guard cell

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guard cell A specialized type of plant epidermal cell (see EPIDERMIS), 2 of which surround each stoma. Changes in their turgidity cause stomatal opening and closing. This procees is not fully understood but includes:
a. the effects of the overall water potential of the plant on the stomata;

b. a feedback mechanism, whereby when water is plentiful abscisic acid is formed and stomata open and when water is scarce abscisic acid is not released and the stomata close;

c. low carbon dioxide levels promote stomatal opening;

d. except in CAM plants, stomata normally open in light and close in darkness;

e. generally, an increase in temperature tends to increase stomatal opening.

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