ABA Transport and Distribution in Relation to Its Function in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 67

hydraulic signal. However, Blackman and Davies ( 1985 ) conducted a root-split
experiment that demonstrated that stomatal closure could occur when no percep-
tible changes occurred in leaf water potential under soil-drying conditions, imply-
ing the presence of chemical signals involved in root-to-shoot communication.
As ABA is known to play an important role in the regulation of stomatal move-
ments, there has been much discussion as to whether ABA is a root-to-shoot sig-
nal. Numerous studies have found that drought stress induces a dramatic increase
in ABA content in both the root and xylem sap, and the increase in ABA is closely
correlated with a decrease in leaf stomatal conductance (Zhang and Davies 1989 ,
1990 , 1991 ). Feeding xylem sap collected from drought plants to detached leaves
was able to inhibit stomatal movement, and this inhibitory effect was relieved
when ABA was removed from the xylem sap by an ABA-affinity column (Zhang
and Davies 1991 ; Davies et al. 1994 ). These findings strongly suggest that ABA is
capable of acting as a long-distance signal to regulate stomatal movement under
drought stress conditions (Davies and Zhang 1991 ).
While ABA is a well-known mediator of stomatal closure through long-dis-
tance transport, numerous reports suggest that root-sourced ABA cannot medi-
ate stomatal closure under some conditions. For instance, in some plant species,
root-sourced ABA was found to have no significant effect on stomatal movement
(Holbrook et al. 2002 ; Munns and King 1988 ). Supposing that drought stress is
able to increase the xylem ABA concentration significantly, a reasonable explana-
tion for the failure of root-sourced ABA to regulate stomatal movement is that the
root-sourced ABA cannot be effectively accumulated at its target sites of the guard
cells. As discussed above, pH is a major factor affecting the distribution and accu-
mulation of ABA, and hence, it is of particular interest to establish whether pH
modulates the root-to-shoot signaling of ABA. In agreement with this proposal,
it has repeatedly been shown that drought stress is able to induce an increase in
xylem sap pH in many plant species (Jia and Davies 2007 ; Wilkinson 1999), and
the pH increase in the xylem is proposed to render the apoplast of the leaf more
alkaline, and this would contribute to the sequestration of more ABA in the apo-
plast of guard cells, thereby promoting stomatal closure (Wilkinson 1999; Jia and
Davies 2007 ; Wikinson and Davies 2002 ). By feeding artificial xylem sap buffered
to different pH to detached leaves of Commelina communis , Wilkinson and Davies
( 2002 ) found that an increase in pH could indeed lead to a great reduction in tran-
spiration rate in the presence of low concentrations of ABA. These studies strongly
suggest that pH can function as a signal that coordinately regulates stomatal move-
ment with the ABA signal under drought stress conditions. Given that the regu-
latory effect of root-sourced ABA on stomatal movement is strongly affected by
apoplasitc pH, any factors that affect apoplastic pH would be expected to have an
impact on stomatal movement in relation to root-to-shoot ABA signaling. This has
been demonstrated by a study of Jia and Davies ( 2007 ), which showed that dif-
ferent forms of nitrite nutrition could modify stomatal sensitivity to root-sourced
ABA, owing to modifications of the apoplastic pH in the leaves of some species.
Water deficit occurs initially from the top of roots and gradually extends toward
the tip. Once the rate of water transport from soil into roots can no longer balance