Environmental conditions can affect stomatal openings. Drought conditions, which induce water stress, can affect gas exchange because the lack of water moving through the plant causes the guard cells to lose turgor and close the stomata. When the temperature becomes too warm, stomata also tend to close. In some instances, the higher temperature causes water to leave the leaf more rapidly, which leads to water stress.
In other cases, the increase in temperature causes an increase in cellular respiration that, in turn, increases carbon dioxide levels. Internal high carbon dioxide concentrations both reverse the carbon dioxide pressure gradient and cause the stomata to close.
The percentage of relative humidity can drastically affect the rate of water evaporating from the leaf surface. As the humidity increases, the higher water content of the air decreases the rate of water loss from the leaf because the water pressure gradient no longer favors evaporation from the leaf surface. The amount of solar radiation can also influence gas exchange. As the amount of light increases, the stomata open faster and wider, resulting in a more rapid rate of gas exchange. Wind currents will also increase gas exchange rates: As the wind blows across the leaf, it carries water vapor away and, in a sense, reduces the humidity at the leaf surface. Because of this lower humidity, the water evaporates from the leaf surface more rapidly.
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