DIATOM MINERALIZATION OF SILICIC ACID IV. KINETICS OF SOLUBLE Si POOL FORMATION IN EXPONENTIALLY GROWING AND SYNCHRONIZED NAVICULA PELLICULOSA1

Silicic acid taken up from the growth medium by Navicula pelliculosa (Bréb.) Hilse was shown to enter at least two compartments: i) soluble pools; ii) insoluble fraction comprised predominantly of the silica frustule. Soluble Si pools were extracted by a variety of agents from cells uniformly labeled for ten generations in medium containing ⁶⁸Ge-Si(OH)₄. 100 C water soluble and 0 C perchloric acid (PCA) soluble Si pools of 680 mM Si·l^?1 and 490 mM Si·l^?1 cell water represented 13 and 9%, respectively, of total, cell Si in exponential growth phase cells. Uniformly labeled cells synchronized by the combined synchronization technique accumulate at the cell cycle stage where silica frustule development is initiated. These cells contain water and PCA soluble pools of 10 nmol Si·10⁶ cells^?1 and-8.8 nmol Si·10⁶ cells^?1, respectively. On addition of Si(OH)₄, a rapid uptake ensues allowing the Si pool to expand 2.5-fold, apparently to provide precursors of the silica frustule.     

Spatial patterns in species–area relationships and species distribution in a West African forest–savanna mosaic

Aim To investigate the relationship between the slope z of the species–area relationship (SAR) and the intensity of spatial patterns in species number and dissimilarity for woody plants with different modes of seed dispersal. According to island theory we expect, for any given archipelago, steeper slopes and more pronounced spatial patterns for groups of less dispersive species. Location Ivory Coast, West Africa. Methods In a West African forest–savanna mosaic we collected presence–absence data for woody plant species in 49 forest islands. The parameters of the SARs were fitted by nonlinear regressions and then compared for plant species aggregated according to their mode of seed dispersal. We used the Mantel test to calculate the intensity of spatial patterns in species number, i.e. residual deviation from SAR, and species dissimilarity. Results The z‐value for bird‐dispersed species was lower (0.11) than that for wind‐dispersed species (0.27), with mammal‐dispersed species taking an intermediate value (0.16). This result suggests that, as a group, bird‐dispersed species are better colonizers. The spatial pattern in species number as well as species similarity was more pronounced for bird‐ compared with wind‐dispersed species. Main conclusions The standard interpretation of the theory of island biogeography claims that shallow slopes in the SAR imply low isolation of islands, i.e. good dispersal abilities of species. The results of our study appear to contradict this statement. The contradiction can eventually be resolved by a more detailed account of the colonization process, i.e. by distinguishing between dispersal and consecutive establishment of populations.     

ULTRASTRUCTURE OF A NEW SAND-DWELLING DINOFLAGELLATE,SCRIPPSIELLA ARENICOLA SP. NOV1

A new dinoflagellate, Scrippsiella arenicola Horiguchi et Pienaar sp. nov., is described from tidal pools with sandy substrates along the east coast of South Africa. S. arenicola exhibits a vertical migratory rhythm which is in synchrony with the tidal cycle. It is a medium-sized armoured dinoflagellate with many rod-shaped chloroplasts. Thecal plate arrangement is pp, x, 4′, 3a, 7′, 6c, 5′, 2″ and 4s. The 2a and 3a plates are separated from each other. S. arenicola has several unique ultrastructural features. Electron-dense fibres are found on the protruded part of the thecal plates, such as on the ornamental projections or extremities of the lists. In addition to the 9 + 2 axoneme, additional fibres are found in the free moving part of the longitudinal flagellum. The portion of the transverse flagellum covered by the left sulcal list possesses a dense array of mastigonemes which connect the flagellum and the cell. The flagellar pore platelets differ from ordinary thecal plates in their thickness and fibrous nature. The ultrastructure of the apical stalk and its associated structures is described. The vertical migration and mode of cell division is also described.     

Increase of Lipid Hydroperoxides in Liver Mitochondria and Inhibition of Cytochrome Oxidase by Carbon Tetrachloride -Intoxication in Rats

The cellular localization of lipid hydroperoxides was determined for the first time in mitochondria, microsomes and cytosol of rat liver using a specific method involving chemical derivatization and HPLC. Mitochondria contained the highest level of hydroperoxides. After 6h of intragastric administration of carbon tetrachloride (CCl₄) to rats (2 ml/kg body weight), the concentration of lipid hydroperoxides increased significantly in liver mitochondria and cytochrome oxidase activity was inhibited to 35% of the control rats. The mitochondrial content of haem a decreased to 60% of the control at 12 h of CCl₄ administration. In vitro reaction of mitochondria with CCl₄ caused inactivation of cytochrome oxidase. These observations suggested that cytochrome oxidase and haem a in mitochondria were targets of CCl₄.     

The role of seed rain in determining the assembly of a cliff community

Abstract. Cliff-face communities of the Niagara Escarpment in Ontario, Canada, are dominated by long-lived Thuja occidentalis and a consistent assemblage of other plants. Our objective was to determine whether seed rain plays a role in determining why these species are dominant. Seed rain was collected from the cliff face and from the surrounding plateau-and talus communities at two sites over a 2-yr period in order to compare these three adjacent, but different communities. Multivariate Discriminant Analysis first separated the two sites: primarily due to the importance of Betula papyrifera at one site. The three community types were also separated, although there was still substantial overlap. There was a predictable array of species associated with each community although the seed rain on the cliff faces differed slightly. When characterized by univariate ANOVAs, seed rain in the cliff faces and plateaus had a lower species richness and lower total seed density than the talus sites. Seeds of two of the 11 species analysed individually showed an influence of habitat type on their number. Seed morphology did not influence patterns of seed rain. Finally, there was no correlation between the seed rain and above-ground vegetation in any of the communities. We conclude that the seed rain patterns that exist do not act to filter the plants that form the mature vegetation of cliffs.     

Ecophysiological studies on orchids of Madagascar: incidence and plasticity of crassulacean acid metabolism in species of the genus Angraecum Bory

The present study is an investigation on photosynthetic options in an orchid taxon and deals with the mainly epiphytes comprising genus Angraecum Bory. The incidence of crassulacean acid metabolism (CAM) in Angraecum species collected at various habitats in Madagascar was surveyed by analysis of stable carbon isotope composition (¹³C values). The values showed both inter- and intraspecific variability and suggest that in situ about 50% of the analysed species perform C3 photosynthesis, 20% moderate CAM (fixation of external CO₂ during day-and night-time) and 30% pronounced CAM (CO₂ uptake entirely during the night). The photosynthetic behaviour of the species indicated by the ¹³C values was clearly related to the habitat from where the samples derived. In A. eburneum, A. sororium and particularly in A. sesquipedale the stable carbon isotope analysis was complemented by measurements of CAM performance under controlled conditions. The experiments with A. sesquipedale revealed that drought and temperature are important factors modulating CAM, whereas variation of the leaf-to-air water vapor pressure difference was less effective. Altogether, the results of the study support the view that the high biological adaptability and thus the ecological success of the genus Angraecum is largely based on genotypic diversity and intraspecific plasticity of the photosynthetic behaviour.     

Characteristics of transient photosynthesis in Quercus serrata seedlings grown under lightfleck and constant light regimes

Photosynthetic-induction response and light-fleck utilization were investigated for the current-year seedlings of Quercus serrata, a deciduous tree found in temperate regions of Japan. The tree seedlings were grown under three light regimes: a constant low photosynthetic photon flux density (PFD) regime of 50 mol m^–2 s^–1, a constant high PFD regime of 500 mol m^–2 s^–1, and a lightfleck regime with alternated low (lasting 5 s) and high (lasting 35 s) PFD. The photosynthetic-induction response following a sudden increase of PFD from 50 to 500 mol m^–2 s^–1 exhibited two phases: an initial fast increase complete within 3–5 s, and a second slow increase lasting for 15–20 min. Induction times required to reach 50% and 90% of steady-state assimilation rates were significantly shorter in leaves from the constant low PFD than those from the high PFD regime. During the first 60–100 s, the ratio of observed integrated CO₂ uptake to that predicted by assuming that a steady-state assimilation would be achieved instantaneously after the light increase was significantly higher for leaves from the low PFD regime than from the high PFD regime. Lightfleck utilization was examined for various durations of PFD of 500 mol m^–2 s^–1 on a background PFD of 50 mol m^–2 s^–1. Lightfleck utilization efficiency was significantly higher in low PFD leaves than in the high PFD leaves for 5-s and 10-s lightflecks, but showed no difference among different light regimes for 100-s lightflecks. The contribution of post-illumination CO₂ fixation to total carbon gain decreased markedly with increasing lightfleck durations, but exhibited no significant difference among growth regimes. Photosynthetic performances of induction response and lightfleck utilization in leaves from the lightfleck regime were more similar to those in leaves from the low PFD regime. It may be the total daily PFD rather than PFD dynamics in light regimes that affects the characteristics of transient photosynthesis in Q. serrata seedlings.     

The fate of spring applied fertilizer N during the autumn-winter period: comparison between winter-fallow and green manure cropped soil

A field experiment was carried out at a pilot plot that was cropped with oilseed rape, and then left partly fallow and partly cropped with a green manure (mustard) during the autumn after harvest of the oilseed rape. The rape residues were incorporated in the soil. Methods used to quantify the N fluxes from harvest until sowing of the next crop were (1) ¹⁵N balance method, (2) total mineral N analysis and (3) NO emission measurements. Losses of spring applied fertilizer N were negligible in cropped plots and minimal in fallow plots during the following autumn-winter period. Most of the plant-N residues was retained by the organic N pool of the upper 30-cm soil layer. The green manure contributed slightly to soil available N at sowing of the next crop. However, the incorporation of plant material resulted in a nitrate flux that was at risk of leaching on the fallow plots, and on the green manure plots after incorporation of the green manure. This nitrate was largely derived from soil organic N, not from unused fertilizer applied in spring or from immobilized fertilizer. The NO emissions from the green manure plots were significantly higher than emissions from the fallow plots. The plants had a stimulating effect on the NO emission. A relationship between the NO emission and the soil nitrate concentration could not be established. No emissions were measured after green manure incorporation due to the low temperatures at the pilot plot. However, a greenhouse experiment showed an increased emission after incorporation. The NO emissions seemed to be related with the soil ammonium concentration.