Abstract In seed formation the role of ethylene has received little attention. The data available on zygotic embryogenesis suggest an association of the ethylene biosynthetic pathway and seed maturation. Over the course of dicot embryogenesis, ACC-oxidase mRNA can be expressed in the cotyledons and embryonic axis. However, as maturation proceeds, cotyledonary ACC-oxidase expression disappears. In some seeds that develop primary dormancy, ethylene synthesis can be among the prerequisites for breaking dormancy. Moreover, the persistence of dormancy may be related to the difficulty of the embryonic axis to produce the necessary ethylene levels or to low tissue sensitivity. The use of inhibitors of ethylene biosynthesis or its action has provided data implicating an ethylene requirement for seed dormancy or germination in some species. However, the role of ethylene in germination remains controversial. Some authors hold that gas production is a consequence of the germination process, while others contend that ethylene production is a requirement for germination. Furthermore, among seeds that require ethylene, some are extremely sensitive to the gas, while others require relatively high levels to trigger germination. Recent studies with Xanthium pennsylvanicum seeds suggest that β-cyanoalanine-synthase is involved in ethylene-dependent germination. In addition, regulation of the partitioning of S-adenosyl-L-methionine (AdoMet) between the ethylene vs polyamine biosynthetic pathways may be a way of controlling germination in some seeds. Such regulation may also apply to the reversal of seed thermoinhibition, which can occur when polyamine synthesis is inhibited, thereby strongly channelling AdoMet towards ethylene. The biological models and approaches that may shed additional light on the role of ethylene during seed germination are presented.

Non-dormant seeds of Orobanche aegyptiaca were incubated at water potentials of 0 to –1.33 MPa and at constant temperatures from 5 to 29°C. Effects of water potential and temperature on final germination were modelled. In general, germination increased with increased temperature from 5 to 20°C and decreased above 26°C. Maximum germination occurred at 20–26°C and 0 MPa. Germination was reduced as the water potential decreased. Water potential also affected the temperature range over which high germination was observed; at 0 MPa high germination occurred over 9° (17–26°C) compared with 3° at -1.25 MPa (17–20°C). The optimum germination temperature also tended to decrease with a decrease in water potential. Final germination could be accounted for by seed-to-seed variation in the population assuming that each seed had a minimum temperature for germination and a maximum temperature above which it would not germinate. Seed-to-seed variation in these characteristics was assumed to be normally distributed, and it was further assumed that the two characteristics were independent. Effects of water potential on these temperature requirements were quantified, and the resulting empirical model accounted for final germination with reasonable accuracy (R2= 0.96).

The effects of desiccation and temperature on the germination capability of Carica papaya L. were investigated for seeds extracted from three commercial fruit batches. More than 50% of freshly isolated, cleaned (sarcotesta removed) but undried seeds germinated at 26°C. However, desiccation to approx. 20%seed RH reduced germination at this temperature to < 10%. A substantial increase in germination at alternating temperatures (33/19°C) indicated that desiccation induced seed dormancy rather than viability loss. Dormancy could be removed in a large proportion of the population by the application of a single heat shock to rehydrated seeds for 4 h at 36°C, with subsequent return to 26°C for germination. Longer (days) and shorter (minutes) periods of heat shock were less effective for releasing dormancy. Heat shock was generally applied 5 or 14 d after rehydration had started, but the treatment was equally effective after imbibition for only 1 d. Light was always applied during both imbibition and the post-heat shock treatment, but was not essential during the actual heat shock treatment. Rehydration and post-heat shock temperature treatments in the range of 16°C to 36°C revealed the same optima of 26°C. Dormancy was re-imposed in heat-shocked seeds when they were subsequently dried to seed relative humidities of 25 to 75%(5 to 11% moisture content [fresh weight basis]), but this state could be removed by a further heat shock. The longer heat-shocked seeds were held on agar-water at 26°C prior to re-drying, the greater the level of desiccation intolerance.

The viability of Norway maple seeds collected 21 d before mass maturity (68%moisture content, wet basis) and at mass maturity (56% moisture content) was reduced from 52–85% to 0–7% if dried rapidly (at 10–12% r.h. and 15–17°C for 3 d, then 3 d over silica gel) to 4–5% moisture content. Moist storage of the fruits at 15°C improved the ability of the seeds to tolerate rapid desiccation considerably: 10 and 21 d of moist storage enabled seeds collected at mass maturity or 21 d earlier, respectively, to attain maximum desiccation tolerance to 4–5% moisture content. Moist storage and/or subsequent desiccation affected stachyose, sucrose, and to a lesser extent raffinose, concentrations. The oligosaccharide:total sugar ratio showed a similar pattern in relation to ability to germinate after desiccation to 4–5% moisture content among seeds collected on both dates: desiccation tolerance developed from nil to maximal in these seed populations between threshold oligosaccharide:total sugar values of just less than 0.3 and about 0.4.

Priming of sugarbeet (Beta vulgaris L.) seeds induces increased solubilization of the basic B-subunit of 11-S globulin (a major seed storage protein in sugarbeet). Using a sensitive single-seed ELISA, the soluble and total B-subunit contents of individual untreated and primed sugarbeet seeds were measured. With the untreated seeds, there was a 160-fold range of the soluble B-subunit content among individual seeds. The individual primed seeds also exhibited large variations in their soluble B-subunit content, yet only over a five-fold range. Furthermore, the frequency distributions of soluble B-subunit content were markedly different for the primed and untreated seed populations; the primed seed population exhibited a substantially higher median than that for the untreated seed population. In marked contrast, and as expected from results with pooled seed samples, the distributions of total B-subunit content were superimposed when comparing untreated and primed seed populations.

An image analysis system is described to study the process of imbibition in white cabbage (Brassica oleracea L.) seeds. The system consists of two components: 1) an environmental system where seeds germinate; 2) a computer imaging system, composed of a Charged Coupled Device (CCD)-imaging sensor, an image frame grabber, a computer and a video monitor. The measurement software can determine area, perimeter, width, length and eccentricity as descriptors of changes in seed size during swelling. The captured images of imbibing seeds allowed estimation of timing of ‘visible germination’ occurrence without any subjective evaluation. Using the area parameter, the imbibition process can be described graphically by a series of curves similar to the triphasic pattern of water uptake, with extension and rate depending on the degree of seed viability and germination medium conditions. Imbibitional osmotic stress, provided by -1.5 MPa mannitol, induced a large delay in germination and, upon stress removal, a more uniform radicle emergence. The versatility and the sensitivity of the method, together with the feasibility to investigate germination rate in individual seeds within a population, suggest that image analysis techniques have high potential in seed biology studies.

Abstract In isolated embryos from dormant barley grains, synergistic effects of fusicoccin (FC) and gibberellic acid (GA3) were observed on the induction of α-amylase mRNA expression. However, no α-amylase mRNA expression could be induced by both agents in embryos from non-dormant grains. Both light- and electron-microscopy studies demonstrated that there were large numbers of starch granules present in mature embryos (mainly in scutellum) from dormant barley grains but none or almost none in embryos from non-dormant grains. Furthermore, the content of reducing sugars in embryos from dormant grains was about half of that from non-dormant grains. In contrast to GA3, FC was able to induce a strong acidification of extracellular pH (pHe). Clamping the pHe to prevent FC-induced acidification, by using 50 mM MES buffer (pH 5.6), caused an inhibition of GA3- or FC-induced α-amylase mRNA expression but did not affect the germination of embryos from dormant grains. In addition, in MES buffer, addition of FC or a combination of FC and GA3increased the germination rate of embryos isolated from dormant grains, though large numbers of starch granules were still present in these embryos. Based on these observations, the presence of starch granules and a low reducing sugar level in embryos from dormant grains is not a factor for control of grain dormancy and germination.

Seeds of four African tree species from the Sahelian zone (Boscia senegalensis) and the Sudanian zone (Butyrospermum parkii, Cordyla pinnata and Saba senegalensis) lose viability after moisture contents drop below 22 and 30%, depending on the species. Seed longevity in wet and airtight storage does not exceed a few months. Temperatures close to zero elicit symptoms of chilling injury leading to rapid seed death. The optimum storage temperature is 15°C. These results allow the four species to be classified as recalcitrant seed species. Boscia senegalensis is therefore an exception in arid zones where most species have orthodox seeds.