White rust of chrysanthemums

Teleutospores of Puccinia horiana Henn. germinate and discharge sporidia between 4 and 23 °C. At the optimum temperature of 17 °C sporidia discharge starts within 3 h. Maximum germination of the sporidia takes place within 2·5 h between o and 30 °C, there being no clear optimum. High humidity and a film of moisture appear to be necessary for germination of both teleutospores and sporidia. Sporidia can penetrate either leaf surface of chrysanthemum to cause infection between 4 and 24 1°C and within the optimum temperature range, 17–24 °C, effectively penetrate within 2 h. The sporidia are very sensitive to desiccation at below 90 % relative humidity. Methods are described, using leaf discs and whole plants, for screening chrysanthemum cultivars for susceptibility to white rust. Cultivars were placed in five classes ranging from susceptible to immune. Leaf discs of immune cultivars can be distinguished within 30 h by a brown discolouration at the point of inoculation. The early stages of development of the fungus in susceptible, resistant and immune hosts are described. The incubation period in susceptible plants is normally 7–10 days, teleutospores being formed a few days later. Leaves become less susceptible with age but the oldest leaves on 5-month-old plants could still be infected. The maximum survival time of teleutospores in the sori on detached leaves was 8 weeks but was considerably less under moist conditions or buried in soil. Low doses of a mancozeb with zineb fungicide controlled infection by preventing penetration rather than by inhibiting sporidial germination.     

Annuals with transient seed banks: the population biology of indigenous Sorghum species of tropical north-west Australia

Indigenous Sorghum species are a prominent feature of the seasonally-dry tropics of north-west Australia. Studies of the dormancy characteristics of these species were undertaken, and measurements of seedling emergence, of plant survival, growth and reproductive success, and of seed bank changes were made for one Sorghum intrans population and for three S. stipoideum populations over two growing seasons. On wetting, 90% of the non-dormant, viable seeds of nil species germinated within five days. The temperature range for optimal germination was 25 - 35°C, and germination declined with decreasing water potential. The seeds of all species had high innate dormancy at seedfall, and this was broken down over the ensuing dry season. For S. intrans and S. stipoideum height and tiller number increased steadily after emergence up to the time of floral initiation, which occurred simultaneously for primary tillers within each population. Most vegetative tillers died after this leaving a single reproductive culm which rapidly elongated to reach 1 – 4 mat maturity, depending on site and season. Seedfall occurred over a short, c. two-week period before the rainy season had finished. Seeds were dispersed within 2 m of the parent plant. Seeds were buried to less than 2 cm by the screwing action of their hygroscopically-active awns. All these annual Sorghum species appear to have transient seedbanks, formed at seedfall near the end of one rainy season and exhausted by germination following the first rains of the next annual rainy season. We suggest that S. intrans and S. stipoideum persist despite this feature because they mature at the earliest time that the rainy season has been known to end in their respective localities. Persistence is further aided by adaptations which enable individuals to survive the unpredictable short droughts which occur while they are emerging and growing. S. intrans and S. stipoideum have low fecundity (mean ? 8 viable seeds per individual) and high survivorship. They fit Watkinson's (1981) schema relating fecundity to survival for annuals with transient seed banks.     

Utilization of Messenger in Adenovirus-2-infected Cells at Normal and Elevated Temperatures

THE regulation of protein synthesis in eukaryotic cells appears to be distinctly different from that in prokaryotes. The long-lived messenger RNA in eukaryotes suggests that control of protein synthesis occurs partially at the level of translation and regulation at the level of initiation of translation of messenger RNA has been found under several different conditions in mammalian cells^1–6.     

Seasonal variation in the filament morphology of the freshwater diatom Melosiragranulata(Ehrenb.) Ralfs

SUMMARY 1, The seasonal changes in filament dimensions and the occurrence of separation valves in a population of Melosira granulata (Ehrenb.) Ralfs were investigated in a eutrophic lake in southern England. Experiments were carried out in culture to study the effects of various environmental factors on the diatom's morphology.
2. Major blooms of M. granulata occurred from September to October, coinciding with an increase in mean filament length brought about by the high growth rate of the diatom. This increase was controlled by a reduction in the frequency of separation valve production. A similar increase in filament length occurred in rapidly growing cultures.
3. The population was lost from the epilimnion when thermal stratification occurred. Immediately prior to this the population consisted of short filaments.
4. The mean filament length was greatest during occasional summer blooms and was again controlled by a reduction in the frequency of separation valve production. This appeared to be a response to the reduced nutrient concentration in the lake. This is supported by similar morphological changes in low nutrient cultures.
5. Increases in the filament length of the diatom were accompanied by increases in the diatom's sinking velocity. The length increase thus ensured its rapid return to the sediment, after summer growth periods, where the cells could be replenished with nutrients.