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

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 CzH4 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 C02 and increased by 02 enrichment. Both C2H4 and 02 significantly increased osmolarity of the sap extracted from the whole seed, but COz 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.