Studies of Dormancy in the Seeds of Subterranean Clover (Trifolium Subterraneum L.) II. The Interaction of Time, Temperature, and Carbon Dioxide During Passage Out of Dormancy

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Abstract
The dormancy-breaking action of CO2 on imbibed seeds of T. subterraneum) is temperature dependent, the efficiency falling off sharply above 25[degree]C and approaching zero at 30[degree]C. Within the effective temperature range the action is rapid, exposure of seeds to CO2 for 6 hours having significant effect. Freshly harvested seeds of dormant strains do not germinate at 22[degree] C or higher temperatures, but do so at 11[degree] C and lower temperatures. When held at temperatures between 3 and 11[degree] C for 1 and 3 days, and then transferred to 22[degree] C, the germination of freshly harvested seed is lower than if no transfer had been made; but it is higher the longer the duration of cold treatment. The extent of the high temperature inhibition of germination of both young and old seed depends both on strain and age of seed. An hypothesis is advanced in which germination is assumed to be dependent on the level of some carboxylated compound. The dormany-breaking actions of CO2 of activated carbons, and in part at least of low temperature, are interpreted in terms of net carboxylation. In addition to this control mechanism, evidence is presented for the existence of at least 2 other steps in the germination sequence. The first is the passage out of a stage of "ultra-dormancy," during which seeds are sensitive neither to low temperature nor CO2 treatment. Ultra-dormancy has been noted only in the most dormant strains, and only in some seasons. The second is the formation of a precursor of the carboxylated compound.