ETHYLENE BIOSYNTHESIS AND ETHEPHON METABOLISM AND TRANSPORT IN BARLEY

When applied to barley (Hordeum vulgare L.) for lodging control, ethephon (2-chloroethylphosphonic acid) also promotes tillering. These tillers do not mature prior to harvest, and may compete for assimilates and reduce yield. This combined field and greenhouse study was conducted to investigate the possibility that promotion of tillering by ethephon includes (i) an effect of ethephon on ethylene biosynthesis, (ii) ethephon transport and metabolism, and (iii) a direct effect of ethephon in the plant base near the tiller buds. Ethylene release from flag leaves, spikes, and stems was determined by photoionization gas-liquid chromatography. 1-Aminocyclopropane-1-carboxylic acid (ACC) and conjugated ACC were quantified by combined gas chromatography-selected ion monitoring-mass spectroscopy. Ethephon transport was monitored with [C-14]ethephon, and by measuring ethylene release from untreated plant parts. Ethephon treatment resulted in a substantial increase in ethylene release from both flag leaves and spikes, peaking 1 to 2 d after treatment and remaining above controls for at least 15 d after treatment. No effect of treatment was observed on ACC and conjugated ACC levels; thus, ethephon application did not alter endogenous ethylene biosynthesis. Decreasing amounts of [C-14]ethephon were observed (TLC analysis) in the flag leaves and spikes over a 27 d period after flag leaf treatment with [C-14]ethephon. Increasing amounts of [C-14]ethephon metabolites (putative conjugates) were found in these organs over the same period. Breakdown of these conjugates, followed by ethylene release, probably accounts for elevated long-term ethylene emanation from flag leaves and spikes. While ethephon was transported from the flag leaf to the spike, little transport was observed from the flag leaf to the base of the plant. Injections of ethephon into the base promoted tillering, and reduced elongation of the upper stem. Foliar ethephon treatments which reduced plant height also increased post-treatment tillering. It is concluded that the tillering response following ethephon treatment does not include a modification of ethylene biosynthesis, and the amount of ethephon imported into the base is insufficient to be a direct signal for the tillering response. Induction of tiller growth may be a result of ethephon inhibition of upper stem elongation, rather than a direct response of the buds to ethephon.