A primary component in photosynthesis and respiration
Responsible for turgor pressure in cells (Like the air in an inflated balloon, water is responsible for the fullness and firmness of plant tissue. Turgor is needed to maintain cell shape and ensure cell growth.)
A solvent for minerals and carbohydrates moving through the plant
Responsible for cooling leaves as it evaporates from leaf tissue during transpiration
A regulator of stomatal opening and closing, thus controlling transpiration and, to some degree, photosynthesis
The source of pressure to move roots through the soil
The medium in which most biochemical reactions take place
Relative humidity is the ratio of water vapor in the air to the amount of water the air could hold at the current temperature and pressure. Warm air can hold more water vapor than cold air. Relative humidity (RH) is expressed by the following equation:
RH = water in air ÷ water air could hold (at constant temperature and pressure)
The relative humidity is given as a percent. For example, if a pound of air at 75°F could hold 4 grams of water vapor, and there are only 3 grams of water in the air, then the relative humidity (RH) is:
3 ÷ 4 = 0.75 = 75%
Water vapor moves from an area of high relative humidity to one of low relative humidity. The greater the difference in humidity, the faster water moves. This factor is important because the rate of water movement directly affects a plant's transpiration rate.
The relative humidity in the air spaces between leaf cells approaches 100 percent. When a stoma opens, water vapor inside the leaf rushes out into the surrounding air and a bubble of high humidity forms around the stoma. By saturating this small area of air, the bubble reduces the difference in relative humidity between the air spaces within the leaf and the air adjacent to the leaf. As a result, transpiration slows down.
If the wind blows the humidity bubble away, however, transpiration increases. Thus, transpiration usually is at its peak on hot, dry, windy days. On the other hand, transpiration generally is quite slow when temperatures are cool, humidity is high.
Hope that helps.