Abscisic acid inhibits shoot elongation of Scirpus mucronatus

The relationships between free ABA levels and shoot elongation were investigated in shoots of Scirpus mucronatus L. Under submergence, shoot elongation increased but free ABA levels decreased. The extent of the increase in length and the decrease in free ABA in submerged shoots increased with the increase of water depth. When the shoots were transferred to air after 12 days of submergence, they ceased to elongate and the free ABA levels recovered to the values of air-grown shoots. ABA, at concentrations from 1 μ M to 1 m M , inhibited the submergence-induced shoot elongation. In ambient air, fluridone, an inhibitor of ABA biosynthesis, at 7 μ M decreased the free ABA levels in shoots but increased shoot elongation. The effects could be reversed by 10 μ M ABA. These results indicate that ABA is an internal inhibitor of shoot growth in Scirpus .