Dormancy and Impotency of Cocklebur Seeds

Abstract

Dormant seeds of cocklebur (Xanthium pennsylvanicum Walir.) were characterized by the lack of ability to form chlorophyll. Such an inability of cotyledons of the dormant seeds was improved by the application of various factors and reagents which were capable of breaking the dormancy and of increasing cotyledon enlargement. Of these, ethylene, benzyladenine, and high temperature treatments were particularly effective, and, in turn, oxygen enrichment, gibbereHlic acid, thiourea, carbon doxide, and potassium nitrate were also promotive to the greening of the dormant cotyledons. The effects of benzyladenine, oxygen enrichment, and high temperature were reduced in the presence of absorbents for endogenously evolve carbon dioxide and ethylene. &-Aminolevulinic acid could not restore their greening ability.In dicotyledonous seeds, not only a growing embryonic axis but also a pair of enlarging cotyledons are responsible for rupture of testa on germination (8, 9). It has also been demonstrated that the dormancy of the seeds is due to the inability of both the axis and cotyledons to grow (8). The dormancy of cocklebur seeds is characterized by the lack of the aerobic C2H4 production in them (13,19) and is broken by C2H4 applied exogenously, which is capable of restoring their growth (8).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).Concerning the possible actions of plant hormones in breaking seed dormancy or in accelerating seed germination, there have been the two views that, first, they may act by stimulating the biosynthesis of hydrolysis enzymes (2, 29); and second, the contrary is the case in which the primary action of GA3 may not be the activation of de novo synthesis of hydrolysis enzymes, such as amylase (7) and protease (6). 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. Furthermore, the growth inability of the axis and cotyledons of dormant cocklebur seeds implies that seed dormancy is due to the arrest of activity not only of catabolic reaction processes in these tissues but also of anabolic reaction I Present address: Department of Biology, Miyagi University of Education, Aobayama, Sendai 980, Japan.processes. In the dormant cotyledons, the arrested anabolic processes may be symbolized by the failure of Chl biogenesis and chloroplast development, and the various factors which are capable of breaking seed dormancy may pe...