Is uronic acid oxidase involved in the hormonal regulation of abscission in explants of citrus leaves and fruits?

Abstract

The role of uronic acid oxidase in abscission was studied in explants of citrus (Citrus sinensis L. Osbeck; var. Shamouti) leaves and fruits. In leaf explants, activity of uronic acid oxidase prior to onset of abscission and the rate of abscission were markedly accelerated by ethylene and delayed by 2,4‐dichlorophenoxyacetie acid. Similar results were obtained for uronic acid oxidase activity in the exocellular fraction of young fruit explants. In mature fruit explants, treated with ethylene, an immediate increase in activity was evidenty in the non‐active shoot/peduncle abscission zone, whereas in the calyx abscission zone the rise in activity occurred after a prolonged exposure to ethylene, when most of the fruits had already abscised. Whenever ethylene enhanced uronic acid oxidase activity, 2,4‐dichlorophenoxyacetic acid delayed it. A gradient of decreasing activity or uronic acid oxidase was recorded from both sides of the abscission zone in leaves and fruits toward the separation line, where activity was the lowest as compared with the activity found in adjacent tissues. It is suggested that uronic acid oxidase is involved in senescence and cell wall degradation. However, it is yet questionable whether this enzyme is directly related to the control mechanism of abscission.