ROLE OF SEED COAT IN REGULATION OF SEED DORMANCY IN HOUNDSTONGUE (CYNOGLOSSUM-OFFICINALE)

To understand the role of the seed coat in regulation of houndstongue seed dormancy, the effects of manipulation of seed coat integrity on s eed germination and O-2 uptake were studied. The results suggest that the seed coat of this weed regulates dormancy in part by interfering w ith the diffusion of O-2 to the embryo. Scanning electron microscopy s howed a network of ridges on the seed coat surface that were partially dissolved following 1.5 min of sulphuric acid scarification. Mechanic al scarification removed fragments of the seed coat surface. Both scar ification treatments stimulated seed germination. Supply of an elevate d level of O-2 also stimulated seed germination. O-2 uptake by seeds i mbibed in O-2-saturated water was 150% higher than that for seeds imbi bed in air-saturated water. Although all treatments that stimulated se ed germination also stimulated O-2 uptake, there was a lack of a consi stent, quantitative relationship between increases in O-2 uptake and s eed germination in various experiments. This suggests that limitation of O-2 availability to the embryo is not the only factor involved in r egulation of houndstongue seed dormancy by the seed coat. Mechanical r estriction of embryo expansion by the seed coat may also be important. Methanol-insoluble phenolics constituted < 1% of the total phenolic p ool in the embryo. Their potential oxidation could not account for mor e than a small fraction of the previously reported massive stimulation of O-2 uptake by the embryo upon decoating. The present O-2 uptake an d seed germination studies indicate that not all of the large increase in O-2 uptake following decoating is essential for houndstongue seed germination.