Oxygen-Independent Ethylene Action in Cocklebur Seed Germination in Relation to Osmoregulation

Abstract

The pre-incubation of cocklebur (Xanthium pennsylvanicum Wallr.) seeds under the water-stressed conditions markedly enhanced the subsequent germination, showing that cocklebur seeds were also responsive to “seed priming”. This effect was significantly increased by ethylene applied during a period of the priming, and the increment rose with increasing the water stress. The respiration-stimulating effect of ethylene was not found under the stressed conditions and its promotive effect on the seed priming was not affected by various respiration inhibitors, suggesting that ethylene action in this case is not dependent on oxygen or aerobic respiration. Ethylene increased the osmotic pressure (OSP) of both cell saps and water extracts from the whole seeds and from their axial and cotyledonary tissues. The rise of OSP by ethylene occurred even when it was applied to seeds in anoxia. The effectiveness of ethylene in all cases was striking in an increase of amino acid contents, although in some cases ethylene augmented the contents of sugars and soluble proteins. Glutamine, arginine, asparagine, alanine, γ-aminobutyric acid and serine were particularly augmented by ethylene. A possible role of accumulated amino acids in the promotion of seed germination was suggested by the findings that the growth-stimulating effect of ethylene under water stress was reduced by addition of mixed amino acids, and a synergestic relationship between ethylene and carbon dioxide can be replaced by the combination of carbon dioxide with the above-mentioned amino acid mixture.