Movement ofAbscisic Acidinto theApoplast inResponse to WaterStress inXanthium strumarium L.

Theeffect ofwater stress ontheredistribution ofabcisic acid(ABA) inmature leaves ofXawthium strumarium L.wasinvestigated using a pressure dehydration technique. Inbothturgid andstressed laves, the ABAinthexylem exudate, the'apoplastic' ABA,icasedbefore 'bulk leaf' stress-induced ABA accumulation began. Intheinitially turgid leaves, theABAlevel remained constant inboththeapoplast andthe leaf asawhole until wilting symptoms appeared. Following turgor loss, sufficient quantities ofABAmovedinto theapoplast tostimulate stomatal closure. Thus, theinitial increase ofapoplastic ABAmayberelevant to therapid stomatal closure seeninstressed leaves before their bulkleaf ABAlevels rise. Following recovery fromwater stress, elevated levels ofABAremained intheapoplast after thebulkleaf contents hadreturned totheir prestress values. Thisapoplastic ABAmayretard stomatal reopening during the initial recovery period.