In the past there have been scattered reports of breaking of dormancy in seeds by ethylene (4, 11) or by ethylene chlorohydrin (5) which apparently acts on plants through a release of ethylene (12). The advent of gas chromatographic techniques has made it possible to show that many seeds produce ethylene naturally during the germination process (8, 9, 10); these reports raise the interesting possibility that the production of ethylene by the imbibed seed mav contribute to the breaking of dormancy. This possibility lhas been examined for peanuts by Toole et al. (10) and Ketring and MIorgan (6),. who concluded that the ethylene production may actually contribute to the breaking of dormancy. From studies with lettuce, Abeles and Lonski (1)
1994). A mechanism of seed deterioration in relation to the volatile compounds evolved by dry seeds themselves. AbstractSome of the seed-evolved volatiles, which were mainly composed of methanol, acetaldehyde, ethanol and acetone, caused the loss of seed germmabihty during storage In general, the deleterious effects of the volatiles increased with increasing RH and temperature during storage Acetaldehyde had the strongest deleterious effect regardless of RH and temperature, while ethanol caused seed deterioration only at high RHs Acetone was slightly deleterious to some species, while methanol was almost inert in most seeds Various aldehydes applied during storage showed some toxicity to seed germmabihty, which decreased, except for 3-methylbutanal, with increasing molecular size, suggesting that the endogenous volatile compounds with an aldehyde group cause seed deterioration On the other hand, the contents of volatile compounds in seeds were higher when they were stored at 44% RH (water sorption zone 2), than when stored at 12% RH (water sorption zone 1) Acetaldehyde, the most deleterious volatile, was more abundantly accumulated within the seeds stored at -3 5°C than at 23°C Based on these facts, it is suggested that endogenous volatiles, especially acetaldehyde, may be an important factor that accelerates seed deterioration which often occurs under lower RHs and/or temperatures throughout long-term storage
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.
Abstract. The germination of seeds of Xanthium pensylvanicum Wallr. occurs in 2 phases, an initial passive phase of water uptake followed by an active phase of growth. These 2 phases have been separated experimentally, and shown to occur similarly in isolated cotyledons and embryonic axes. Measurements of the physical thrust generated by the entire seed and its separate components of cotyledon and axis reveal that non-dormant Xanthium seeds develop more than twice the thrust of dormant seeds, and that this difference develops principally in the second phase of enlargement of the axis. Measurement of the forces required for piercing the testa of these seeds establishes that whereas the thrust developed by non-dkwmant seed is adequate to cause testa rupture, that developed by dormant seeds is not. It is concluded that the dormancy of Xanthium involves an inadequacy in the embrvo for rupture of the testa.In the fruits of Xanthiuin there are 2 seeds, a smaller dormant one and a larger non-dormant one (1). The dormant condition has been attribtuted by Crocker (3,4) to impernmeability of the testa to oxvgen, although he could not find a great difference in permeability between the 2 types of seeds. On the other hand, Wareing and Foda (17) attributed dormancy to the lesser ability of the embryo of the small seed to oxidize a water-soluble inhibitor.For many vears it has been assumed that the embrvo must generate enough swelling force to overcome the restraining force of the testa or seed coat (2,5,6,13), and this force generation has been recentlv identified in lettuce seeds with the lightsensitive phase of germination (9). We have attempted to measture the forces generated by germinating Xanthiumn. seeds, and the restraining force of the testa, in an effort to define nmore precisely the germinative processes and how they may be restrained in dormancy. Materials and MethodsIn the present experimiients, fruits of cocklebur (Xanhuthin pensylvanicumt)
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