Evidence of a Role for Abscisic Acid in Mediating Stomatal Closure Induced by Obstructing Translocation from Leaves of Pearl Millet (Pennisetum americanum[L.] Leeke)

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Abstract
Application of a heat girdle near the base of the lamina of the fifth, fully expanded leaf of young pearl millet (Pennisetum americanum [L.] Leeke) plants resulted in a decrease in solute potential, an increase in leaf dry matter content, and a decline in stomatal conductance and in the rate of CO2 assimilation. Total water potential was largely unaffected by girdling while turgor potential increased as a consequence of the decrease in solute potential. Abscisic acid (ABA) content of the leaf increased 5 to 6-fold within 1 h of girdling, then declined equally rapidly before increasing again at a slower rate. The decline in conductance was correlated with both the decrease in solute potential and the increase in ABA. To determine which of these factors could be controlling conductance, girdled leaves were exposed either to 14 h of continuous light or to a similar period of darkness followed by a brief light treatment to allow stomata to open. Girdling reduced conductance equally following darkness or light but solute accumulation occurred only in the light. ABA accumulated in girdled leaves in both darkness and light. Simultaneous measurements of conductance and CO2 assimilation showed that intercellular CO2 concentration did not increase following girdling. It was concluded that the decrease in conductance in millet leaves after girdling was most probably mediated by the increase in ABA content.