Are plant populations seed‐limited? A review of seed sowing experiments

We define seed limitation to be an increase in population size following seed addition. Here, we briefly consider how theoretical models deal with seed limitation and how seed sowing experiments can be used to unravel the extent of seed limitation in natural systems. We review two types of seed addition experiments: seed augmentation studies where seeds are added to existing populations; and seed introductions where seeds are sown in unoccupied sites. Overall, approximately 50% of seed augmentation experiments show evidence of seed limitation. These studies show that seed limitation tends to occur more commonly in early successional habitats and in early successional species. Most of the studies have concentrated on simply categorising populations as seed- or microsite-limited, but we believe that seed sowing experiments could be used to reveal much more about community structure, and we discuss possible future directions. In 53% of introduction studies (where seeds were sown at sites from which the species was known to be absent) the introduced species was recorded in at least one of the experimental sites following sowing. However, of the subset of studies where both seedlings and adult plants were recorded, 64% of sites contained seedlings while only 23% contained adults. This implies that, for many species, conditions for establishment are more stringent than conditions for germination. The successful establishment of plants in unoccupied patches indicates the potential for immigration to enhance local diversity (the spatial mass effect). Few studies continued monitoring for long enough to determine whether or not self-sustaining populations were successfully established, and no study attempted to link introduction sites to a putative natural source of propagules, or considered the dynamics of the metapopulation as a whole.     

Seed germination and seedling emergence of three annuals growing on desert sand dunes in China

BACKGROUND AND AIMS: Information on the initial growth characteristics of annuals found in Chinese deserts is very limited. The aim of this study was to investigate seed germination and interactive effects of irrigation and seed burial depth in sand on seedling emergence and seedling survival in three annuals (Agriophyllum squarrosum, Bassia dasyphylla and Aristida adscensionis) commonly growing on sand dunes in these regions. METHODS: Effects of temperature, light and polyethylene glycol-6000 on seed germination were examined by irrigating seeds sown on filter paper in Petri dishes. Seedling emergence was examined for seeds sown on the surface of, or at different depths (5, 10, 20, 30, 40 and 50 mm) in, sand-filled pots, which were irrigated under different regimes. For seeds buried at a depth of 50 mm, seed viability was examined after irrigation of the pots. KEY RESULTS: Seeds of three species germinated at most temperatures recorded between spring and autumn in their native habitats. No seed dormancy was found in any species. For all three species, seedling emergence was most favoured when seeds were buried at a depth of 10 mm. When seeds sown on the sand surface were irrigated, seed germination was considerably suppressed due to water deficiency, but many seeds remained viable. For A. squarrosum and B. dasyphylla, many seeds that were deeply buried and irrigated remained ungerminated but viable, while for A. adscensionis deeply buried seeds germinated, but the seedlings did not emerge due to unfavourable seedling growth in deep sand. CONCLUSIONS: Precipitation is the most crucial factor in determining the seasonal emergence of seedlings of the three tested species in the field. The vertical distribution of seeds in sand determines the proportion of seeds that germinate after precipitation and acts to maintain seed banks over multiple years.     

Population biology of Senecio integrifolius (Compositae), a rare plant in Sweden

In a study of natural populations of Senecio integrifolius in southern Sweden (1979–1983) seeds were found to disperse from mid-June to late July and most of them germinated in autumn. No soil seed bank was observed. On a heavily grazed site few seeds were produced but the percentage of estimated germination was high (c. 75%). In two moderately grazed habitats 8–10% of the estimated number of seeds produced in permanent plots germinated. In a lightly grazed habitat many seeds were produced but only a small percentage germinated (1.4%). In field experiments the average germination was 50–53% when newly harvested seeds were sown where the vegetation had been removed, and 20–33% when sown in undisturbed vegetation at a moderately grazed site. Germination was much lower in a lightly grazed habitat (3–12%). Survival of seedlings was much higher in heavily and moderately grazed habitats than in lightly grazed habitats. Mortality tended to be higher during the growing season (mid-April to early November) than during the winter, and increased markedly during a drought period in the summer of 1982. The half-life of plants established in 1980 varied from 39.3 years at the most intensively grazed site to 7.2 at the lightly grazed site. The number of flowering stems varied between years mainly according to weather. Few plants in the permanent plots flowered every year, the flowering being most frequent at heavily grazed sites. It is concluded that heavy grazing by cattle after seed dispersal is the appropriate management for maintaining S. integrifolius in Sweden.     

Effect of seed depth on slug damage to winter wheat

In a field experiment drilled at two depths on three dates in autumn 1988, with or without methiocarb pellets broadcast on the soil surface immediately after drilling, 26% of seeds of winter wheat sown at c. 20 mm depth were killed by slugs compared with only 9% of seeds sown at c. 40 mm. The protection from slug damage provided by this additional 20 mm of depth was comparable with that provided by methiocarb pellets. The effects of seed depth and pellet application did not interact and were consistent on all drilling dates. Thus, fewest seeds and seedlings were killed where methiocarb pellets were broadcast on a seed-bed with seeds sown at 40 mm depth. Intermediate damage was recorded where seeds were sown at 40 mm depth without pellets, or where pellets were broadcast on seeds sown at 20 mm depth. Most seeds and seedlings were killed where seeds were sown at 20 mm depth without pellets. Sublethal damage to seedlings was not affected by sowing depth but was reduced where pellets were broadcast immediately after sowing.     

Sowing seasons and drying methods during post-harvest influence the seed vigour of soybean (Glycine max (L.) Merr.)

Experiments were conducted to study the influence of sowing seasons and drying methods on the seed vigour of two spring soybean (Glycine max (L.) Merr.) cultivars. Two cultivars, ‘Huachun18’ and ‘Huachun 14’, were sown in three seasons viz., spring, summer and autumn and the harvested seeds were dried using three different methods. The results showed that soybean sown in spring had a higher number of branches per plant, pods per branch and seed weight, and consequently resulted in higher seed yields than that of soybean sown in autumn or summer seasons. Seeds sown in the autumn season had the lowest values of electrical conductivity during seed imbibitions, higher peroxidase (POD) activity in germinated seedlings and lower contamination by the seed-borne fungi on the MS medium, which indirectly improved the seed vigour, which was followed by summer sown seeds. Seeds sown during the spring season resulted in poor seed vigour. In addition, the effect of drying methods on the seed vigour was also clarified. Seeds that hung for four days before threshing and then air-dried had the poorest seed vigour which was determined by germination, electrical conductivity, POD activity and seed borne fungal growth. There was no difference in seed vigour between other methods, i.e. seeds threshed directly at harvest and then air-dried on a bamboo sifter or concrete floor. These results indicated that autumn sowing soybean and the drying method in which seeds were threshed directly at harvest and then air-dried on a bamboo sifter resulted in higher seed vigour.     

紫茎泽兰土壤种子库特征及其对幼苗的影响

研究紫茎泽兰(Eupatorium adenophorum)土壤种子库特征及其对紫茎泽兰种子的萌发、幼苗命运的影响, 在综合治理紫茎泽兰入侵危害及防止紫茎泽兰的继续扩散等方面有着重要的意义。该文在攀枝花紫茎泽兰入侵严重的地区, 通过采集果园、放牧灌丛以及禁牧灌丛3种不同生境紫茎泽兰土壤种子库的样本, 以说明不同干扰程度下紫茎泽兰种子在土壤中的分布状况。通过野外调查并结合盆栽实验, 初步研究了紫茎泽兰土壤种子库基本特征以及光照和种子在土壤中的埋藏深度等对紫茎泽兰幼苗的影响。结果表明: 1)果园、放牧灌丛和禁牧灌丛等3种干扰程度不同生境的深层种子量占总种子量的比例分别为56.44%、46.96%和24.86% (p=0.006), 这说明土壤深层种子量大小与干扰成正比, 干扰越大, 深层次紫茎泽兰种子量占总种子量的比重越大。2)播种在0、1和5 cm深度的种子萌发率分别为64.67%、22.67%和13.33%, 即种子埋藏越深, 萌发率越低, 不同层次种子萌发率差异极显著(p=0.00); 幼苗死亡率分别为27.95%、0和0, 表层种子萌发的幼苗有较高的死亡率, 而由埋藏在深层的种子萌发的幼苗没有死亡, 土壤表层发芽的幼苗与不同埋藏深度种子萌发的幼苗之间死亡率差异极显著(p=0.00)。3)在无遮蔽、半遮蔽和全遮蔽3种不同情况下, 紫茎泽兰幼苗的死亡率分别为72.15%、30.38%和4.87%, 定居率分别为6.66%、33.99%和46.67%, 即遮蔽程度越高, 死亡率越低, 定居率越高, 不同处理之间死亡率和定居率差异均极显著(p=0.00)。研究结果暗示, 强光可能是导致紫茎泽兰幼苗死亡的重要原因, 人类活动的干扰可能导致更多的紫茎泽兰种子进入土壤深层, 从而改变了紫茎泽兰土壤种子库的结构。由于土壤深层种子比表层种子具有更强的抵抗强光照射等不良环境因子影响的能力, 所萌发的幼苗成活率高, 表明其具有更高的繁殖效率, 因此人类活动干扰是紫茎泽兰入侵后难以根除的原因之一。     

Seed Dormancy,Seedling Establishment and Dynamics of the Soil Seed Bank of Stipa bungeana (Poaceae) on the Loess Plateau of Northwestern China

Studying seed dormancy and its consequent effect can provide important information for vegetation restoration and management. The present study investigated seed dormancy, seedling emergence and seed survival in the soil seed bank of Stipa bungeana, a grass species used in restoration of degraded land on the Loess Plateau in northwest China. Dormancy of fresh seeds was determined by incubation of seeds over a range of temperatures in both light and dark. Seed germination was evaluated after mechanical removal of palea and lemma (hulls), chemical scarification and dry storage. Fresh and one-year-stored seeds were sown in the field, and seedling emergence was monitored weekly for 8 weeks. Furthermore, seeds were buried at different soil depths, and then retrieved every 1 or 2 months to determine seed dormancy and seed viability in the laboratory. Fresh seeds (caryopses enclosed by palea and lemma) had non-deep physiological dormancy. Removal of palea and lemma, chemical scarification, dry storage (afterripening), gibberellin (GA₃) and potassium nitrate (KNO₃) significantly improved germination. Dormancy was completely released by removal of the hulls, but seeds on which hulls were put back to their original position germinated to only 46%. Pretreatment of seeds with a 30% NaOH solution for 60 min increased germination from 25% to 82%. Speed of seedling emergence from fresh seeds was significantly lower than that of seeds stored for 1 year. However, final percentage of seedling emergence did not differ significantly for seeds sown at depths of 0 and 1 cm. Most fresh seeds of S. bungeana buried in the field in early July either had germinated or lost viability by September. All seeds buried at a depth of 5 cm had lost viability after 5 months, whereas 12% and 4% seeds of those sown on the soil surface were viable after 5 and 12 months, respectively.     

A comparison of Brussels sprout seedling establishment from ungerminated and low-moisture-content germinated seeds

The moisture content of newly-germinated Brussels sprout seeds (radicles 1 ± 0·5 mm long) was reduced to 16% without loss of viability when compared with untreated newly-germinated seeds. Seedling emergence and growth from these low-moisture-content germinated (LMCG) seeds and ungerminated seeds were compared in cellular trays and seedbeds. A greater number of seedlings emerged more uniformly and more rapidly from LMCG seeds than from ungerminated dry seeds, leading to larger, more uniform seedlings for subsequent transplanting. Post-germination separation in sucrose solutions before drying resulted in a seed batch with 98% of germinated seeds. As there was a positive relationship between the rate of germination and subsequent seedling growth, it is suggested that germinated-seed separation techniques could be used to select the most vigorous seedlings.     

Experimental demography of the rare Gentianella germanica: seed bank formation and microsite effects on seedling establishment

The persistence of populations of short-lived species requires regular reproduction and seedling establishment. A persistent seed bank can buffer populations against extinction in unfavourable years. We experimentally investigated seed fate in Gentianella germanica, an endangered biennial species characteristic for species-rich nutrient-poor calcareous grasslands in central Europe. We studied the effect of experimental gaps on seedling establishment from sown seeds and the fate of seeds buried in bags over two years. In December 1993 experiments were established at seven calcareous grassland sites in the Swiss Jura mountains. In spring 1994 seedlings emerged in all plots where seeds had been sown, including previously unoccupied patches. This suggests that limited dispersal within sites contributes to small population sizes. Significantly more seedlings emerged at sites with current populations of G. germanica than at unoccupied sites (5.95% vs 3.40%). Because this difference was not explained by germinations from the natural seed bank it indicates differences in habitat quality. Clipping of the vegetation and disturbance of the soil reduced vegetation cover in the following spring and enhanced seedling emergence. In undisturbed plots 4.5% of seeds sown produced a seedling in spring 1994, whereas in plots with clipped vegetation 9.9% and in disturbed plots 12.7% produced seedlings (p>0.01). This suggests that management measures which create gaps in the vegetation (e.g. grazing) could positively influence population size and persistence of G. geymanica. On average, we recovered 7.55% viable seeds after one year of burial in bags, and 4.05% after two years, indicating that G. geymanica has a persistent seed bank. The demographic data indicate that the number of viable seeds in the seed bank exceeds the number of established plants in a population at least by a factor of 20. Restoration of extinct populations of the species from the seed bank may thus be possible if appropriate management measures are taken within a few years.     

Long-term variability in seed size and seedling establishment of <Emphasis Type="Italic">Maianthemum bifolium</Emphasis>

Relatively few studies conducted in natural plants populations focus on the relationship between seed size and their germination ability and seedling establishment. Maianthemum bifolium is a perennial herb that spreads vegetatively through rhizomatous growth and reproduces through seeds. However, this species is characterized as seed and microsite limited, and under undisturbed conditions seedlings are not noted. The studies were conducted in two populations of M. bifolium in six subsequent seasons. The mean seed mass was negatively correlated both per ramet as well as in the fruit with the number of seeds, and positively with its height and the number of flowers. The long-term mean annual production of seeds in the populations was 37 and 56 seeds per m². The seeds from both populations had similarly high germination abilities that were approximately 90% under laboratory conditions, 60% in garden, and 55% in the natural habitat. Seeds from four size classes were sown and a positive correlation was noted between seedling establishment and the mass of the seeds from which they grew (r_S = 0.64). Seedling survival was also significantly correlated with seed mass.     

Effects of experimental and real land use on seedling recruitment of six fen species

Seedling emergence and early establishment of six fen species differing in seed mass and growth form were investigated under experimental land use with changed vegetation structure and under real land use in a calcareous fen. Seeds of all six species were sown in plots with different experimental land-use treatments: summer and autumn mowing with or without litter removal, trampling and abandonment. Additionally, emergence and survival of experimentally sown seeds was investigated under real land use on adjacent sites managed by mowing, grazing, intense trampling or abandonment.On abandoned plots and on plots without litter removal of the land-use experiment, emergence rates of all species were negatively affected either by high litter and moss cover or by tall canopy. No differences were found between autumn and summer mowing. Gap creation by experimental trampling did not increase germination rate. Under real land use, establishment of seedlings of most species was positively affected by litter cover and tall canopy. Trampling, in contrast, had a severe negative effect on seedling survival.The investigated species differed in their germination ability which was tested in the germination chamber and in their response to land use. Succisa pratensis with the highest seed mass germinated well in the chamber and in the field more or less regardless of land use. The low germination rate of Parnassia palustris in the germination chamber indicated a limitation of viable seeds. In the field, however, seedling emergence was additionally limited by microsite availability. Seeds of Serratula tinctoria and Primula farinosa germinated well in the germination chamber, but seedling recruitment in the field was hampered in the presence of a high litter or moss cover. Seeds of Tofieldia calyculata and Pinguicula vulgaris were strongly dependent on the availability of suitable microsites in the field. They hardly germinated under natural conditions, in spite of a high number of germinable seeds in the germination chamber.     

The effect of ants on the seed dispersal cycle of the typical myrmecochorous <Emphasis Type="Italic">Ricinus communis</Emphasis>

The directed dispersal hypothesis has two components: (1) non-random seed deposition by a predicted vector, which removes greater amounts of seeds to specific sites, and (2) higher seed survival and seedling establishment in these specific sites. Several studies suggest that ants perform both tasks. This study was designed to cover the processes from post-dispersed seeds to established juvenile plants of a typical ant-dispersed species. Our main objective was to determine whether Ricinus communis benefits from directed dispersal by ants to maintain its populations in previously colonized habitats. We examined whether there were differences between ant nest pile mounds and their vicinities in the: (1) densities of seeds with and without elaiosome, seedlings and juveniles; (2) performance of post-dispersed seeds (without elaiosome), which may be affected by seed density, a key feature attracting seed predators; (3) nutrient quantities; (4) number of germinated seeds and juvenile biomass measurements; and (5) ant protection of seedlings from herbivores. There were more seeds without elaiosome, seedlings and juveniles in pile mounds, and seeds with elaiosome were equally distributed. There was no difference in the number of non-removed seeds in pile mounds and in their vicinities, and there was no tendency for this difference to increase or decrease with time or with initial seed density. Apparently, there was no difference in nutrient contents in soils of pile mounds and their vicinities. Likewise, there was no difference in the number of seeds germinated and in the biomass measurements of juveniles in both soils. Ants did not provide differential protection for seedlings in pile mounds against potential herbivores. The dispersal of Ricinus seeds by ants had a marked effect on the distribution pattern of the seeds, seedlings and juveniles of this species. However, there were no additional advantages for the seeds, seedlings and juveniles in pile mounds and, therefore, Ricinus does not benefit from directed dispersal by ants to maintain its populations in the study sites.     

Germination as a determinant of seedling distributions among natural substrates in Picea engelmannii (Pinaceae) and Abies lasiocarpa (Pinaceae)

In Rocky Mountain (USA) subalpine forests, seedlings of Picea engelmannii (Engelmann spruce) colonize logs more frequently than seedlings of its codominant associate Abies lasiocarpa (subalpine fir). We hypothesized that spruce germinates more readily on logs than fir, perhaps because small spruce seeds are more likely to lodge in log crevices than larger fir seeds. Our objectives were to test this hypothesis and compare both species' germination among several substrates to assess germination influences on natural seedling distributions. Spruce and fir seeds were sown on field-collected logs, litter, and soil in the greenhouse and monitored for 36 d. To test the crevice hypothesis, seeds were either scattered on logs or wedged into crevices, assuming that if both species were placed in crevices, interspecific germination differences on logs would decrease. Spruce mean germination percentages were significantly greater than fir's in all substrate treatments except when seeds were wedged in log crevices. The difference in means between the two log treatments was greater for fir (68%) than spruce (21%). Spruce germinated more rapidly than fir on all substrates. We suggest that large seed size reduces fir's success in colonizing logs, and that germination and establishment factors interact to determine natural seedling distributions for these subalpine conifers.     

A survey of seedling establishment in sugar-beet crops in 1980 and 1981

One hundred and twenty five crops distributed throughout the UK sugar-beet growing area were examined to identify the growth stage at which failures occurred during the seedling establishment phase. Establishment varied between 15 and 90% and averaged 64 and 67% in 1980 and 1981 respectively. It exceeded 70% in 46% of fields. Overall, the actual seed spacing was only 0·15cm wider than the 17·0cm intended but the number of seed stations in individual fields varied from 27% less to 19% more than the target. On average the drill failed to place a seed at 5% of seed stations but up to 14% misses were recorded. Germination in the field was 92% - only 1% less than in the standard laboratory germination test. However, on average, 16% (range 2 - 42%) of seeds which germinated failed to develop much further; the cause of only a quarter of these losses was identified as due to soil pests, diseases or being caught under stones or clods. There was a tendency for more seeds to fail between germination and emergence as the proportion of empty seed stations increased. Rapid emergence usually resulted in greater final emergence. There were post-emergence losses in 122 of the 125 fields. These losses averaged 6% and exceeded 14% on five fields. Birds were considered to have killed plants in 49 fields, soil pests caused losses in 20 fields and herbicides, fungal disease or grazing by rabbits or hares decreased plant stands in about 10 fields.     

Germination biology and seedling growth of Clusia hilariana Schltdl., a dominant CAM-tree of drought-prone sandy coastal plains

The spatial and temporal fluctuations of water availability can be an obstacle for recruitment of many species in the restinga and might restrict seed germination and seedling growth in specific regeneration safe-sites. Clusia hilariana is one of the most dominant species of Restinga de Jurubatiba. This species has a high proportion of seedling establishment occurring inside the tanks of soil bromeliads underneath vegetation patches. Given the thin seed coats, the fast germination time and seed dispersal of C. hilariana during the dry season, we hypothesized that their major regeneration niche (the tanks of soil bromeliads) is related to susceptibility of seed germination and also seedling growth to low water availability. To test this hypothesis, seeds were germinated under decreasing water potentials using PEG 6000 solutions and seedlings were grown under varying water regimes. The percentage of seed germination progressively decreased at lower water potentials. After 38 days in ?1.0 MPa no seeds germinated. However, approximately 90% of seeds germinated when transferred to Ψ = 0 MPa. The relative growth rates of seedlings of C. hilariana did not differ between water treatments. Thus, the major regeneration niche of C. hilariana is not a consequence of a high sensitivity of seeds and seedlings to water shortage. Nonetheless, C. hilariana showed an array of seed and seedling traits that may help to overcome establishment constraints of the harsh environment of restingas.     

Comparative habitat susceptibility to invasion by Chinese ash (<Emphasis Type="Italic">Fraxinus chinensis</Emphasis>: Oleaceae) in a tropical Andean landscape

Chinese ash (Fraxinus chinensis) is an exotic tree species that has been used in non-commercial monospecific plantations for revegetation programs in the Central Andes of Colombia. At the Otún river watershed, these plantations occur in patches intermixed with old pastures, oak forests, and successional forests. In this heterogeneous landscape, the ash has been able to invade some of its surrounding habitats. This study evaluates the invasion patterns of ash to each of these habitats and experimentally quantifies seed and seedling survivorship and seedling growth as three processes that could determine ash establishment. Of the four habitats examined, old pastures were the most vulnerable to invasion, followed by oak plantations, and successional forest. Ash plantations exhibited recruitment levels intermediate between pastures and oak plantations. Abandoned pastures showed the highest seed germination, seedling survivorship, and seedling growth. In the ash plantations, recruitment seemed negatively affected by the low number of germinated seeds, high mortality of seedlings, and low growth. Invasion in oak plantations was constrained by high seed mortality and burial of seedlings by leaf litter, although the seedlings that did survive grew fast and produced a high number of leaves. Within the successional forest, Chinese ash seeds germinated but establishment was constrained by a reduced seedling survivorship and low growth. This research offers new evidence of how different processes affecting the establishment of an invader may differ among landscape elements, and it also yields important information for the management and control of ash in these Andean landscapes.     

SOME EXPERIMENTS AND FIELD OBSERVATIONS ON THE GERMINATION OF WILD OAT (AVENA FATUA AND A. LUDOVICIANA) SEEDS IN SOIL AND THE EMERGENCE OF SEEDLINGS

The distinguishing characters of Avena fatua and A. ludoviciana are described.
Pot experiments and field observations showed that most seeds of A. fatua germinated in spring and a few in autumn; hardly any germinated in summer or winter. Seeds of A. ludoviciana germinated in winter only. The greatest depth of sowing from which seedlings of either species reached the surface was 9 in., but seedlings from this depth were weak and yellow when they first appeared. A. ludoviciana gave more and sturdier seedlings than A. fatua from 6 and 9 in. There was no evidence of induced dormancy in seeds of A. fatua buried at depths down to 20 in. Germination of this species was hastened by monthly cultivation of the soil. The maximum survival of A. fatua was 3 years in pots and slightly longer in the field; seeds of A. ludoviciana in pots survived only 2 years. The two or three seeds of each spikelet of A. ludoviciana germinated in turn, starting with the largest, but the interval between germination of successive seeds varied.
Seedlings from freshly sown seeds of both species were more vigorous than seedlings from seeds which had been buried for a year or more. Germination and subsequent growth of both species took place in soil of pH 4.5 to 7.0 approx.     

Germination studies ofMolucella laevis

Seed of Molucella laevis (L.) was gathered in the years 1963-66 and germinated soon after harvest and at various intervals subsequently. All seeds showed dormancy on gathering. There was a noticeable fluctuation in the percentage of seeds germinating during storage. Dormancy persisted throughout the years of the experiment. It appears that a number of factors are operative in the inhibition of germination of M. laevis seeds. One factor is an inhibitor which may be adsorbed by active charcoal or heavy loam, involving some process which requires fluctuating temperature. Rupture of the seed-coat also improved the germination of dormant seed, and a subsequent water rinse for 24 hrs. further increased the percent of germinating seed. Immersion of the seed for 48 hrs. in gibberellic acid (G.A.), 400 ppm, greatly improved germination, but it did not completely overcome dormancy. The maximum effect (93% germinating seed) was obtained when seed pre-treated with G.A. was germinated on top of active charcoal. The optimal germination temperature was found to be a daily alternating one of 16 hrs. at 15°C and 8 hrs. at 30°C with light supplied at the latter temperature. Treated seed was sown in the screen house and found to develop normal plants and seed. The dormancy-breaking effect of G.A. lasts for at least 180 days. The fluctuations in germination of seed pre-treated with gibberellin were similar to those of the untreated ones. The effect of dormancy-breaking factors differed with year of gathering and date of application. Thus, M. laevis seeds display much heterogeneity in their germination potential.     

Emergence and early establishment of Chondrilla juncea L. (skeleton weed) in the Western Australian wheatbelt

When sown over consecutive years at two sites in the Western Australian wheatbelt, a maximum of 34.5% of viable Chondrilla juncea seeds gave rise to emerged seedlings. During the early winter months at one site, waterlogging on a duplex soil type led to an almost complete failure of emergence. Very low levels of seed persistence (maximum 3.1%) between years were noted. The protection provided by stubble enhanced the survival of C. juncea seedlings which emerged during May, but for seedlings which emerged later, the effects of stubble upon seedling survival were strongly dependent upon the abundance of the red-legged earth mite (Halotydeus destructor) At the observed densities of winter annuals, the presence of seedlings and young plants appeared to reduce seedling mortality in C. juncea. Young plants of C. juncea were highly susceptible to simulated cultivation practices until 12 weeks following emergence. It is argued that seedling establishment of C. juncea is most likely to occur during the pasture phase of crop/pasture rotations in the Western Australian wheatbelt and that pastures immediately following cropping years are most prone to invasion.     

Seed transmission of broad bean stain virus and Echtes Ackerbohnenmosaik-Virus in field beans (Viciafaba)

When grown in a glasshouse during spring or autumn field bean (Vicia faba minor) seedlings infected with seed-borne broad bean stain virus (BBSV) or Echtes Ackerbohnenmosaik-Virus (EAMV) usually showed symptoms on some leaves within 4 wk of emergence. Symptoms caused by each virus were indistinguishable. The viruses were transmitted as often through unblemished seeds as through seeds with necrotic patches or stains on the seed coat, and sometimes as often through large as through small seeds. Soaking seeds for 24 h in solutions of 8-azaguanine or polyacrylic acid did not decrease transmission. Both viruses were detected in nearly mature seeds by inoculation to Phaseolus vulgaris but neither virus was detected in fully ripened seeds by inoculation or serological tests. The percentage of seeds from field plots that produced infected seedlings when sown in a glasshouse was closely related to the percentage of parent plants that showed symptoms of BBSV and/or EAMV at the end of flowering. The relationship seemed similar in different cultivars. On average EAMV was transmitted through more seeds than BBSV, probably because more parent plants were infected with EAMV. Inspection of seed crops for symptoms of BBSV and EAMV at the seedling stage and again at the end of flowering is probably the most practicable way of identifying progeny seed lots with little or no infection.