E8 Humidity

Topic

The humidity regime of the air surrounding the leaf can be summarized by the vapor pressure deficit—the difference between the amount of water vapor the air could hold if saturated and the amount it actually contains. The greater this difference, the higher the air's vapor pressure deficit; this deficit represents the evaporative demand that drives water loss from the leaf into the atmosphere.

The air's vapor pressure deficit acts as the thermodynamic engine of transpiration: the higher the deficit, the more strongly the air "pulls" water vapor exiting the leaf through the stomata, resulting in faster transpiration—even if the leaf's stomatal aperture remains unchanged.

Of all environmental variables, the air's vapor pressure deficit is most directly linked to plant water stress, particularly under high radiation conditions. When high radiation heats the air and simultaneously lowers its relative humidity, the vapor pressure deficit rises further; this combination of a high deficit and high radiation most readily pushes the plant toward water stress.

Air humidity affects the leaf not only by drawing out water through transpiration during the day; at night, when air temperatures drop sufficiently, water vapor condenses on the leaf surface to form dew. This nocturnal condensation serves as a supplementary water source for the plant—a factor of particular importance in arid ecosystems where rainfall alone is insufficient to offset the vapor pressure deficit the plant faces during the day. In addition to governing transpiration and dew formation, atmospheric relative humidity influences foliar pathogens that can affect the leaf: the higher the relative humidity and the longer the leaf surface remains wet—whether from dew or rain—the more favorable the conditions become for foliar pathogens to germinate and colonize leaf tissue.

Ultimately, the atmospheric moisture regime affects the plant in three distinct ways: the air's vapor pressure deficit drives transpiration and, under conditions of high radiation, pushes the plant toward water stress; nighttime condensation—promoted by high relative humidity during the night—provides the plant with a supplementary water source, particularly in arid ecosystems; and that same relative humidity, when sustained at high levels on the leaf surface, favors foliar pathogens that compete with the plant for the very leaf tissue that the atmospheric moisture regime helps keep hydrated.

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