The Effects of Temperature on 86Rb Uptake by Two Species of Chlamydomonas (Chlorophyta, Chlorophyceae)

⁸⁶Rb uptake was examined in two species of unicellular greenalgae, Chlamydomonas nivalis isolated from snow, and a cellwall-less mutant of the temperate freshwater Chlamydomonas reinhardii.In C. reinhardii cells grown at 20°C and cooled rapidlyto 0°C, ⁸⁶Rb uptake was abolished. Cells cooled rapidlyto –5°C in the absence of ice accumulated ⁸⁶Rb veryrapidly but the time course of this uptake suggested non-selectiveaccumulation through a damaged plasmalemma. Cells grown at 8°Cwere viable, able to divide and motile; they showed no signsof cold-shock and ⁸⁶Rb uptake, albeit slow, was measurable at–5°C in the absence of extracellular ice. Cells ofC. nivalis grown at 20°C were damaged at sub-zero temperaturesalthough they did show an enhanced ⁸⁶Rb uptake at 0°C. Cellsgrown at 5°C were able to accumulate ⁸⁶Rb from media undercooledto -5°C in the absence of extracellular ice, and again showedenhanced uptake at 0°C. The process of acclimation to lowtemperature appears to differ in the two species. Key words: Chlamydomonas, temperature, ⁸⁶Rb uptake, membrane     

Development of a new piosphere in arid chenopod shrubland grazed by sheep. 2. Changes to the vegetation

The initial development of a piosphere created by stocking a new watering trough with ca. 200 sheep at a near-pristine site in arid chenopod shrubland near Whyalla, South Australia, was studied. The following variables were monitored for up to 8 years using fenced plots as controls:shrub mortality; forage biomass and defoliation of the dominant shrubs and short-lived grasses; linear dimensions of Rhagodia ulicina; the density of short-lived species; and the invasion of exotics. Patterns in these variables were looked for which reflected the attenuation of stocking pressure with distance from water; that is, ‘piosphere patterns’. Piosphere patterns were detected for many variables. Those with values increasing towards the trough were mortality of the main forage shrub Atriplex vesicaria. the density of three short-lived taxa, viz. the bassias (Sclerolaena obliquicuspis, S. patenticuspis. Maireana sclerolaenoides), Tetragonia tetragonoides, and annual Zygophyllum spp. (Z. crenatum. Z. ovatum), and growth of short-lived grass species when sheep stocking pressure was low. Variables with values decreasing towards the trough were grass biomass, percentage of A. vesicaria individuals in flower, the percentage foliation of A. vesicaria and of the forage shrub Maireana sedifolia, and shrub forage biomass of A. vesicaria and M. sedifolia. The rate at which these patterns were registered varied. After only 3 months, a piosphere pattern was evident for grass biomass. Marrubium vulgare, an introduced weed, invaded the trough site within the first 6 months. Almost no A. vesicaria shrubs had died after 2 years, but after 8 years there was a marked piosphere pattern in their mortality. The piosphere patterns in forage biomass and defoliation of A. vesicaria and M. sedifolia became more pronounced with time. Some variables did not exhibit a piosphere pattern, such as the dimensions of the shrub R. ulicina and the densities of successive cohorts of the forb Erodium spp., even though there was an overall effect of grazing in reducing their values. There was no mortality of two dominant shrubs, M. sedifolia and R. ulicina, during the first 8 years.     

Leaf anatomy in a Venezuelan montane forest

A survey has been made of the leaf anatomy of 43 woody species in three montane vegetation types of Isla Margarita, Venezuela, differing in levels of cloud cover (and hence wetness) and exposure to wind. The only character that varies significantly with increasing probability of periodic drought is specific leaf area, which can be related to the higher proportion of deciduous species in the drier habitat. Leaves become significantly smaller and thicker with increased exposure to wind, and have thicker outer epidermal walls and cuticles. Most characters, including stomatal density, guard cell length, palisade: non-palisade ratios, and the incidence of such features as hypodermis, sclerenchyma, crystals and secretory structures are highly variable and show few or no trends according to habitat. Some of these characters are apparently more constrained by phylogeny than by immediate ecological circumstances. As a contribution to the continuing debate on the significance of xeromorphy in tropical montane forest leaves, it is suggested that low light levels due to cloud cover can be discounted as an important determining factor; in the case of the Isla Margarita vegetation, the need to avoid excessive leaf temperatures may be of greater significance.     

Range of Variation in the Somatic Infraciliature and Contractile Vacuole Pores of Tetrahymena pyriformis

SYNOPSIS. Meridian ranges and contractile vacuole pore patterns were determined for 36 strains of Tetrahymena pyriformis, including most of the “classic” amicronucleate and micronucleate strains used by many investigators, plus a dozen other strains not previously characterized with respect to these two morphological features. Uniform axenic culturing and staining techniques were employed. Meridian ranges and modes are analyzed with strain-by-strain comparisons to the reports of previous investigators. The overall inclusive range of the 36 strains was 15–25 meridians. Modes for individual strains varied from 16 to 23. Broader ranges and, especially, higher modes and higher limits are recorded than those reported by Corliss (1953) for strains grown in bacterized hay cultures. Two postoral meridians are characteristic, although a single individual having only one and a few having three postorals were observed. Three postorals were found only in those strains having the higher number of total meridians. A few individuals, showing an anterior half or a posterior half of a bipolar meridian missing, indicate a possible explanation of how a single dividing individual could produce a proter and an opisthe differing in the total number of meridians. Meridian ranges of individual strains do not overlap each other in all cases, but they do constitute a progressively overlapping series which does not warrant taxonomic separation of strains at either extreme from each other. Contractile vacuole pores varied from 1 to 3, with 2 pores (1 each in meridians 5 and 6) being characteristic of 22 of the strains. The 2-pore 6-7 pattern was modal for 8 of the strains. Presence of 3 pores (in 5, 6 and 7) was typical of only 2 strains, but 3 pores were found occasionally. Another pattern of 3 pores (in 6, 7 and 8) was observed occasionally in 10 strains. Double pores (in 6 and 7 or 7 and 8) and the triple pore patterns were observed only in the strains having the highest total number of meridians. These variations are not considered adequate justification for taxonomic separation. Strains Ki and Aq are affirmed to be T. pyriformis. Strains V¹ PP and V, formerly regarded as strains of T. vorax, are considered to be T. pyriformis, supporting the view of Shaw and Williams (1963).     

The Respiratory Metabolism of Strelitzia juncea Ait. Seeds. The Effect of Dormancy Release by Oxygen on certain Glycolytic Enzyme Activities and Metabolite Concentrations

The activity of the glycolytic enzymes PFK, PFP, PK and aldolaseas well as the content of glucose, fructose, glucose-6-phosphateand fructose-6-phosphate were compared in the embryos of airand oxygen-incubated seeds of Strelitzia juncea. Determinationswere made during the first 4 d of incubation, prior to radicleemergence, which commences on day five for oxygen-treated seeds. No difference in PFK activity was found for the two treatments,and for both treatments PFK tended to increase with the incubationperiod. The fr2, 6P₂-stimulated PFP activity was slightly higherfor oxygen-incubated seeds, and showed a significant increasein activity over the 4 d incubation period for both treatments.No significant change in the general trend of PK and aldolaseactivity resulted from incubating the seeds in oxygen. The almost equimolar glucose and fructose contents of the embryoswere lower after 1 d of oxygen incubation of the seeds, andthe content decreased sharply during the incubation period.It is concluded that a moderate increase in the glycolytic capacityof embryos resulted from oxygen treatment of S. juncea seeds. Key words: Dormancy, glycolysis, Strelitzia juncea     

Herbivory and patch dynamics on rocky reefs in temperate Australasia: The roles of fish and sea urchins

Sea urchins are widely considered to be the major grazers in temperate subtidal systems, with herbivorous fish being browsers of minor importance. This paper reviews spatial and temporal patterns in these herbivores on rocky reels in temperate Australasia, with the aim of assessing their relative impacts on patch structure and dynamics. Herbivorous fishes are widespread and make up a significant numerical component the reel fish fauna. Sea urchins are also abundant, but not all geographic locations support actively grazing species. Both fish and sea urchins exhibit distinct patterns of distribution among depth strata. Within depth strata, all herbivores are restricted to (sea urchins) or forage preferentially in (fish) particular habitat patches, causing a mosaic of different feeding activities. These patches are either related to specific features of the habitat (e.g. Kelp patches, topography) or behavioural interactions. Foraging by sea urchins and demersal-nesting damselfishes is intense and persistent, whereas in the kelp-feeding fish Odax cyanomelas, foraging reaches greatest intensity at predictable locations during a few months of every year. Many fish and sea urchins consume some algae in preference to others. However, feeding preferences may determine the nature of the impact only in fishes. For sea urchins, preference may occasionally determine the order in which algae are consumed, but at high densities they consume all available macroalgae. Impacts of both types of herbivore on the abundance of algae have been recorded. Some sea urchins (e.g. Evechinus chloroticus, Centrostephanus rodgersii) appear to severely modify biogenic habitat structure by maintaining ‘barrens’ (areas devoid of macroalgae) over long periods. In contrast to this, the effects of fishes may be more transitory (e.g. seasonal impact of Odax cyanomelas on brown algae) or occur at smaller spatial scales (e.g. nest sites maintained by male Parma victoriae) Herbivorous and other fishes appear to respond to spatial patterns in algal distributions, rallier than having it major impact upon them. The relative effects of fish and sea urchins on the long-term dynamics of kelp forests are unknown, hut temporal patterns in herbivore abundance and behaviour, and algal demography arc urgent targets for research.     

Ants remove virtually all western myall (Acacia papyrocarpa Benth.) seeds at Middleback,South Australia

Abstract Ants can have a profound effect on the numbers of seeds present in the seed-bank. We investigated the removal of seeds of an important arid zone tree species, the western myall (Acacia papyrocarpa Benth.). The species has seed characteristics typical of other Acacia species: a hard seed-coat and a small, white aril or elaiosome; the latter is reported to be attractive to ants. An experiment was conducted to see how rapidly and completely the seeds would be removed under various caging treatments. Observations were also made to discover the major seed removers and the fate of the seeds. Seed removal was rapid (< 12h) and was almost entirely by ants, and while our observations were not conclusive, we speculate that much of the seed supply is effectively destroyed by seed harvester ants which thus act as true granivores, not seed dispersers. Whether this matters for western myall in the long term is problematic because the species is so long-lived; rare but successful recruitment events may well be sufficient for its long-term survival.     

Population Interactions and the Determinants of Population Size

Abstract The flux of individuals within populations is dependent upon six controlling processes: the intrinsic rate of increase of the plant, intraspecific competition for resources, interspecific competition, natural enemies, mutualisms and refuge effects such as the immigration of seeds from other populations. Although population interactions are generally believed to play a major role in determining the flux of individuals within populations, little attempt has been made to quantify the strength of these interactions and their role in the dynamics of populations. In this paper we examine the role of competition, herbivory and mutualistic interactions in determining the dynamics of a range of annual plant species. Firstly, it is shown that the dynamics of three weed species ( Bromus sterilis, Galium aparine, Papaver rhoeas ) in an experimental community in an arable cropping system of winter wheat are determined primarily by the rapid population growth of B. sterilis . Interactions between the species play a minor part in the dynamics of the system. Secondly, it is shown that current levels of grazing by overwintering populations of brent geese have a minor impact on the abundance of Salicornia europaea , but that increased grazing has the potential to reduce abundance and increase the instability of S. europaea populations. This is a consequence of the aggregative response of the geese, which results in an increasing proportion of the seeds of S. europaea being eaten as plant density increases. Thirdly, it is shown that there is a complex interaction between root pathogenic and arbuscular mycorrhizal fungi in natural field populations of Vulpia ciliata and that the benefit of mycorrhizal fungi to the plant is in providing protection against pathogens.