As mentioned earlier, water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of the stomata, thus regulating transpiration.
Water also provides the pressure to move a root through the soil.
Among water's most critical roles is that of a solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage. By its gradual evaporation of water from the surface of the leaf, near the stomate, helps stabilize plant temperature. Relative Humidity is the ratio of water vapor in the air to the amount of water the air could hold at a given temperature and pressure expressed as a percent. If the amount of water in the air stays the same and the temperature increases the relative humidity decreases. Water vapor will move from an area of high relative humidity to one of low relative humidity.