Studying guard cells in the intact plant: modulation of stomatal movement by apoplastic factors

Here, we discuss why guard cells in intact plants respond to environmental signals in a different way than guard cells in epidermal strips, or protoplasts thereof. In intact leaves stomatal opening is counteracted by epidermal cells that press against the guard cells. Changes in the turgor of epidermal cells therefore can alter the stomatal aperture. In addition, stomatal opening may be modulated by the solute composition of the guard cell wall. Changes in apoplastic K⁺ , Cl^- and Ca²⁺ occur after light-dark transitions, but not in such a way that it would support stomatal opening. Organic anions may play a role, since they enhance the open probability of anion channels in the plasma membrane. Furthermore, studies with auxin-resistant and abscisic acid-insensitive mutants show that light-induced stomatal opening is modulated by these hormones. Using the newly developed method in which guard cells in the intact plant are impaled with double-barreled electrodes, the role of these apoplastic factors now can be studied on single guard cells that are still in their natural environment.