Effect of Ethephon, Indole Butyric Acid, and Treatment Solution pH on Rooting and on Ethylene Levels within Mung Bean Cuttings

Abstract

Light-grown mung bean (Phaseolus aureus Roxb.) cuttins were treated with buffered and nonbuffered solutions of Etbepbon, imnole butyric acid (IBA), and the combination of both. Ethepbon treatment resulted in Increased tssue ethyene levels with increasing solution pH, but had no effect on rooting. IBA treatment had no effect on tissue ethylne levels, but sbtongy promoted rooting. Combnatios of Ethep_on and IBA had no effect on rootig of mung bean cuttigs beyond that obtained by IBA alen. Ethephon has been reported to promote (6, 8, 13, 16, 18, 20), inhibit (11, 12), or have no effect (14, 15, 17,18) on rooting of cuttings of various species of plants. The release of ethylene from Ethephon has been shown to be pH-dependent (5, 10). When Ethephon does not promote rooting, the possibility exists that the cause is the ineffective release of ethylene gas from Ethephon due to the low pH of Ethephon solutions, rather than a lack of response to ethylene gas. Several authors have assumed that the generation of ethylene from Ethephon occurs primarily intracel-lularly (2, 3), where it would occur rapidly at the higher pH of the cell (pH 5-6) than at the pH of the treatment solution (pH 2-3) (5, 10). This assumption is based on a single study by Warner and Leopold (19) involving Bryophyllum cruentum in which the rate of ethylene evolution from long-day-and short-day-grown leaves was compared. Long-day-grown leaves, which have a higher tissue pH than short-day-grown leaves, also had higher ethylene levels. However, they did not rule out the possibility that the difference in the rate of Ethephon decomposition was due mostly to differences in extracellular rather than intracellular pH. That the relatively nonbuffered extracellular water spaces (cell walls and xylem elements) are an important site of Ethephon degradation seems likely, since the uptake of a "foreign" anion-like Ethephon by plant cells would not be expected to be rapid. At the low pH of a typical Ethephon treatment solution (eg. a 10 sl/1 solution has a pH of 3.7) the decomposition of Ethephon would be quite slow (5, 10). This suggests that treatment solution pH is limiting with respect to the rate of ethylene production from Ethephon within treated tissues. If ethylene does promote rooting of cuttings as indicated (6, 8, 13, 16, 18, 20), the use of buffers to increase the pH of Ethephon treatment solutions might increase rooting of cuttings.