Mechanism of Action of Ethylene in Promoting the Germination of Cocklebur Seeds I. Osmoregulation

Abstract

The germination potentials of upper and lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.) were increased by pre-exposure to water stress imposed by mannitol or polyethylene glycol (PEG) solution. Both types of seeds were responsive to this 'seed priming' which was further enhanced by C₂H₄ applied exogenously for at least 1 day. The C₂H₄ effect that occurred during seed priming increased with increasing water stress. It was not influenced by respiratory inhibitors, such as cyanide, azide, benzohydroxymate or propyl gallate, but it was counteracted by CO₂ and increased by O₂ enrichment. Both C₂H₄ and O₂ significantly increased osmolarity of the sap extracted from the whole seed, but CO₂ reduced it. Similar events occurred in seed axial and especially in cotyledonary tissues, and in water extracts of these tissues. Increased osmotic pressure in response to C₂H₄ stimulated subsequent cotyledonary growth. These findings were applicable also to the action of C₂H₄ under anoxic conditions, in which the increased potential of seed germination caused by O₂ shortage was further enhanced by C₂H₄ treatment. The higher osmolarities were linked to a stimulation of cotyledonary growth.