(+)-abscisic Acid content of spinach in relation to photoperiod and water stress.

Levels of (+)-abscisic acid present in the long-day plant spinach (Spinacia oleracea L., cv. Savoy Hybrid 612) grown under different photoperiodic regimes were measured in purified extracts by optical rotary dispersion. When plants were transferred from short to long days, the abscisic acid content increased 2- to 3-fold. This rise in the level of abscisic acid took place during the 1st long day. Abscisic acid levels of plants under short days as well as under long-day conditions were higher at the end of the 8-hour high intensity light period than at its beginning.The growth retardant AMO-1618 [2'-isopropyl-4'-(trimethyl-ammonium chloride)-5'-methylphenyl piperidine-1-carboxylate], which strongly reduces the gibberellin content of spinach under long days, did not affect the abscisic acid content.When water was withheld from plants until wilting symptoms appeared, the abscisic acid content increased more than 10-fold over that of turgid plants. There was no evidence that the sudden rise of abscisic acid level during wilting was due to release from a water-soluble bound form.Bioassays of crude acidic extracts in the wheat coleoptile section test did not indicate the presence of other specific growth inhibitors besides abscisic acid. It is concluded that abscisic acid does not function as an endogenous regulator of stem growth and flower formation in the long-day plant spinach.