Abstract
Recent research suggests that chemicals sent from roots in the transpiration stream could control leaf expansion, and that xylem sap from plants in dry or saline soil could contain increased amounts of a growth inhibitor, or decreased amounts of a growth promoter. In order to test these possibilities, a bioassay that could detect the presence of growth regulators in xylem sap was developed using whole shoots of wheat and barley seedlings. The bioassay showed that xylem sap collected from intact, transpiring plants in a drying soil contained a strong growth inhibitor. The inhibitory substance was not abscisic acid: while the concentration of abscisic acid in the sap rose as the soil dried, the highest concentration found, 4 × 10-8 M, was too low to inhibit leaf expansion. The identity of the new inhibitor is unknown.
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