Is abscisic acid responsible for abnormal stomatal closure in coconut palms showing lethal yellowing?

Lethal yellowing induced permanent stomatal closure in coconut palms (Cocos nucifera L.) is central to the development of the disease's symptoms such as decreased photosynthesis, leaf yellowing and palm death, but the cause of this abnormal permanent stomatal closure is unknown. Since abscisic acid (ABA) can cause stomatal closure, ABA concentrations were measured in the roots, leaves, and leaf xylem sap to test for a correlation with the degree of stomatal closure as disease severity advanced. Concentrations of ABA in leaf and xylem sap extracts increased with disease progression but not until the later stages of the disease, well after abnormal stomatal closure. ABA concentrations in the roots were also not correlated with stomatal closure. Transpiration declined proportionally in response to increasing ABA concentrations (10(-8), 10(-6), 10(-4) mol L-1) in a detached coconut leaf bioassay but all extracts from healthy and diseased coconut palms reduced transpiration to similar levels. Likewise, all coconut palm leaf extracts reduced stomatal aperture 60-70% in Commelina communis leaf epidermal strips with no notable differences between healthy and diseased palms. Thus, the results indicate that bulk ABA concentrations in leaf xylem sap or leaf tissue are not responsible for the abnormal stomatal closure in lethal yellowing disease.