Effect of obstructed translocation on leaf abscisic Acid, and associated stomatal closure and photosynthesis decline.

Pod removal or petiole girdling, which causes obstruction of translocation, was found in our previous study to cause reduced rates of photosynthesis in soybean leaves due to stomatal closure. The purpose of this research was to determine the involvement of photoassimilate accumulation and leaf abscisic acid (ABA) levels in the mechanism of stomatal closure induced by such treatments.Leaf glucose and sucrose levels increased during the initial 12-hour period after depodding or petiole girdling. Starch, which represents a much larger pool of leaf carbohydrate, was not perceptibly increased above control levels during the 12-hour posttreatment period.When leaflets were exposed to nonphotosynthetic environments (shading or CO(2)-free air) for a 24-hour period after the translocation-obstructing treatments were applied and then returned to normal light or CO(2) concentration, stomatal diffusive conductivity was reduced 65% and 85% with depodding and girdling, respectively. These reductions were comparable to those previously observed without an intervening nonphotosynthetic exposure, thus indicating that photosynthate accumulations were not necessary for the observed response.Free and bound ABA (released on alkaline hydrolysis) were determined by gas liquid chromatography with electron capture detection following preparative high performance liquid chromatography. Free ABA in monitored leaves increased almost 10-fold 48 hours after complete depodding and 25-fold 24 hours after petiole girdling of such leaves. By 3 hours after treatment, in a time course study, free ABA had increased 2-fold above control values in depodded and 5-fold in girdled leaves. Leaf concentrations of bound ABA did not appear to be related to the treatment effects on stomata.Thus, the translocation-obstructing treatments cause an increased level of ABA by a mechanism not involving accumulation of photoassimilate. Increased leaf ABA levels, which were independent of water stress or leaf water potential, appear to be involved in the stomatal closure response. It is suggested that the mechanism of increased leaf ABA levels following translocation-obstruction may be due to an interference with normal translocation of ABA out of leaves.