Localization of Mechanisms Involved in Hydropassive and Hydroactive Stomatal Responses of Sambucus nigra to Dry Air

The response of stomata to a reduction of air humidity is composed of a hydropassive opening followed by active closure. Whereas the mechanisms behind the hydropassive opening are largely understood, the location and physiological basis of the sensing mechanisms leading to active closure are not yet known. This study attempts to evaluate the importance of a single pore's transpiration on its own response and that of adjacent pores. Selected stomata on attached intact leaves of Sambucus nigra were sealed with mineral oil and the response to a reduction of humidity was continuously observed in situ. Blocking a pore's transpiration had no appreciable effect on hydropassive opening and subsequent stomatal closure. If the adjacent stomata were additionally sealed, the closing response was reduced, but not the hydropassive opening. On the other hand, sealing the entire leaf surface, except a small area including the observed stomata, also reduced stomatal closure. These results indicate that strictly local processes triggered by a pore's own transpiration are not required to induce stomatal closure. To describe the effect of one pore's transpiration on the hydropassive and hydroactive responses of neighboring stomata, a simple spatial model was constructed. It suggests that 90% of the closing effect covers an area of approximately 0.5 mm2, whereas the effect on hydropassive opening affects an area of approximately 1 mm2. This divergence may suggest mechanisms other than or in addition to those involving changes of local leaf water potential.