A metal ion involved in stomatal regulation is
a)	Iron	b)	Potassium	c)	Zinc	d)	Magnesium

(b)

According to proton transport theory, proposed by Levitt (1974), the regulation of stomata (i.e., opening and closing) depends upon the entry and exit of potassium ions () in the guard cells. In guard cells, during the day time, the malic acid is formed at first, which dissociates into cations and anions.