In the higher plants, S-adenosylmethionine (SAM) synthesized from methionine serves as a common substrate for the biosynthesis of ethylene and polyamine. To clarify the relationship of ethylene and polyamine metabolism during the anthesis and senescence of Hibiscus syriacus L. flowers, ethylene production and changes in the concentration of 1-aminocyclopropane-1-carboxylic acid (ACC), ACC-conjugate, and polyamines in the petal were examined. A very low level of ACC was detected in the petal, and a very low ethylene evolution was observed, even before flower opening. Ethylene production greatly increased just around the beginning of petal in-rolling and paralleled an increase of ACC-conjugate in the petal. With regard to polyamine concentration, spermine decreased with petal senescence in contrast to the sharp rise in ethylene production. Aminoethoxyvinylglycine, which is a potent inhibitor of ACC synthesis from SAM and thereby represses ethylene formation, extended the flower longevity and maintained a high spermine concentration in the petal. In contrast, methylglyoxal-bis (guanylhydrazone), which is a potent inhibitor of SAM decarboxylation and also inhibits spermine biosynthesis, promoted ethylene production and shortened the flower longevity. These findings suggest that a competition for SAM between ethylene and polyamine biosynthesis may play a key role in determining flower longevity in H. syriacus L.
A convenient method was devised for the fractionation of aliphatic polyamines (PAs), 1-aminocyclopropane-1-carboxylic acid (ACC), and their conjugated forms using a cation exchange resin, and applied to the floral organs of Hibiscus syriacus L. 'Diana'. A batch-wise use of the cation exchange resin to the acid extracts of the Hibiscus flower effectively separated the ACC-conjugate from free ACC, free PAs, and PA-conjugates. Good recovery rates, showing over 90% for PAs and 76-97% for ACC, were obtained when known amounts of ACC and PAs were added to the tissue extract. The amounts of these cellular compounds were determined in the petal, sepal, ovary, and style with stigma (+ stamen) collected at two different stages (flower opening and flower senescence showing complete petal in-rolling). Both ACC and ACC-conjugate, which are generally associated with tissue senescence, were consistently detected in all organs even immediately after flower opening, but their concentrations, especially that of the ACC-conjugate in the ovary, greatly increased in the senescent flowers. As regards the free PA levels, a high concentration of spermidine was found in the ovary, and its level was maintained even when the petals wilted. PA-conjugates bound to small molecules decreased in the ovaries of senescent flowers, while the PA-conjugates bound to macromolecules remained very low in all organs at the two different flower stages. The present method seems applicable to a quantitative analysis of these physiologically important compounds in a variety of plant tissues, despite the fact that their extracts contain highly viscous materials that generally reduce the recovery rate of ACC.
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