SITE-SPECIFIC SEED DORMANCY IN SPARTINA ALTERNIFLORA (POACEAE)

Seed dormancy in Spartina alterniflora was studied with regard to: 1) the specific site of the dormancy mechanism, and 2) the influence of three growth-regulating substances: gibberellic acid, fusicoccin, and abscisic acid. The site of the mechanism was determined by assessing the germinability of surgically altered dormant seeds. Effects of the growth substances were determined by assessing the germinability of seeds incubated in their presence. Results revealed that dormancy could be broken surgically by altering the scutellum or chemically by applying fusicoccin. Dormancy could be restored by abscisic acid in the former case but not the latter. It is concluded that the dormancy mechanism is located in the scutellum, that it consists of at least two sequential steps, and that it involves a leachable chemical inhibitor.     

SEED DORMANCY IN UNIOLA PANICULATA

Seeds of Uniola paniculata L., a sand-dune grass of the Southeast coast, showed an increasing dormancy under laboratory germination conditions that reached 98–100% six months after maturity. The embryos were viable and nondormant and the seeds absorbed water readily. Chemical treatments were only moderately effective, but leaching the seeds after cutting into the endosperm gave 100% germination. The presence of a diffusible growth inhibitor was demonstrated by this response and by tests of endosperm leachate. Temperature, particularly sand temperature above 30 C, appears to be the major factor in normal germination of the seed.     

MECHANISM OF SEED DORMANCY IN AMBROSIA ARTEMISIIFOLIA

Dormancy in Ambrosia artemisiifolia seeds was broken by 8 weeks of stratification. Germination of nondormant seeds was greater in light than in continuous darkness. Embryos of freshly harvested seeds were nondormant. Leaching and scarification did not stimulate germination of the dormant seeds. Exogenous gibberellin (GA₃) slightly increased germination of intact dormant seeds, and the effect was greatly increased by scarification. Germination was greater in the light in both tests. Exogenous indoleacetic acid did not stimulate germination of dormant seeds. Endogenous gibberellin and auxin content increased during stratification, and there was also a significant increase in GA during post-stratification at a favorable germination temperature. Inhibitors in the dormant seeds decreased during stratification and post-stratification. The high concentration of chlorogenic acid present in dormant seeds increased slightly during stratification. An unknown phenol very similar to chlorogenic acid in fluorescence and U.V. absorption significantly increased after 2 weeks of stratification. A significant decrease in the concentration of a second unidentified phenol occurred after 2 weeks of stratification. It is proposed that dormancy in Ambrosia artemisiifolia may be controlled by an inhibitor-promoter complex. The dormant seed is characterized by high inhibitor and low promoter levels. In the nondormant seed the balance was shifted to favor the promoter. Evidence suggests that the inhibitor involved may be abscisic acid and the promoters may be gibberellin and auxin. The content of auxin may be partially controlled by the concentration of phenols.     

SEED DORMANCY AND GERMINATION IN MELAMPYRUM LINEARE

Immature seeds of Melampyrum lineare Desr. have very high germination percentages and dormancy is induced in a variable fraction of the seed crop during ripening. Correlated with this is the endogenous gibberellin-like activity which is found in considerable amounts in immature seeds, less in batches of ripe seeds, and is not detectable in batches containing only dormant seeds. For germination dormant seeds require activation followed by cold storage. In the laboratory activation is produced by allowing moist, dormant seeds to respire freely for several weeks at 20 C, or by treatment with exogenous GA₃. Dormancy appears to be most directly related to inability of the embryo to hydrolyze the thickened, mannan-containing endosperm cell walls. Embryos excised from dormant seed can be grown on agar enriched with whole macerated dormant seeds or with the ethanol-extractable materials from these (mostly sucrose and a glycoside). However, dormant seed material does not support growth when extracted to remove benzene- and ethanol-soluble materials.     

GERMINATION AND GROWTH INHIBITORS IN GUAYULE (PARTHENIUM ARGENTATUM GRAY) CHAFF AND THEIR POSSIBLE INFLUENCE IN SEED DORMANCY

Guayule (Parthenium argentatum Gray) is a rubber-producing plant native to the semi-arid region of north central Mexico and southwestern Texas. As a member of the family Asteraceae, the seed dispersal unit in guayule is comprised of an achene enclosed by two staminate florets and a bract. The seeds in this condition are dormant and need special treatment in order to germinate. Threshing, prolonged washing, and/or chemical treatments prove effective in improving the germination. Experiments showed that chaff influenced germination and seedling emergence. Bioassays of aqueous extracts of guayule chaff and the seed coat showed a significant inhibition of germination and radicle growth of guayule, lettuce, and tomato seeds. High-performance liquid chromatography enabled the identification of at least seven phenolic acids from guayule chaff extract. These, in order of decreasing concentrations, included p-hydroxybenzoic, protocatechuic, p-coumaric, ferulic, benzoic, vanillic, and cinnamic acids. All of them are well-known germination and growth inhibitors, and seem to play an important role in the dormancy of guayule seed.     

DORMANCY FACTORS IN IRIS (IRIDACEAE) SEEDS

Utilizing a newly developed radish seed bioassay, it has been demonstrated that Iris seed dormancy is caused by water-soluble inhibitor(s). Leached Iris seeds germinate freely when planted in soil without previous chilling. The inhibitor(s) can prevent germination of representative species of both monocotyledonous and dicotyledonous seeds.     

AN ELECTROPHORETIC AND SEROLOGICAL INVESTIGATION OF SEED PROTEINS IN GALEOPSIS TETRAHIT L. (LABIATAE) AND ITS PUTATIVE PARENTAL SPECIES

Seed proteins from the tetraploid Galeopsis tetrahit L. and its putative parental species G. pubescens Bess. and G. speciosa Mill., were examined using disc gel electrophoresis (PAGE), Ouchterlony double diffusion, and Immunoelectrophoresis. Forty-three inbred cultivated lines of G. speciosa, G. pubescens, and G. tetrahit (both the naturally occurring species and an artificially produced hybrid) and two wild populations of G. tetrahit were examined. PAGE gels were stained for total protein or malate dehydrogenase. Although variability in the total protein pattern was detected within and between lines for all three species, there was much similarity among the taxa. No bands appeared in the hybrid that were not present in the parents. The band pattern for synthetic G. tetrahit was very similar to that of natural G. tetrahit suggesting that G. tetrahit originated from a cross in nature between G. pubescens and G. speciosa. The total protein PAGE results were analyzed using numerical taxonomic techniques. Although these analyses did not clearly separate the three species, the general tendency was toward grouping G. pubescens lines together, natural G. tetrahit lines together, and synthetic G. tetrahit lines together. Natural and synthetic G. tetrahit lines were also interspersed among each other along with some G. speciosa lines. The malate dehydrogenase pattern showed little variation among the three species. Qualitative serological methods did not distingush among the three species, but confirmed the similarity observed among the taxa in the results of electrophoresis.     

POLARITY OF TRACHEARY REGENERATION IN YOUNG INTERNODES OF COLEUS (LABIATAE)

After Jacobs’ recent discovery of almost absolute basipetal polarity of IAA-¹⁴C movement through young internodes of Coleus, tracheary regeneration around a wound in the stem was re-investigated to see if it showed parallel changes from the less strict polarity of IAA-¹⁴C described decades ago. As determined from either counts of “complete regenerated strands” or from finer details of regeneration, tracheary regeneration was very strongly polar. If leaves were present only below the wound, no regenerated strands developed unless there was a sizeable length of leafless stem remaining above the wound. If there were leaves below the wound as well as above it the amount of regeneration was usually reduced. The short strands of acropetally regenerating tracheary cells, previously interpreted as resulting from acropetal IAA movement, were observed in plants with leaves above but not below the wound, and were not seen in plants with leaves only below the wound. Hence, they are more likely to result from basipetally moving IAA. Isolated patches of tracheary regeneration were observed under several conditions. The wound interfered with development of the leaf directly above the wound.     

POLLINATION ECOLOGY OF LABIATAE IN A PHRYGANIC (EAST MEDITERRANEAN) ECOSYSTEM

This study was conducted in a phryganic (East Mediterranean) ecosystem at Daphni, near Athens, Greece. The Labiatae, represented by ten species belonging to nine genera, dominate in this ecosystem type. They flower from February to July. Both flowering time and nectar quantity are related to the species ability to tolerate intense water stress. Labiatae are visited by 201 insect species. Of these, 43 are exclusively supported by the family and 37 are monotropous. Solitary bees (mainly Anthophoridae, Megachilidae, Halictidae) constitute 47.3% of pollinators. The family is important in hosting specialized bees (15 species) in phrygana, particularly late in the flowering season. Labiatae species form two equally represented groups in this system; namely, the late winter-early spring (early) flowering, visited by relatively few pollinator species, and the late spring-summer (late) flowering species, visited by numerous pollinators. This temporal distinction is accompanied by different pollination profiles that include duration of anthesis, reward to pollinators, floral attractiveness, and flower character differentiation. All of these attributes are maximized in the early flowering period. This strategy suggests a mechanism for resource partitioning at a time when the pollinator resource is limited and competition for the services of pollinators is expected to be intense. Contrary to the current theory concerning cornucopian species, the copiously rewarding flowers of Labiatae in phrygana are not those abundantly serviced by pollinators.     

DORMANCY IN SEEDS OF FRASERA CAROLINIENSIS (GENTIAN ACEAE)

In freshly matured seeds of the long-lived monocarpic perennial, Frasera caroliniensis Walt., the embryos are underdeveloped and physiologically dormant. Dormancy was broken by a long period of stratification (chilling) at 5 C. Seventy six percent of the seeds germinated at 20 C (day)/10 C (night) after 98 days of chilling at 5 C, while seeds kept at 5 C germinated to 87% after 205 days. A warm, moist pretreatment was not required for subsequent breaking of dormancy at 5 C. Embryos in fresh seeds averaged 1.3 mm long, but after 12 weeks of chilling they averaged 4.1 mm. Thus, the embryos require a period of chilling to become fully developed, after which seeds can germinate at the afterripening temperatures (5 C) or at some higher temperature. Seeds of F. caroliniensis fit Nikolaeva's (1977) morpho-physiological complex dormancy type.