The effect of different concentrations of abscisic acid on the growth of Spirodela oligo~vhiza has been studied. Abscisie acid effectively permanently arrests growth at concentrations down to 10 -1 rag/1 {one part per 10 million). Normal growth tends to be resumed at concentrations of 10 -3 and 10 -S rag/1 between 9 and 12 days after treatment. A concentration of 10 -s rag/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 abscisie acid has shown no effect whatsoever in controlling growth of Saprolegnia, a water-mould of the Phycomycctes to which Spirodela apparently is the host.
Recent investigation on the endogenous regulator abscisic acid (ABA), have clearly demonstrated its role as a major inhibitor of plant growth.
VAN OV]~HB~]~ et al. (1967) found that Lemna minor was particularly sensitive to ABA; concentrations as low as one part per 1,000 million (3.8 x 10 -9 M) reduced its rate of growth, while one part per million (3.8 β’ 10 -6 M) was sufficient to keep it indefinitely in a state of dormancy. This reduction in growth apparently is a consequency of the inhibiting effect of ABA on nucleic acid metabolism. In fact WAH~ING (1968) and VrLLI~S (1968) showed that ABA was responsible for the inhibition of nucleic acid synthesis in Taraxacum and Fraxinus, respectively.
In the light of abscisic acid's role in plant growth regulation, it was decided to include this substance in a study of the effects of certain inhibitors of protein synthesis on the growth of Spirodela oligorrhiza, a pondweed in the family Lemnaceae.
Spirodela oligorrhiza was grown in 50 m] erlenmeyer flasks in 30 ml sterile, full-strength I-Ioagland's solution, pH 4.6, with Fe-EDTA as source of iron. Each flask, containing six uniform fronds, was aerated for 45 minutes twice daily with air sterilized by passing it through a millipore filter fitted with a GS 0.22 B filter disc. Attempts to rid these plants of algae and particularly of the saprophytie fungus, Saprolegnia, proved successful only after harsh treatment by a method modified from that of GoaHA~ (1945). The plants were first submerged for four minutes in 0.1% I-IgCl~ and then in 50% ethanol for 30 seconds, after which they * The authors gratefully acknowledge R.J.