Immature embryos isolated from fruits of Acer pseudoplatanus L. at various stages during a 24-week development period, showed a sharply defined transient accumulation of endogenous free ABA, with peak levels at 10 weeks postanthesis (wpa). When this ABA peak occurred isolated embryos showed a high level of germinability when incubated in sterile culture; however, germinability fell rapidly thereafter, reaching minimum values at 14 wpa before recovering as maturity approached. Halfembryos and excised embryonic axes, although showing consistently higher germinability than whole embryos, also showed a similar progressive reduction in germination capacity up to 14 wpa, followed by a recovery in germinability. This peak of free ABA in the embryo also correlated with the start of the most rapid phase of both embryo mass and volume increase. The possibility that endogenous ABA is the causal agent of this short-lived embryo dormancy in A. pseudoplatanus is discussed, as is the possibility that by suppressing precocious germination, ABA frees the embryo to initiate or accelerate its normal developmental processes.

We have studied by means of cytology and autoradiography the effect of benzyladenine (BA, a synthetic cytokinin) on DNA metabolism during early maize germination.The data indicate that BA stimulates nuclear DNA replication. The doubling of the amount of nuclear DNA in BA-treated axes occurs earlier than in nontreated axes, and there is a three-fold increase in the mitotic index at 24 h of germination. These results provide further corroboration for the suggestion that the stimulation of DNA synthesis observed relates to a nuclear replicative type of synthesis. Possible mechanisms of action of BA are discussed.

Loss of vigour in leek (Allium porrum L.) seed lots is accompanied by the appearance of damage to ribosomal RNA in quiescent embryo tissue. Polyethylene glycol osmopriming treatments of such low-vigour seed permit replacement of this damaged ribosomal rRNA over a 7-day priming period. Low-vigour leek seeds germinated for 6 days without a prior osmopriming treatment still exhibit evidence of damaged ribosomal RNA in embryo tissue. Osmoprimed leek seeds dried back whilst still in the desiccation-tolerant state retain most of the benefits conferred by priming treatments. Osmoprimed leek seeds dried back when having reached the desiccation-sensitive state germinate poorly, if at all, and exhibit much reduced rates of protein synthesis in embryo tissue upon rehydration compared to desiccation-tolerant seeds. Embryo tissue from dried back, desiccation-tolerant seeds exhibits the capacity for much higher levels of protein synthesis than embryo tissue from unprimed seeds at equivalent stages of imbibition. RNA levels continue to increase in embryo tissue upon rehydration of dried back, desiccation-tolerant leek seeds but not in desiccation-sensitive seeds. Loss of protein synthetic capacity in desiccation-sensitive seeds during rehydration is accompanied by increasing levels of ribosomal RNA degradation. Such ribosomal RNA degradation may be indicative of the germinative capacity of leek seeds dried back after osmopriming treatments.

The capacity for protein synthesis in embryonic axes of Phaseolus vulgaris L. cv. Top Crop seeds was reduced markedly by soaking in airsaturated and CO2-saturated water. The former treatment induces soaking injury while the latter treatment prevents injury. After soaking, the capacity for protein synthesis in axes of seeds soaked in air-saturated water declined and the pattern of synthesis remained almost similar to that at the time of termination of soaking. However, the capacity and pattern of protein synthesis in axes of seeds soaked in CO2-saturated water recovered subsequent to soaking and approached that of non-soaked seeds.

Priming in polyethylene glycol (PEG, −1.5 MPa) for 7 d followed by drying, led to a 4–5 fold increase in mean longevity when achenes (seeds) of Ranunculus sceleratus L. were subsequently stored at 35°C and 9.2% moisture content on a fresh weight basis (67.1% equilibrium relative humidity). The increase in longevity was due to an increase in the standard deviation of the frequency distribution of individual seed lifespans (decrease in the slope of transformed survival curves) and to a lesser extent to an increase in the intercept of survival curves. Priming for 1 d resulted in a smaller but significant increase in longevity independent of whether seeds were primed in PEG, distilled water or a saturated atmosphere (100% RH).

The effects of priming were not due to the prevention of imbibition injury, and there was no evidence that the water relations of whole achenes was affected by priming. The effects of a 7 d priming treatment were dependent on the temperature and nature of the priming medium. However, no differences were recorded between corresponding treatments which were primed in PEG or distilled water. Moreover, the survival and responsivity to priming of R. sceleratus seeds was not related to dormancy status. The effects of priming on the longevity of R. sceleratus seeds are apparently unrelated to seed quality as the response of seeds previously aged for 8 d at 35°C and 7.9% moisture content was similar to that for unaged seeds.

Priming followed by drying did not increase seed longevity in the related species R. acris L. The promotive effects of priming on seed survival appear to be species specific and may be related to ecological factors.

Galactomannan hydrolysis in isolated endosperms of fenugreek seeds is promoted by the presence of the embryo. Incubation in a large volume in the absence of the axis also results in endosperm mobilization and an increase in endo-β-mannanase activity. This is prevented when endosperms are incubated in a small volume, or when abscisic acid is present in the large volume. Fenugreek endosperms contain abscisic acid which is present in greater concentrations when these tissues have been incubated in a small volume rather than in a large volume. Antibodies prepared against purified tomato endo-β-mannanase detect the fenugreek enzyme on Western blots. Endosperms contain equal amounts of this enzyme whether they are incubated in a large volume, or in a small volume, or in the presence of abscisic acid. Thus, increased enzymic activity may not be related to increased synthesis, but rather to activation or release from the cells into the galactomannan-containing cell walls.

Identification of discrete stages during embryogenesis is important for the consistent and repeatable selection of seeds having similar developmental characteristics. A timetable for staging developing seeds of alfalfa (Medicago sativa L.) has been developed. Morphological characteristics, fresh and dry weights, SDSpolyacrylamide gel electrophoretic protein patterns and total protein content were recorded at various times between fertilization and 36 d after pollination (maturity), stages I–IX. A full complement of storage proteins (2S, 7S, 11S) is synthesized in both developing cotyledons and radicles. Low-salt soluble (S1) and high-salt soluble (S2) storage proteins first appear during embryo elongation. The proportional amounts of some S1 storage proteins change during alfalfa seed development. Markers are thus provided as uniform reference points for staging from the time of anthesis to seed shedding.

This paper is a résumé of research on imbibitional chilling injury of seeds. The following summarizes the experimental results obtained and some views and concepts derived from the studies conducted at Beijing Botanical Garden during the last decade. A lot of evidence has supported and confirmed the hypothesis for imbibitional chilling injury in seeds, i.e. the primary target of harmful effects during the chilling rehydration in seeds is the membrane systems, and the subsequent degree of lasting damage depends highly on the repair of the membrane systems themselves. The two processes of repair, physical and physiological–biochemical processes, can complement each other to a certain extent.

Embryogenesis was induced on cotyledons of immature zygotic embryos of soybean (Glycine max (L.) Merrill) placed on solid medium containing 62.5 mm glutamine, soybean seed growth medium salts and vitamins, and 40 mg I−1 2,4-dichlorophenoxyacetic acid (2,4-D) plus 175 mm maltose, or 8 mg I−1 α-naphthaleneacetic acid (NAA) plus 88 mM sucrose. Somatic embryo development was continued in liquid medium containing 0.16 mg I−1 indole-3-butyric acid and 2.64 mg I−1 abscisic acid, glutamine and salts as above, and 88–438 mM sucrose in progressively increasing steps. Germination was on solid half-strength Murashige-Skoog medium. During maturation, somatic embryos mimicked zygotic embryos in colour, protein concentration, water and solute potentials, and respiration. Protein and lipid accumulated to 329 and 86 g kg−1 dry weight in somatic embryos. Fatty acid composition was similar to that of axes of mature seeds. Before desiccation, the water and solute potentials of maturing somatic embryos declined to −1.13 and −1.99 MPa while turgor increased to 0.86 MPa. Concomitantly, a 60% reduction in activity of the cytochrome oxidase pathway of respiration occurred with somatic embryo maturation at 600 g water kg−1 fresh weight. Although small (about 8 mg per embryo), 60% of the somatic embryos formed roots and shoots during germination after maturation without drying and 30% germinated after drying to 60 g water kg−1 fresh weight. In the greenhouse, somatic plantlets grew to mature plants with seeds.

Amaranthus albus L. seeds able to germinate in darkness can be prevented from germinating by prolonged irradiation with incandescent light (CIL). The germination in darkness is controlled by pre-existent Pfr. Allowing the seeds to remain in darkness for about 16 h allows an escape from the inhibition by CIL, about comparable with the time required to escape from pre-existent Pfr control. By examining escape from low fluence far-red (FR), it was found that no escape occurred in CIL. It was concluded that the mode of action of CIL in inhibiting A. albus seed germination involved prevention of the action of the pre-existent Pfr and a subsequent dormancy which does not allow a response to the Pfr generated by the incandescent light. Other actions of CIL were found but were considered less likely as causative for the inhibition of germination. One was a direct inhibition of radicle elongation and another involved prevention of an increased responsiveness to Pfr induced by treatment at 40°C.

Cotyledon length in onion (Allium cepa, cv. White Lisbon) was linearly related to time from radicle emergence until the formation of the cotyledon hook, but thereafter increased exponentially with time. Within each phase the rate of cotyledon elongation was linearly related to temperature, and these relations were similar among seeds from several percentiles of the germinating populations of three contrasting seed lots. The mean base temperature was 1.4°C. These thermal time relations closely predicted the elongation of cotyledons of seedlings from cohorts of early- and lategerminators from a further lot of this cultivar at two suboptimal temperatures. Eleven cohorts of seedlings from seeds of differing quality were transplanted to the field on both the same date and at the same time from visible germination. The plants were harvested 59 days later. No effect of seed quality on mean plant weight, mean bulb diameter, or interplant variation in bulb diameter was detected. It is concluded that the rate of pre- and post-emergence seedling growth in onion is independent of environmentally induced differences in seed quality (seed vigour). Thus, any effect of this factor on plant size or crop yield, at a given density, is solely a function of time to germinate.

Prolonged imbibition and incubation of lettuce seeds at a supraoptimal temperature induces secondary dormancy. Such seeds no longer germinate when returned to conditions optimal for germination of non-dormant seeds. The possibility that inhibitors are involved in the induction of thermodormancy was investigated.

Washing of thermodormant seeds restored germination to a small extent. However, continuous leaching of seeds during high-temperature treatment with distilled water, largely prevented the induction of thermodormancy. Such seeds were qualitatively similar to nondormant seeds, i.e. they required only light to germinate at a lower temperature and germinated in the dark if given GA3. The germination water from thermodormant seeds inhibited the germination of non-dormant lettuce seeds. The inhibition was not an osmotic effect. Absorbance in the ultraviolet region was higher in germination water from thermodormant seeds than that of non-dormant seeds. It is concluded that inhibitors are involved in the induction of thermodormancy of lettuce seeds.

Chromosome integrity and DNA polymerase activity are negatively affected by improper storage of maize seeds. We have extended our studies on the effect of improper storage on DNA metabolism by analysing the effect of hot–dry and hot–humid conditions on DNA ligase activity. An assay to measure ligase activity was established by using linearized plasmid DNA as substrate and protein extracts from maize axes. Optimal conditions were established for the in vitro assay. Enzyme activity was importantly reduced under both storage conditions. Activity recovered 6–9 h after imbibition. A model for loss of viability and vigour due to damage of DNA metabolism is proposed.

Seeds of papaya (Carica papaya L.) stored for 12 months at 15°C with 7.9–9.4% moisture content maintained their original germination. In contrast, many seeds stored cooler or drier lost viability, the losses occurring more rapidly at −20°C than at either 0°C or 15°C. The results are not compatible with the definitions of either orthodox or recalcitrant seed storage behaviour.