Heat and drought go hand in hand. Evaporation from moist soils and plants cools the land just as perspiring cools us when it's hot. Hot sun increases plant transpiration, drawing moisture from the soil; the soil dries, temperatures rise as soil dries because the land can no longer use evaporation as a cooling mechanism and the cycle goes round and round. Even mulched areas of retentive soil under trees become dry as mature trees can draw some two or three hundred gallons (9.09 - 13.63 hl) of water through their systems per day.
Drought stress results when water loss from the plant through transpiration exceeds the ability of the plant's roots to absorb water, reducing the water content in the plant enough to interfere with normal processes.
Once this lack of water starts to limit normal functions, plants try to conserve water by reducing cell turgor and closing the stomata (leaf pores) to reduce further water loss. Continued drought stress can alter or damage photosynthesis, leading to long-term reduction in vigor.
When water stressed, some plants appear to change color, usually from blue to green, others roll their leaves, change their leaf orientation or reduce leaf and stem growth and fruit yield. Eventually, foliage wilts. If no water is supplied, leaves may fall off and the plant will eventually die.
The Astilbe leaves in this scan show what happens when a plant reaches its permanent wilting point. The tips of the leaves were so damaged that they couldn't recover once the plant received water. 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.
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