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How fast does stomatal resistance respond to environmental changes? Correct me if I'm wrong, but I assume that atmospheric humidity would be the fastest changing factor (e.g. rain storm on a dry day, or a warm change coming through). If the relative humidity changes from say 50% to 100%, how quickly will stomata open? And how fast in the opposite direction?

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Stomatal speed varies depending on a lot of factors, especially anatomy even within species, but especially between species.

Evidence from several studies has also suggested that smaller stomata respond faster than larger stomata

The paper I just cited from (first link below) includes is a long review of the what is currently known and also original research into the process. It includes a really detailed, lengthy chapter on the stomatal speed, but which is mostly lacking in easily citable numbers except for closing times:

Stomatal aperture responds more slowly, typically with half-times of 10 to 20 min, reaching a new stable, (near) closed state after 45 to 60 min

A few more clear numbers come from the second paper I link to below, however this is in response to changes in light, not humidity.

It includes a table (table 1) on mean stomatal closing and opening time delays in four plant groups and plants adapted to two climate conditions. In most groups, closing happens slower, taking about 50 percent longer, with mean times varying between 6 and 18 minutes. Opening is faster in every group but one and takes between 4 and 30 minutes. In plants adapted to dry climate conditions stomata respond faster.

Sources:

Stomatal Size, Speed, and Responsiveness Impact on Photosynthesis and Water Use Efficiency

Effects of stomatal delays on the economics of leaf gas exchange under intermittent light regimes

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