Development of seed coat-imposed dormancy during seed maturation in Cynoglossum officinale. -Physiol Plant, 97: 28-34,The relationship between seed phenolics and appearance of seed coat-imposed dormancy during seed development in Cynoglossum officinale L, was studied. Up to 24 days after anthesis, seeds failed to genninate upon imbibition in Petri dishes at 25°G, At 44 days after anthesis, seeds were fully geiminable; removal of seed coats did not improve their germination or O2 uptake. At 72 days after anthesis. mature seeds at the base of the cyme did not germinate unless their coats were removed. Removal of seed coat also stimulated O2 uptake at this harvest date. The methanol-soluble phenolic content of the seeds increased during the early stages of seed development, in both the seed coat and the embryo. As seed development continued, the methanol-soluble phenolic content of the embtyo stabilized, but that of the seed coat declined. This decline was associated with an increase in the thioglycohc acid-soluble phenolics, presumably lignins, in the seed coat. These results suggest that polymerization of methanol-soluble phenolics into lignins in the seed coat during later stages of seed development renders the seed coat of C. officinale impermeable to Oj, and thus keeps the seed dormant.

To understand the role of the seed coat in regulation of houndstongue seed dormancy, the effects of manipulation of seed coat integrity on seed germination and O2uptake were studied. The results suggest that the seed coat of this weed regulates dormancy in part by interfering with the diffusion of O2to the embryo. Scanning electron microscopy showed a network of ridges on the seed coat surface that were partially dissolved following 1.5 min of sulphuric acid scarification. Mechanical scarification removed fragments of the seed coat surface. Both scarification treatments stimulated seed germination. Supply of an elevated level of O2also stimulated seed germination. O2uptake by seeds imbibed in O2-saturated water was 150% higher than that for seeds imbibed in air-saturated water. Although all treatments that stimulated seed germination also stimulated O2uptake, there was a lack of a consistent, quantitative relationship between increases in O2uptake and seed germination in various experiments. This suggests that limitation of O2availability to the embryo is not the only factor involved in regulation of houndstongue seed dormancy by the seed coat. Mechanical restriction of embryo expansion by the seed coat may also be important. Methanol-insoluble phenolics constituted < 1% of the total phenolic pool in the embryo. Their potential oxidation could not account for more than a small fraction of the previously reported massive stimulation of O2uptake by the embryo upon decoating. The present O2uptake and seed germination studies indicate that not all of the large increase in O2uptake following decoating is essential for houndstongue seed germination.

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