Influence of factors affecting germination on respiration of Phacelia tanacetifolia seeds.

Germination of the seeds of Phacelia tanacetifolia is inhibited by light. Removal of that part of the covering structures of the seeds which directly covers the radicle allows full germination in light. The rate of O2 uptake in the seeds increases following imbibition, and reaches the same steady rate in light and in darkness after 3 hr. From the 14 th hour on, dark-imbibed seeds show a linear increase in the rate of respiration. This increase is not observed in "dormant" seeds incubated in light. In normal dark germination, protrusion of the radicle begins at 12 th hour following soaking, and by the end of 18 th hour approximately 60% of the seeds have germinated. The seeds which have been scarified at the radicle end and germinate readily in light show a steady increase in Q O 2. If scarified seeds are allowed to imbibe 0.3 M mannitol and are then incubated in light, the embryo does not grow and the pattern of O2 uptake becomes identical with that of intact seeds in light. Mannitol, however, does not inhibit respiration by itself. These observations indicate that the increased O2 uptake is the result rather than the cause of seed germination, and that light does not cause dormancy by inhibiting O2 uptake. Measures effective in releasing dormancy (dark incubation, mechanical scarification, gibberellin treatment) do not induce germination by facilitating oxygen entry.