Transpiration is the process by which moisture is pulled through plants. Water and nutrition travels from the smallest root hairs to the underside of leaves, where small pores (called stomata) regulate the release of saturated (100% humid) vapour to the less humid air. Since the external atmosphere has less vapour, it exerts a pulling force (a pressure) on the saturated air when these pores are open. Hence Vapour Pressure Deficit.
Vapour Pressure Deficit dictates how efficiently a plant might balance its internal energy with that of the wider environment. Thus translating into just how well a plant can pick up or lose water, pump nutrients throughout, remain turgid and upright, essentially grow. VPD directly influences a plants’ hydraulic capacity and this, in turn, impacts rates of growth and overall plant health.
Plants are high in moisture content (they process up to 95% more water than they use for growth) and the gases in all healthy plants are always fully saturated. Since the air in the growing space is almost never fully 100% saturated, there is a difference in the pressure of the vapour in the air and the vapour in the leaf.
It seems natural that a healthy leaf should constantly be drying and so be cooler than the surrounding atmosphere, but this is not always the case.
On the one hand, if the air is cold and/or damp, less moisture is pulled from the plant, meaning fewer nutrients go into the plant and the plant might develop deficiencies and mould or freeze and die.
But if the air is hot and/or dry, the Vapour Pressure difference can be too great and can cause plants to become stressed under rapid transpiration. This can result in nutrient toxicity due to excessive uptake of nutrients (even if fed with just water). One of these nutrient overloads may be calcium, which can lead to chlorosis and stunted growth.
If the air is too hot and/or dry, the plant stops growing to save water and closes the cells that release moisture and enable photosynthesis (located on the underside of the leaf, called the stomata cells).