Evidence on the Auxin-Ethylene Balance Hypothesis of Foliar Abscission

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Abstract
A quick petiole test employing the lanolin-paste technic was used to study the role and effectiveness of indoleacetic acid (IAA), ethylene chlorohydrin, trans-cinnamic acid, Shed-A-Leaf (Na chloratepentaborate), and Endothal (disodium 3,6-endoxo-hexahydrophthalate) in the abscission process. Debladed but intact Coleus and cotton petioles were utilized in the study. The defoliation of mature cotton plants by means of ethylene gaff, when used singly and in various combinations with the Na salt of naphthaleneacetic acid, was also investigated. IAA completely inhibited abscission of petioles of both plants, while Endothal and ethylene chlorohydrin induced complete abscission in the case of Coleus and a high % of abscission for cotton. Trans-cinnamic acid effected a higher % of abscission than occurred in the controls (debladed leaves) of both spp., indicating the possibility that the compound offsets the inhibitory action of auxin. The more extensive expts. with cotton showed that when Endothal, Shed-A-Leaf, and trans-cinnamic acid (which promoted leaf fall by themselves) were each combined with IAA, the reduction in % abscission was somewhat proportional to the concn. of IAA. However, when ethylene chlorohydrin was combined with IAA, or when mature cotton plants were exposed to ethylene gas after they were pre-treated with naphthaleneacetic acid, the results are more suggestive that it is the relative balance of ethylene to auxin which determines the amt. and rate of abscission effected. Detn. of the amt. of ethylene and CO2 evolved by untreated cotton leaves and comparison with leaves treated with Endothal, trans-cinnamic acid, and Shed-A-Leaf showed a general stimulation in ethylene production by treatment. In all cases ethylene production was closely correlated with CO2 evolution. In vitro production of ethylene by action of enzymes on pectin and 1-arabinose was inhibited by IAA, and it was proposed that this is an important effect of auxin in preventing abscission. It is concluded that artificially supplied ethvlene probably inactivates auxin as previously suggested by others, as well as accelerating hydrolase and respiratory enzymes. The data of this study are presented as evidence that the natural cause of abscission is the result of the auxin-ethylene balance in the petiole.