Tomato seeds that have been dried, imbibed and redried (primed) develop internal free space between the embryo and endosperm. Seeds of the ABA-deficient sitiens(sitw) tomato mutant can exhibit internal free space at the completion of seed development even without priming. Both primed and sitwseeds germinate more rapidly than untreated wild-type seeds. To determine whether internal anatomy predicts germination physiology, individual sitwand primed wild-type seeds were sorted into three categories based upon the extent of internal free space observed non-destructively using X-radiography. Category 3 (C3, extensive free space present) sitw seeds completed germination more rapidly than all other seed categories and genotypes in water, in abscisic acid (ABA) or under far-red illumination. The force necessary to puncture the endosperm caps (and testa) of C3 sitw seeds was less, and the percentage of nuclei in C3 sitw radicle tips in the G2 stage of the cell cycle was greater than for all other seed categories. Wild-type seeds exhibited free space following long-term priming, but germination was still prevented by far-red light and ABA, and endosperm cap strength and nuclear DNA contents were not altered. Endo-β-mannanase activity of individual endosperm caps was not consistently related to their resistance to puncture. While internal free space is diagnostic for primed tomato seeds and occurs in a fraction of sitw seeds, it is not predictive of many aspects of germination physiology.

The role of abscisic acid (ABA) in seed germination of two cultivars of lettuce (Lactuca sativa L.; light-sensitive Ritsa and light-insensitive Strada) was investigated. The inhibition of Ritsa seed germination by exogenous ABA was higher than that of Strada seeds, the extent of inhibition of both cultivars being reduced by a short light break. At 25°C the sensitivity of both cultivars to exogenous ABA was higher than at 15°C. The endogenous level of ABA was similar in dry seeds of both cultivars, increasing temporarily in Ritsa seeds during the first 4 h of imbibition in darkness but not in Strada seeds, nor in Ritsa seeds exposed to a short light break. The transitory increase of ABA content in Ritsa seeds imbibed in darkness was accompanied by increased expression of the gene responsive to ABA (Rab) under these conditions. Zorial (Norflurazone), an ABA-biosynthesis inhibitor, decreased ABA content and allowed dark germination of the light-requiring Ritsa seeds. A short light break induced germination of Ritsa seeds when applied at 24 and 48 h after imbibition onset, i.e. after the transitory increase of ABA. GA3, on the other hand was effective when applied at the beginning of imbibition. It seems that light induces germination of the photoblastic Ritsa seeds by both inhibiting ABA synthesis and decreasing seed sensitivity to ABA and inhibitory processes induced by it.

Seed desiccation sensitivity was studied in nine species of the genus Coffeaby measuring seed viability after equilibration over various saturated salt solutions. A quantal response model based on the logistic distribution was developed in order to describe the typical S-shaped patterns observed. The closeness of fit of the desiccation sensitivity model was shown, and the assumption that seed desiccation sensitivity follows a continuous distribution within species was verified. For each species, the water content at which 50% of initial viability was reached, WC50, and a specific parameter describing the intra-specific variability, β, were calculated using a non-linear regression. A simplified water sorption model was developed which allowed easy calculation of water activity and water potential corresponding to WC50 (aw50 and Ψ50) for relative humidities ranging between 10 and 100%. Distribution of WC50and Ψ50 (or aw50) in the genus Coffea was homogeneous within the following intervals: from 0.05 to 0.38 g H2O.g−1dw for WC50 and from −168 to −11 MPa for Ψ50. Different classifications of the coffee species studied as regards to their desiccation sensitivity were obtained depending on whether WC50 or Ψ50was used for classification. The continuum for desiccation sensitivity observed within the nine species studied confirmed that coffee is an appropriate material for studying desiccation sensitivity.

Seeds of two grass (Arrhenatherum elatius and Festuca rubra), two herb (Plantago lanceolata and Rumex acetosa) and two legume (Lotus corniculatus and Trifolium repens) species were sown in summer 1995 at four densities (no seed, 1000, 10 000 and 50 000 seeds m−2) into an established rabbit-grazed grassland given factorial combinations of rabbit fencing (with and without fences) and soil disturbance (with and without cultivation). On plots where no seeds were sown, only the species with persistent seed banks (P. lanceolata, L. corniculatus and T. repens) showed enhanced seedling emergence in response to disturbance. In disturbed soil, seedling densities of all species increased with increasing density of sown seeds, the effects of which were still evident for plant cover 2 years after seed sowing. In undisturbed vegetation, A. elatius, F. rubra, P. lanceolata and R. acetosa showed increased seedling densities following seed sowing; but in each case, there was an upper asymptote to seedling recruitment, presumably due to microsite limitation. Rabbit grazing reduced seedling densities, with this reduction being more pronounced with disturbance than without. However, the effect of rabbit grazing did not persist for some species; seedling mortality of R. acetosa, P. lanceolata, L. corniculatus and T. repens was higher on plots without rabbit grazing, so that plant densities of these species in summer 1996 and plant cover in summer 1997 were greater on grazed plots. The results indicate interactions between soil disturbance, propagule availability and herbivory, rather than disturbance alone, will play an important role in controlling seedling recruitment and species habitat distributions in grasslands.

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.

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.

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.