Roles of Abscisic Acid and Indoleacetic Acid in the Stunted Growth of Water-Stressed, Etiolated Squash Hypocotyls

The endogenous contents of indoleacetic acid (IAA) and abscisicacid (ABA) were assayed simultaneously in etiolated squash hypocotylsthat had been subjected to water stress applied to squash rootsby immersing them in 60 mM polyethylene glycol solution. Thegrowth rate and suction force of the hypocotyl under this water-stresscondition were determined for comparison with the two hormonecontents. Water stress retarded hypocotyl growth and increased the suctionforce of hypocotyl cells, as the ABA content increased, by 5-foldof the initial content one day after treatment with polyethyleneglycol. The ABA contents thereafter remained high up to day4. The increase in ABA content was detected in stressed hypocotylas early as 6 h after treatment, when water stress had not affectedhypocotyl growth. When the water stress was terminated one dayafter treatment began, hypocotyl growth immediately recovered,and the ABA content decreased to the value for unstressed hypocotylsin one day. The IAA content gradually decreased in unstressedhypocotyls, but was maintained at a high value in stressed hypocotyls. Logarithmic concentrations of endogenous ABA (ng/g fr wt) werecorrelated significantly with the suction force of the hypocotyls(r=0.92) and with the growth rates (r=–0.78). IAA contentwas correlated neither with the growth rate nor the suctionforce. These results suggest that the endogenous ABA contentwas associated with the stunted growth of the etiolated squashhypocotyls produced by water stress but that the endogenousIAA content did not have an important role in growth regulationunder water stress conditions. (Received April 11, 1984; Accepted October 2, 1984)