THE EFFECT OF ABSCISIC ACID ON STEM ELONGATION AND CORRELATIVE INHIBITION

Abstract

SummaryThe mature length of the second internode (which was undergoing rapid vacuolation at the time of treatment with ABA) was not appreciably affected by doses of ABA below 20 μg per plant although the rate of elongation was depressed for the first 2 days after treatment, the depression being as much as 50% in tall varieties. Doses of ABA in excess of 20 μg not only depressed elongation rate immediately, but resulted in shorter internode length at maturity. Etiolated seedlings showed a stimulation of elongation rate and mature length in the dwarf ‘Meteor’, hut no statistically significant differences in the tall variety ‘The Pilot’ with doses below 20 μg ABA. Higher doses of ABA caused immediate cessation of elongation in vacuolating internodes, but only postponed the onset of vacuolation in younger internodes. Lighted plants also showed this postponement of the onset of vacuolation in the younger internodes at higher doses; in both lighted and etiolated seedlings the normal growth rate was resumed after a delay of between 3 and 7 days.Decapitation reduced elongation rate during the subsequent 48 hours either with or without ABA. In the absence of ABA there was a subsequent recovery in growth rate of the dwarf variety so that the final length of the second internode was the same for both intact and decapitated plants. But decapitation of the tall variety produced a marked reduction in mature length of the second internode, as it did in all ABA treated plants both dwarf and tall, intact and decapitated. The only significant interaction between decapitation and ABA on elongation rate occurred in the tall variety: during the first 24 hours all doses of ABA produced approximately the same big reduction in elongation rate in intact plants but had no statistically significant effect on decapitated plants; during the second day only the higher doses of ABA reduced the elongation rate of intact plants while all three levels produced the same big reduction in elongation rate of the decapitated plants. In the dwarf variety there was no significant interaction between the decapitation and ABA effects. Lateral bud outgrowth in decapitated plants was inhibited by ABA, and 100 μg per plant achieved inhibition comparable to that produced by the apex in intact plants. Hence ABA may possibly be the inhibitory substance demonstrated to be responsible for the phenomena of correlative inhibition.