Dormancy and Impotency of Cocklebur Seeds

Abstract

Germination of nondormant but impotent small cocklebur seeds (Xanthium pennsylvanicum Walr.) was promoted profoundly with thiourea or benzyladenine, and slightly with gibbereUlic acid. GibbereUic acid was ineffective in causing the germination of dormant cocklebur seeds, although thiourea and benzyladenine were effective. Experiments with excised seed pieces showed that the promotive effects of thiourea, benzyladenine, and gibbereUlic acid on cocklebur seed germination were associated with the enhancement of growth of seed parts; thiourea stimulated predominantly the axial growth, whereas benzyladenine stimulated predominantly the cotyledonary growth.Potassium nitrate or indoleacetic acid had little effect on the initial growth of either axes or cotyledons. Except for gibberellic acid, all of the compounds employed enhanced ethylene production, but in general, the ethylene production seemed more likely to be a consequence of growth rather than a cause of it. We conduded that the chemical regulation of seed germination may be a consequence of the alteration of growth capabilities in either the axes or cotyledons, or both.In a previous paper (10), we observed that cocklebur seeds respond in different manners to various germination stimulators: seeds treated with TU' or CO2 exhibited normal germination, but 02 enrichment caused germination in which the seed coat was mostly broken at the cotyledonary side rather than at the axial end. On the other hand, the seeds germinated with BA, C2H4, and GA3 exhibited an intermediate pattern in which approximately half of the germinations were normal. Granting that seed germination in dicot plants is a phenomenon occurring when the axis and the cotyledons generate enough thrust to overcome the restraint by the seed coat (8), these diverse response patterns might result from the differential growth responsiveness of the axis and cotyledon: for example, TU could preferentially enhance growth of the embryonic axis proper, whereas BA could more greatly enhance growth of the cotyledons and hence the abnormal lateral breaking of the seed coat. We have previously presented some evidence that C2H4, unlike CO2, strongly stimulates both the axial and cotyledonary growth (7).These findings caused us to examine the relative effectiveness of various known germination stimulators (15), such as TU, KNO3, BA, GA3, and IAA, in stimulating the growth of axial and cotyledonary seed pieces. Experiments were also directed toward a comparison between the dormant and nondormant seeds in response to these stimulators.' Abbreviation: TU: thiourea. MATERIALS AND METHODSSeeds of cocklebur (Xanthium pennsylvanicum Wallr.) were used in this experiment. For germination tests, dormant small seeds newly harvested in 1973, and the nondormant but impotent small seeds fully after-ripened since 1972 were exposed to various drugs on two filter paper discs in 5-cm Petri dishes with 2 ml water. For growth tests, the embryonic axis and cotyledon segments were excised from large seeds according to the procedur...

Dormancy and Impotency of Cocklebur Seeds

“…We have suggested that the plant hormones may exert their primary actions by stimulating the growth of an embryonic axis rather than by supplying hydrolysates (11). The above mentioned facts obtained in the experiments about cocklebur seed germination suggest the plausibility of the latter view.…”

“…Of the factors listed in Table III, enriched 02, high temperature, BA, and thiourea were effective to break cocklebur seed dormancy and to increase cotyledon enlargement, whereas GA3 and KNO3 were not effective to break the dormancy but promotive to the enlargement. IAA was ineffective on both (11).…”

“…the cotyledons to restore the Chl formation. Therefore, several experiments were designed to examine the effects on the cotyledon greening of many diverse factors and reagents which accelerate the loss of the cocklebur seed dormancy and hence initiate the enlargement of the dormant cotyledons (11).…”

“…Cytokinins and thiourea, besides C2H4, are able to release the dormancy of cocklebur seeds by restoring the growth of both the dormant embryonic axis and cotyledons (11). However, GA3 and KNO3, which were often effective to break dormancy in other seeds (2,29), failed to terminate the dormancy of cocklebur seeds, since they were incapable of causing the axial growth which played a leading role in rupturing the testa, although slightly capable of restoring the cotyledonary growth (11).…”

“…However, GA3 and KNO3, which were often effective to break dormancy in other seeds (2,29), failed to terminate the dormancy of cocklebur seeds, since they were incapable of causing the axial growth which played a leading role in rupturing the testa, although slightly capable of restoring the cotyledonary growth (11).…”