Inhibition and promotion by abscisic acid of growth in Spirodela.

The effect of different concentrations of abscisic acid on the growth of Spirodela oligorrhiza has been studied. Abscisic acid effectively permanently arrests growth at concentrations down to 10(-1) mg/l (one part per 10 million). Normal growth tends to be resumed at concentrations of 10(-2) and 10(-3) mg/l between 9 and 12 days after treatment. A concentration of 10(-8) mg/l, however, results in a significant increase in dry weight at both 9 and 12 days after introduction to the culture medium. It is suggested that the resumption of growth 12 days after treatment at those concentrations which inhibit growth up to 9 days, is due to a possible progressive inactivation of abscisic acid resulting in a lowering of its concentration to a level that is promotive. Sterile plants must be used since abscisic acid has shown no effect whatsoever in controlling growth of Saprolegnia, a water-mould of the Phycomycetes to which Spirodela apparently is the host.