Seed germination of the wetland emergent perennial Phragmites australis is stimulated by diurnally fluctuating temperatures. A germination experiment in darkness and light at different temperature regimes showed that P. australis germinated as well in darkness as in light over most of the temperature regimes tested. The germination requirements could partly explain why this species, despite a large annual production of small seeds, does not accumulate a persistent soil seed bank. A second experiment examined the effect of one to four diurnal temperature cycles with amplitude of fluctuations ranging from 0 to 30°C; diurnal mean temperature of 15°C. Germination in the absence of fluctuations was low, and logistic model estimates showed a positive effect of number of temperature cycles which was strongly influenced by amplitude size. For seeds that have fallen on moist ground during the winter, these laboratory results predict that a few large diurnal temperature fluctuations could be sufficient for onset of germination in the subsequently warmer spring period. Seeds that are located under water, however, are predicted to remain ungerminated until the water level falls. Hence, we suggest that the requirement for fluctuating temperature, in addition to being a sensor for ‘exposed’ seed sites, is also effectively acting as a germination timing mechanism.

Seed (= seed plus endocarp) morphology and physical dormancy were studied in seven North American Rhus species: R. copallina, R. glabra, R. typhina, R. aromatica (var. aromatica), R. microphylla, R. trilobata, and R. virens (var. virens). Seeds of Rhus glabra and R. typhina, of subgenus Rhus, were gray, approx. 3 mm long, 2 mm wide, and weighed approx. 7 mg, whereas those of R. aromatica, R. trilobata, and R. virens, of subgenus Lobadium, were brown, >4 mm long, approx. 4 mm wide, and weighed approx. 14.5 mg (R. trilobata) to approx. 23 mg (R. aromaticaand R. virens). Dormancy in all seeds was due to a water-impermeable endocarp, but depth of dormancy varied greatly among species and seedlots. After 4 weeks, 29–34% of the seeds from all five seedlots of R. trilobata, R. microphylla, and R. virens incubated on moist substrate had imbibed, compared to 0–14% of the seeds of all 16 seedlots of R. aromatica, R. copallina, R. glabra, and R. typhina. After 1 yr, imbibition among seedlots of R. aromaticavaried from 28 ± 2% to 69 ± 5%, whereas 93 ±2% to 100 ± 0% of the seeds from all seedlots of R. microphylla, R. trilobata, and R. virens did so. Neither dry laboratory storage for up to 4 yr (even 29 yr in R. aromatica) nor dry heating at 100oC or at 120oC effectively broke dormancy in any of the species tested (R. aromatica, R. glabra, R. trilobata, R. virens). Immersion in boiling water was the best method to render seeds of R. glabra and R. typhinapermeable, yet it was ineffective for those of R. aromatica, R. trilobata, and R. virens. In contrast, a 1 h-soaking in concentrated H2SO4 led to complete loss of endocarp impermeability in the latter three species, but mostly was ineffective in R. glabra and R. typhina. Thus, there seems to be a tendency for seeds of subgenus Rhusto respond well to boiling in water, but not to soaking in H2SO4, whereas the opposite is true for those of subgenus Lobadium.

The relationships among water content, relative humidity and temperature were documented in both seeds and excised embryos of Coffea spp. using water sorption isotherms. Isotherms were constructed at 5, 15 and 25°C and calculated for lower temperatures. There were no apparent differences in sorption characteristics among whole seeds of several cultivars of C. arabica and among different species of Coffea. Excised embryos of genetically diverse Coffea germplasm also exhibited similar sorption characteristics, though there were substantial differences observed between embryos and whole seeds. The shape of isotherms of coffee seed tissues was intermediate to the reverse sigmoidal shape observed for orthodox seeds and the monotonic shape observed for desiccation intolerant plant tissues. The heats of sorption calculated for RH ≤ 25% for whole seeds of Coffea spp. were similar to orthodox seeds. In contrast, the heats of sorption calculated in the same RH range for excised embryos were intermediate between those of orthodox and recalcitrant embryos. Our observations are consistent with earlier observations that desiccation sensitivity or poor longevity is linked with low levels of water sorption at relative humidities less than 25%. An explanation for this remains elusive.

The prediction of the viability of stored seeds is important both for the management of germplasm collections and for the management of commercial seed production and storage. The Ellis and Roberts model for seed viability during storage is examined, and an inadequacy of the model highlighted. A modification is proposed, based on the ‘control mortality’ probit model developed for insecticide bioassays, to take proper account of variation in initial viability. This new ‘control viability’ model, relating seed viability to storage time, is fitted to data from a carrot seed storage experiment and found to fit well for a range of storage environments. A relationship, similar to that proposed by Ellis and Roberts for the effects of storage conditions on the rate of loss of viability, is fitted to the estimated rates from this new model. Data from a second carrot seed storage experiment are used to validate this relationship.

We have been studying a seed aspartic proteinase, termed AtAP, from Arabidopsis thaliana. In previous work, we purified the proteinase, analysed its activity and isolated the cDNA sequence. In this paper, the expression of the mRNA for the aspartic proteinase was analysed in seed tissues both by Northern blots for overall regulation and by in situ hybridization to follow cell-specific localization of message. We found a 1.9 kb aspartic proteinase message in dry seeds and seed pods. This message was expressed in many different cell types of the mature dry seed. The localization of the protein within these cells was also determined. Antibodies were raised against the AtAP and purified using affinity chromatography on an AtAP–immobilized-pepstatin A–agarose column. This purified antibody recognized several AtAP peptides in seeds. To localize the enzyme in cells, we isolated protein bodies from the dry seeds of Arabidopsis using a non-aqueous isolation method. The AtAP activity and antigenic peptides were found to be highest in the protein body fraction and co-localized with the seed storage protein 2S albumin and the vacuolar marker enzyme α-mannosidase. This protein body localization of the AtAP was confirmed with immunocytochemical localization by electron microscopy and shows that the protein is not secreted by these cells.

The effects of temperature, oxygen availability and salinity on seed germination in a population of Zostera marina L. from the Fleet, a tidal lagoon in southern England, were investigated in the laboratory. The fate of seeds under natural conditions was also monitored throughout the year from core samples and from seed samples buried in the sediment layer in nylon sachets. Supported with temperature, salinity and oxygen level measurements at the study site, field data confirmed that following seed dispersal in September/October, the majority of seeds germinated in the Fleet during the winter and early spring. In the laboratory, seeds germinated more rapidly and to a higher final percentage under anaerobic conditions compared to aerobic conditions. Under anaerobic conditions, rapid, near complete germination was possible at full salinity (30 g l-1) or when the salinity was reduced by 50%(15 g l-1). Under aerobic conditions, seeds germinated more rapidly at lower salinity. The effects of temperature were strongly dependent on the presence or absence of oxygen. Under anaerobic conditions, there was a pronounced optimum constant temperature for germination around 6°C, corresponding closely to the average water temperature recorded during the period mid-October to the end of March. On the other hand, under aerobic conditions, very low levels of germination were recorded at low temperatures. These findings support previously published evidence that seeds of Z. marina are well adapted to germinate in anaerobic sediments and that water temperature is the key environmental variable that determines the timing of germination under natural conditions. Previous misuse of dormancy terms to describe seed populations of Z. marina is discussed.

Seeds of watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai var. Congo) were subjected to mild accelerated ageing treatments of 30 and 60 d at 30°C at a moisture content of 7.2%. In comparison to a cold-stored control, normal germination and viability were slightly (but not significantly) decreased by these treatments. Subsequent construction of survival curves during storage at 40°C, at seed moisture contents ranging from 10–18%, revealed that the prior 30°C ageing treatments prolonged survival under these conditions.

Germination responses to 45 combinations of diurnal mean temperature and amplitude were examined in freshly collected seeds of two wetland perennials: Typha latifolia L. and Phragmites australis (Cav.) Trin. ex Steudel. Nearly all seeds (>95%) germinated in favourable temperature regimes. Mean temperature (range 10–30°C) and amplitude (range 0–20°C) affected final germination of both species. P. australis required a high amplitude (> 10°C) for germination over the entire range of mean temperatures. Final germination of T. latifolia was more sensitive to mean temperature than P. australis. The germinated proportion of T. latifolia had a maximum around 20°C, above which it decreased, and amplitudes were more stimulating at low than at high levels of mean temperature. The germination rate was rapid and increased with mean temperature for both species. More than 50% germination was achieved within 1–3 d at favourable temperatures. It is proposed that the thermal requirements provide the non-dormant seeds with a season-sensing mechanism which postpones germination of seeds dispersed during autumn, winter or early spring, until the soil surface is heated by the sun in the spring and sufficiently large diurnal fluctuations of temperature occur. Furthermore, the amplitude requirement implies a strong avoidance mechanism for germination of P. australis in sites with small temperature fluctuations (e.g. below water tables), whereas seeds of T. latifolia appear to be less exacting in the requirements when the soil or water becomes warmer.

The correlation between physiological markers for seed maturity (acquisition of germinability, definitive morphological traits and desiccation tolerance) and timing of the shutdown of nuclear replication activity and β-tubulin synthesis has been investigated in developing pepper seeds. Nuclear replication activity was analysed by flow cytometry. Expression of β-tubulin, a cell-cycle related structural protein, was detected in western blots of total protein extracts. In pepper embryos, DNA synthesis stopped 42–49 days after anthesis (DAA), at which time seeds started to acquire desiccation tolerance. A clear β-tubulin signal was detected up to 63 DAA, only 1 week before the acquisition of complete desiccation tolerance. Both nuclear replication and β-tubulin synthesis can therefore be used as markers for the physiological stage reached by developing pepper seeds. Detection of β-tubulin signals in embryonic tissues can also be helpful in discriminating immature seeds within a commercial seed lot.

We tested whether seed mass and shape are good predictors of seed persistence in the soil of subhumid temperate montane grasslands of central Argentina. We plotted seed mass against variance of seed dimensions of 71 herbaceous species and obtained information on their persistence in the soil from a previous experiment on germinable seed banks. We identified a certain seed mass/shape combination beyond which no seed was persistent in the soil, with the exception of the annual Tagetes minuta. Our results confirmed the patterns previously reported for British species: small and compact seeds tend to persist in the soil for a longer time than big and elongated or flattened seeds. We suggest that the value of seed mass and shape as predictors of persistence in the soil can be extended beyond the British flora to herbaceous floras from other humid temperate regions.

Scaling of water absorption and water loss by seeds on various soil surfaces was simulated using seed analogues constructed with paper pulp. Three sizes of analogue seeds (large, medium and small) were laid on three types of soil surface (coarse, medium and fine texture). To estimate the amount of water absorbed by a seed during a fixed time interval, the difference in seed weight from the start of the experiment was used. The scaling of water absorption necessary for germination was also studied using actual seeds of 14 species representing a range of seed sizes. Scaling coefficients between the amount of absorbed water by an analogue seed (net water gain) and seed mass were usually lower than 1: small seeds absorbed water more rapidly than large ones. The water loss of analogue seeds was also correlated with seed mass with a scaling coefficient lower than 1, but the amount of water loss itself was far smaller than the absorption. On the other hand, the germination of actual seeds revealed that the amount of water necessary to start germination was proportional to seed mass. Thus, smaller seeds have an advantage over larger seeds in more rapidly attaining the water content necessary for germination. Moreover, small seeds can penetrate through small cracks in the soil surface and thus enjoy a double advantage in a microsite that promotes water absorption and minimizes desiccation.

Cell cycle activity in dry and germinating untreated and treated (soaked in water and subsequently in fungicide) seeds of two sugarbeet cultivars, collected at commercial harvest time (late mature seeds) and about 2 weeks before this (immature seeds), was investigated by flow cytometry, and by immuno-detection of β-tubulin and the B-subunit of the 11 S globulin. Germination capacity and field emergence were tested. With dry seeds of both cultivars, higher G2 / G1 ratios were observed in the radicle tips of late mature seeds, as compared with those from immature seeds. The late mature seeds contained more partly degraded (soluble) B-subunit of 11 S globulin, typical of germinating or primed sugarbeet seeds. Thus events associated with the onset of germination had occurred in the seed lots collected at commercial harvest time. The cytoskeleton protein β-tubulin was not detectable in dry seeds from either harvest. Western blotting revealed an accumulation of β-tubulin during germination and this was faster in the late mature harvested seeds which was correlated with the onset of DNA replication. Soaking enhanced the rate of cell cycle activation during germination as well as vigour, germination capacity, and field emergence. There was positive correlation between the G2 / G1 ratio and the traits examined in laboratory and field tests. It is concluded that a combined analysis of proteins and cell-cycle-related events can be used in understanding and predicting sugarbeet seed quality.

The relationship between Calluna vulgaris seedbank density and climate was investigated using regression analysis and two related techniques (factor-ceiling analysis). The seedbank data originated from published and unpublished studies. Low seedbank densities were associated with sites in the relatively dry, sunny and warm south and east of Great Britain. However, as the climatic variables used in the study were highly intercorrelated, it was not possible to determine which climatic variable had the greatest influence on seedbank density. A hypothesis is described suggesting that the limits of seedbank density can be described for single environmental factors. The operation of other environmental and management factors may reduce the seedbank density to less than this maximum. This is illustrated for a range of climatic variables using factor-ceiling analysis. The processes following seed deposition and the longevity of seeds in the soil are highlighted as possible factors controlled by climate which determine seedbank density.