How important is seed predation to recruitment in stable populations of long-lived perennials?

Summary The many evolutionary modifications to seed biology in response to seed predation do not necessarily imply that seed predators have an important impact on population recruitment. This is because competition between individual plants for rare safe sites can cause an oversupply of seeds so far as a population is concerned. The importance of seed losses to population recruitment at any point in time is related to the abundance of safe sites (Fig. 1): it is zero when safe sites are absent, negligible when safe sites are rare, and greatest when safe sites are numerous enough for recruitment to be limited by seed supply. Here I interpret the impact of severe seed losses on population recruitment in four species of long-lived perennials (Eucalyptus baxteri, Leptospermum juniperinum, Casuarina pusilla and L. myrsinoides) by considering these losses in terms of the overall seed dynamics of the populations. I focus on seed supply and seedling survival, as a measure of the current abundance of safe sites, and the maintenance of seed banks, as a measure of the ability of populations to exploit any future changes in safe site abundance. Insect seed predators destroyed about 95% of total seeds in each case. However, these losses do not necessarily have an important impact on population recruitment, because: (i) in most years recruitment appears to be limited by a rarity of safe sites, and not by seed supply (which was as high as 43 germinable seeds/m²/yr); and (ii) the losses did not prevent the establishment of large seed banks (ranging from >30 to >1000 viable seeds/m²) potentially capable of exploiting temporary conditions favourable for recruitment. In contrast to the situation with many annual plants, patterns of recruitment in stable populations of long-lived perennials are often extremely complex, and the significance of seed losses therefore difficult to determine.     

The seed dimorphism of Spergularia marina in relation to dispersal by wind and water

Summary The seeds of the halophyte Spergularia marina differ both within and between individuals in that they either possess or lack a membranaceous border. This paper presents a morphological study of the length, weight and area of the seed types, and their dispersal characteristics under experimental conditions of wind and water dispersal. The winged seeds are shown to be larger both by length and by weight. Their rate of descent increases with wing loading. If the wing is lacking, however, the rate of descent increases with weight only. The distance of dispersal is equal for both seed types except at low wind speeds, when the winged seeds disperse farther. If the seed wing is removed, the excised seeds have shorter dispersal distances. When dispersed by water, a difference in the distance seeds are dispersed can only be detected in the presence of vegetation. The winged seeds are more frequently trapped in the vegetation as compared to the unwinged seeds. The hypothesis that the seed dimorphism is an adaptation for differential dispersal distances is discussed.     

Limiting factors for seed production in Cynoglossum officinale

Summary Over three years of study, small plants of Cynoglossum officinale consistently produced more flowers per unit of dry weight than large plants. In contrast to earlier results, weight of all seeds tended to increase more than proportional to size. As a result a positive correlation existed between seed set per flower and plant size. The correlation between the mean number of pollinator visits per flower and size was positive but not significant. In a field experiment we found that resources rather than pollen were limiting seed set. Thus, it is unlikely that enhanced pollination of the largest plants causes the size-dependency of seed set per flower. Alternative hypotheses are discussed briefly.Publication of the Meijendel Comité, New Series No. 96     

Purification and partial characterisation of two abscisic-acid-responsive proteins induced in cultured embryos ofPisum sativum L.

When pea (Pisum sativum L.) embryos were cultured on low osmotica, with or without added abscisic acid (ABA), there was very little change in the total mRNA translation products resolved by one-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The only marked alteration was an increase in production of two low-molecular-weight proteins. The purification and partial characterisation of these two ABA-responsive seed proteins (ABR17 and ABR18) is described. Both proteins were purified to homoeneity, as judged by SDS-PAGE, from embryos cultured in the presence of ABA. Antisera were raised against both proteins. Each serum cross-reacted with the other protein, indicating that the proteins are closely related. Their apparent molecular masses (M_rs) were estimated to be 17200 (ABR17) and 18100 (ABR18) by SDS-PAGE, and 26000 by gel filtration. Both proteins were heterogeneous on isoelectric focusing. Neither protein was detected (by immunoblotting or immunoprecipitation of cell-free translation products) in embryos grown in vivo at early to mid-development stages but both were present in embryos late in development. These proteins appear to be produced late in seed development but are capable of being induced early in development by culturing embryos in vitro and are markedly enhanced by ABA.     

Maternal effects and generation mean analysis of seed-oil content in cotton (Gossypium hirsutum L.)

Summary The nature of gene action and of maternal influence governing cottonseed oil attributes were determined with four lines, two each with high and low seed-oil percentage. For this purpose, P1, P2, F0, F1, F2 and alternative sets of BC1 and BC2 generations were analysed in six cross-combinations and their reciprocals. Marginal extents of heterosis for seed-oil percentage were noticeable in F1, with inbreeding depression in F2. Data from reciprocal backcrosses provided evidence in favour of maternal rather than cytoplasmic effects of seed-oil development. Relatively higher extents of heterosis, sizeable inbreeding depression and reciprocally unequal F2 averages were characteristic of the seed index trait, which often showed a reversal of effects from F1 to F2. Reverse reciprocal backcrosses exhibited some differences, including greater resemblance between the types, (A/B)A and (B/A)A, in addition to variable dose effects in seed index. Thus, the differences between F1 seed index values were not due to cytoplasmic influence. Positive heterotic effects for seed-oil index, especially among the backcrosses, ranged between 16.08% and 47.29% over midparent averages. Genetic component estimates from analysis of similar sets of crosses differing only in reciprocal backcrosses, and also from sets of reciprocal crosses between any two parental combinations, were inconsistent. Scaling tests detected presence of epistasis within and between a majority of cross-combinations. Despite reciprocal differences, additive gene effects for seed-oil percentage were significant in 7 out of 24 crosses, representing high x low, low x high and low x low seed-oil parents. Those were, however, accompanied by significant dominance effects of higher order. In crosses involving low seed-oil percentage parents SA1060 and SA229, all six components were detected significant, with opposite effects of dominance and dominance x dominance epistatic components. Significant additive components were also detected for seed index and seed-oil index in 7 and 5 out of 24 crosses, respectively. In the inheritance of seed index and seed-oil index, dominance effects were more important. Epistatic components of additive x additive, and to a lesser extent, those of dominant x dominant were found significant.     

Quantitative genetics of seed size variation inPhlox

Summary Because seed size is often associated with survival and reproduction in plant populations, genetic variation for seed size may be reduced or eliminated by natural selection. To test this hypothesis we assessed genetic sources of variation in seed size in a population ofPhlox drummondii to determine whether genetic differences among seeds influence the size they attain. A diallel cross among 12 plants from a population at Bastrop, Texas, USA allowed us to partition variance in the mass of seeds among several genetic and parental effects. We found no evidence of additive genetic variance or dominance genetic variance for seed mass in the contribution of plants to their offspring. Extranuclear maternal effects accounted for 56% of the variance in seed mass. A small interaction was observed between seed genotype and maternal plant. Results of this study support theory that predicts little genetic variation for traits associated with fitness.     

SEM observations on seeds ofStriga spp. andBuchnera americana (Scrophulariaceae)

Seeds of the root parasitesStriga (several spp.) andBuchnera americana were examined by means of SEM. The surface patterns of the seeds in both genera resemble each other closely, especially those ofS. angustifolia andB. americana. SomeStriga spp. can be clearly distinguished by their surface characteristics, while this is quite difficult in others. The taxonomic value of the seed surface features ofStriga andBuchnera is discussed.     

Longitudinal age changes in a fishermen population of West Bengal

The longitudinal anthropological age changes in a sample of coastal fishermen in West Bengal are discussed. The results show that some traits follow the trends observed in ageing of Western populations, for instance nasal length and breadth continue to increase throughout life in both sexes. Stature follows also the trend generally observed: a decrease occurs indeed from 35 years of age in both sexes, the decline is becoming higher with increasing age. The typical ecological conditions of our population influence the rate of change in other traits, such as weight and biacromial diameter. Weight is known to be influenced by the nutritional status and the way of life. It explains that in our study of males an early increase of weight and biacromial diameter is observed first and a decrease afterwards. In females, however, a decrease is observed already from 35 years of age. This kind of analysis was only possible with the help of longitudinal data and in a well defined population: only longitudinal data is free from the effect of secular changes and of selective death.     

Germination traits and seed-bank dynamics of a biennial plant,Oenothera glazioviana Micheli

A study was conducted on the germination traits and seed-bank dynamics ofOenothera glazioviana (=O. erythrosepala), which sets seed in August in sand-dune systems in Japan. More than 90% of freshly matured seeds germinated over a wide range of temperature in light, but less than 10% did so in continuous darkness. Stratification (chilling under moist conditions) was ineffective in diminishing the light-requirement for germination. When fresh seeds were imbibed for 24 h including a 12-h light period, followed by 7-day air-drying, 94% of them became germinable in the dark at 25°C, but remained dormant at less than 15°C. of seeds collected in March from capsules of dead plants, 58% germinated in the dark at 25°C. After four cycles of alternatc 1-day wetting followed by 2-day drying or 1.5-day wetting followed by 1.5-day drying under a 12-h photoperiod, the fraction of viable seeds declined from 76% to 40% and 22%, respectively, due to germination during the wet periods. Seed-bag experiments were conducted in the field, using seeds given and not given a light-stimulus. Forty percent of the light-stimulated seeds germinated in the soil, whereas the seeds without a light-stimulus remained dormant throughout the experiment. When seeds were placed on the soil surface or at a depth of 0.5-1 cm, the proportion of germinable seeds declined during late spring and autumn, but not during winter and early spring. The seed-bank size of a natural population just prior to current seed dispersal was 2–3% of the seed production in the previous year, suggesting a high turnover rate of the seed-bank.     

Seed coat morphology and evolution in Celtidaceae and Ulmaceae (Urticales)

The seed coat surface morphology of Celtidaceae and Ulmaceae (Urticales) indicates a significant evolutionary diversity.Celtis, Chaetachme andPteroceltis (Celtidaceae) have a unique sculpturing with many crateriform holes; such holes occasionally sparsely occur in seeds ofAphananthe, Gironniera (Celtidaceae) andPlanera (Ulmaceae), but not in those of the nine remaining genera of the two families. The perforated seed coat further occurs in at least some genera of all other urticalean families. A pattern of its occurrence in families and genera suggest that the perforation represents a common archaic feature of all Urticales, rather than a feature derived many times independently within the order. The seed coat of Celtidaceae and Ulmaceae seems to have lately lost the holes probably by a neotenic evolution: one or more times within Celtidaceae, and one time in an ancestral line leading to all Ulmaceae. The derived reticulate seed coat surface sculpturing, which is shared byGironniera (Celtidaceae) and some Ulmaceae, is probably the result of parallel evolution. On the basis of evidence from seed coat morphology and other sources, close relationships ofLozanella, Parasponia andTrema within Celtidaceae, as well as variously distinct positions ofAmpelocera, Aphananthe andGironniera, are also discussed.