Carbohydrates Stimulate Ethylene Production in Tobacco Leaf Discs

Abstract

Various naturally occurring carbohydrates, applied at a concentration range of 1 to 100 mM, stimulated ethylene production for several days in IAA-treated or untreated tobacco (N. tabacum L. cv. ''Xanthi'') leaf discs. The lag period for this sugar-stimulated ethylene production was 8-12 h after excision in the untreated leaf disks but < 2 h in the IAA-treated ones. Among the tested carbohydrates, 12 increased synergistically ethylene production, with D-galactose, sucrose, and lactose being the most active; mannitol and L-glucose had no effect. The extent and duration of the increased ethylene production was dependent upon the type of sugar applied, the tissue''s age, and the existence of both exogenous IAA and sugar in the medium. Sucrose apparently elicited continuous IAA effect for 48 h, as expressed by increased ethylene production, even when IAA was removed from the medium after a 4-h pulse. Sucrose stimulated both the uptake and decarboxylation of [1-14C]IAA, as well as the hydrolysis of the esteric and amide IAA conjugates formed in the tissue after application of free IAA. This gradual hydrolysis was accompanied by a further accumulation of a 3rd IAA metabolite. Moreover, synthetic indole-3-acetyl-L-alanine increased ethylene production mainly with sucrose, and this effect was accompanied by its increased decarboxylation and turnover pattern suggesting that release of free IAA was involved. An esteric IAA conjugate, tentatively identified by gas chromatography retention time was found to be the major component (84%) of the naturally occurring IAA conjugates in tobacco leaves. Accordingly the sucrose-stimulated ethylene production in tobacco leaves can be ascribed mainly to the sucrose-stimulated hydrolysis of the esteric IAA conjugate.