No abstract

Non-dormant upper cocklebur (Xanthium pensylvanicum Wallr.) seeds germinated bimodally, in response to low temperature as well as to high temperature. At low temperature, the process was aerobic. Increase in germination potential by pre-exposure to low temperature was termed 'chilling induction'. Similarly to anaerobic induction of cocklebur seed germination, chilling induction required a certain time of presoaking to be effective. The germination pattern was identical in both cases, the seed coat being broken at the axial end. In contrast to anaerobic induction, however, chilling induction was not affected by exogenous ethylene and the effect of chilling was cumulative within 3-4 days, but decreased with increasing duration of chilling beyond these times. The effect of anaerobic induction was enhanced by a pre- ceding chilling, as described in a previous paper and, similarly, the effect of chilling induction for fully presoaked seeds was additively increased by a preceding period of anaerobiosis. However, the effect of the chilling was decreased by a subsequent anaerobiosis.

Germination of non-dormant small cocklebur (Xanthium pennsylvanicum Wallr.) seeds was improved by immersing them in water, suggesting that during their germination endogenous germination inhibitors are leached out. However, the same effect could be obtained by the quasi-anaerobic pre-incubation of the seeds. When seeds were fully imbibed, moreover, water immersion could no longer potentiate them to germinate, and only anaerobiosis increased the germination potential, thus raising a question against the "inhibitor hypothesis" of seed dormancy.

Non-dormant, upper cocklebur (Xanthium pensylvanicum Wallr.) seeds, incapable of germinating under ordinary conditions, can germinate when previously subjected to anaerobiosis; this has been termed the anaerobic induction of seed germination. Aerobic presoaking of the seeds was also required for successful anaerobic induction, and exerted two counter-acting effects on seed germination. When the time period of aerobic presoaking was sufficiently prolonged, the increasing duration of an anaerobic treatment resulted in proportional increase of germination potential but, when it was short, the effect of the anaerobiosis was saturated in a few days. Prolonging the aerobic presoaking period caused less response of the seed to the anaerobic induction, suggesting the development of some germination-inhibiting system during the aerobic presoaking period. This system could not develop in the absence of O2 or at low temperature. Thus, low temperature during prolonged presoaking produced a maximal response to anaerobic induction. Various germination stimulants, CO2, ethylene, gibberellic acid and benzyl adenine, did not significantly alter the effects of the presoaking.

C2H4 production of the embryonic axes and cotyledons excised from dormant and non-dormant cocklebur (Xanthium pennsylvanicum Wallr.) seeds was examined in relation to ambient O2 tensions. There were two kinds of C2H4-producing systems, quasi-anaerobic and aerobic, in both organs. Regardless of the organ, the former activity was high in the dormant state and, particularly in axes, declined with after-ripening. On the other hand, the latter activity was almost insignificant in the dormant state, but increased with release from dormancy and the non-dormant axes exclusively produced C2H4 through this system. In the cotyledons, however, the former was still predominant even after they were fully after-ripened. Thus, the C2H4-producing systems were different in the seed organ and in the dormancy state.