Inhibition of carnation petal inrolling by growth retardants and cytokinins

Excised carnation petals induced to senescence by ethrel (an ethylene-releasing compound) exhibited morphological changes that closely resembled those of senescing petalsin situ in cut flowers. The sensitivity of the excised petals to ethylene was reduced by exogenous cytokinin and this type of hormonal interaction in the control of plant development is discussed. Using the excised petals, a number of known and potential growth inhibitors were compared for ability to prevent petal inrolling induced by ethrel. Cycloheximide and 6-methylpurine were the most effective and inhibited inrolling almost completely, but purine, purine riboside, lauric acid, L-azetidine-2-carboxylic acid and n-decyl alcohol were also very effective. All these compounds were considerably more effective than any cytokinin tested. When supplied through the transpiration stream to short-stemmed carnations, cycloheximide, 6-methylpurine and purine inhibited inrolling nearly completely and the flowers finally senesced by water loss. 6-Methylpurine inhibited ethylene production in cut flowers and RNA synthesis in excised petals very markedly. Degradation of exogenous zeatin riboside by cytokinin oxidase, and the level of activity of the enzyme in petals, were reduced by 6-methylpurine. These biochemical changes probably account for the strong inhibition of inrolling induced by this compound.

Changes in Responsiveness to Ethylene at the Molecular Level during Carnation Petal Development

Differences in responsiveness and sensitivity to plant hormones between tissues and in specific tissues during development have been observed. Increased sensitivity to ethylene during development has been demonstrated in fruits and flowers. In order to study the molecular changes in responsiveness to ethylene during flower development carnation petals of three developmental stages were treated with 0.1, 1, 10, 100, pr 1000 ppm of ethylene for 0, 0.5, 1, 3, 6, 9, 15, or 24 h. Northern blot analysis was performed on total RNA extracted from these treatments. Hybridizations were carried out with the senescence related cDNAs SR5, SR8, SR12, DCACO1, DCACS1, and DCCP. The respective transcripts showed distinct patterns of accumulation in response to ethylene. SR5 transcripts, encoding for a putative β-glucosidase, accumulated significantly faster and at lower ethylene concentrations than all the other transcripts. DCACS1, an ACC synthase, on the other hand showed a delay in the accumulation of its mRNA when compared to the other genes. As the petals develop each mRNA also showed a unique pattern of increased responsiveness to ethylene. This increase in responsiveness is expressed as a decrease in the ethylene-time constant. The ethylene-time concept, similar to degree-days, states that it takes a certain time for a given ethylene concentration to induce a response, 50% maximum transcript level in this study. This allows for a quantitative assessment of changes in ethylene responsiveness during petal development.