlox have had a really bad case of it this year. To understand the visible results of heat stress on our plants, we really have to understand a bit about how they are constructed.