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Abscisic acid (ABA) was quantitated by enzyme-linked immunosorbent assay (ELISA) in water-stressed leaves from control apple seedlings, and also from apple seedlings treated for 28 days with paclobutrazol (12RS, 3RSI-144-chlorophenyll-4,4-dimethyl-2-11,2,4-triazol-1-ylI pentan-3-ol).The ELISA quantitative estimates were also validated by gas chromatography-electron capture detector and lettuce seed germination inhibition bioassay. Paclobutrazol treatment reduced endogenous ABA levels by about one-third, and prevented the marked accumulation of water-stressinduced ABA that occurred in untreated seedlings. The presence of ABA in the apple leaf extracts was confirmed by gas chromatography-mass spectrometry.The accumulation ofABA in the leaves ofwater-stressed plants has been shown by numerous investigators (12,18,22,23). Our previous reports showed that the accumulation of water stressinduced 1 -aminocyclopropane-1 -carboxylic acid, 1 -(malonylamino)cyclopropane-l-carboxylic acid, ethylene production, and polyamines also occurred in water-stressed apple leaves, but these increases were prevented by the treatment with the plant bioregulant ([2RS,

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