Some effects of suction on the hatching eggs of Meloidogyne javanica

Development and hatch of eggs of Meloidogyne javanica was independent of suction between pF o and 3·6 but decreased rapidly between pF 3·6 and 4·2. Water was lost from the extracellular fluid of eggs containing developing embryos when the suction was above pF 3·6, causing a decrease in the mean volume of the eggs. The vitelline membrane of an egg is probably semipermeable and the osmotic pressure of the contents about 4 atmospheres (pF 3·6). The volume of the cellular contents of an egg did not increase during development and so the hydraulic conductivity of the soil is probably unimportant in hatching. The results suggest that eggs of M. javanica hatch over a wide range of soil moistures. The inhibition of hatch at high suctions, involving the reversible removal of water, is a likely survival mechanism.     

Phosphorous availability influences the dissolution of apatite by soil fungi

Apatite (Ca₁₀(PO₄)₆(OH,F,Cl)₂) is the primary inorganic source of phosphorus in the biosphere. Soil fungi are known to increase plant‐available phosphorus by promoting dissolution of various phosphate minerals. Yet no apatite dissolution studies exist using fungi as weathering agents, and regulation of fungal weathering activity in response to different levels of phosphorus availability is largely unknown. Fungi were isolated from a grassland soil in northern California. Three pathways of tri‐calcium phosphate (Ca₃(PO₄)₂) (TCP) dissolution in liquid culture were identified among biogeochemically active fungi: (1) acidification (pH 3.3 ± 0.16), (2) moderate acidification (pH 4.9 ± 0.11) and (3) no acidification. Isolates representing pathway 1 and 2 were Zygomycetes in the order of Mucorales. All non‐acidifying isolates in pathway 3 were Ascomycetes and cleared the media by altering TCP into hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and sequestering it within mycelial spheres. One isolate representing each pathway was used in fluorapatite dissolution experiments either with the fungi present or under abiotic conditions using cell‐free liquid media conditioned by fungal growth at different phosphorus and calcium availabilities. Both Mucorales isolates acidify their substrate when growing in the presence of phosphorus. Mucorales exudates were mainly oxalic acid, and conditioned cell‐free media with phosphorus induced fluorapatite dissolution at a rate of 10^?0.9±0.14 and 10^?1.2±0.22 µmol P m^?2 s^?1. The ascomycete isolate on the other hand, induced fluorapatite dissolution at a rate of 10^?1.1±0.05 µmol P m^?2 s^?1 by lowering the pH of the media under phosphorus‐limited conditions, without producing significant amounts of low molecular weight organic acids (LMWOAs). Oxalate strongly etches fluorapatite along channels parallel to [001], forming needle‐like features, while exudates from the ascomycete‐induced surface rounding. We conclude that while LMWOAs are well‐studied weathering agents, these do not appear to be produced by fungi in response to phosphorus‐limiting growth conditions.     

Interocular Transfer of Adaptation to Spatial Frequency during Retinal Ischaemia

PROLONGED inspection of a grating pattern of repetitive light and dark contours produces a transient reduction in the visibility of low-contrast gratings the spatial frequency and orientation of which are similar to that of the inspected grating. This after-effect, often referred to as adaptation, occurs even when one eye performs the inspection and its partner is tested. Certain properties of this after-effect, with other data, suggest that its locus is visual cortex^1–7, but the evidence is largely indirect because of the difficulty of identifying sites of action within the stages of the human visual system. One of the few techniques available for inferentially partitioning the visual system stages is retinal pressure blinding, a heroic but effective method for functionally uncoupling the retina from higher stages^8–11.     

The behavioural responses of juvenile signal crayfish Pacifastacus leniusculus to stimuli from perch and eels

• 1 Experiments were designed to determine the relative importance of chemical and visual stimuli in eliciting predator avoidance behaviour in juvenile freshwater crayfish Pacifastacus leniusculus (Dana).
• 2 Crayfish placed in visual and/or chemical contact with one of two predators exhibited marked avoidance behaviour, spending less time walking and climbing and more time within shelters.
• 3 The combined effects of both visual and chemical stimuli increased crayfish shelter use and reduced walking and climbing activity to a greater degree than either stimulus when presented alone.
• 4 Crayfish exhibited avoidance behaviour in response to chemical stimuli during periods of light and darkness. Visual detection of predators elicited avoidance behaviour during the day.
• 5 It is suggested that the behavioural response of P. leniusculus to chemical stimuli reduces the likelihood of being detected by visual predators, and that chemical stimuli lower the response threshold for avoidance behaviour in crayfish reacting to visual stimuli. The adaptiviry of using chemical cues to detect predators is emphasized.

     

The Association between Soil and Xylem Water Potential, Leaf Resistance, and Abscisic Acid Content in Droughted Seedlings of Douglas-fir (Pseudotsuga menziesii)

During progressive stages of drought and subsequent recovery through rewatering, foliage of 2-year-old seedlings of Douglas-fir was sampled for water potential, stomatal resistance, and abscisic acid content (ABA). Simultaneous measurements of soil matric potential were also made. There was a rapid rise in stomatal resistance in the range of 1500–2000 ng ABA g^?1 dry weight of foliage corresponding with a plant water potential of –10 to –12 bars. Soil matric potential seemed more closely associated with ABA increase than did plant water potential. After rewatering, seedlings returned to normal ABA levels within 6 days.     

Carbon pathways to zooplankton: insights from the combined use of stable isotope and fatty acid biomarkers

1. Numerous studies have quantified the relative contribution of terrestrial‐ and phytoplankton‐derived carbon sources to zooplankton secondary production in lakes. However, few investigated the pathways along which allochthonous and autochthonous carbon (C) was actually conveyed to consumers. 2. We suggest that the combined use of fatty acid and stable isotope biomarkers could solve this issue. We conducted a field study on two oligotrophic lakes, in which primary production increased significantly between 2002 and 2004. We used modelling to estimate the contribution of terrestrial‐ and phytoplankton‐derived C to particulate organic C (POC) and zooplankton production from their δ¹³C values in 2002 and 2004. 3. According to the isotope model, phytoplankton‐derived C accounted for a major part of the POC pool in both lakes and supported more Daphnia sp. production in 2004 than in 2002. Fatty acid data revealed that increased contribution of algal‐C to Daphnia production, although common between both lakes, was achieved through C pathways that were different. In one lake, Daphnia grazed more intensively on phytoplankton, whereas in the other there was greater grazing on bacteria. In the latter case, the increased primary production resulted in greater release of algal‐derived dissolved organic C (DOC), which may have supported extra bacterial and eventually Daphnia, production. 4. This is the first study illustrating that the combination of fatty acid and stable isotope biomarkers could further our understanding of the factors controlling the relative magnitude of food webs pathways conveying organic matter to zooplankton.     

CELLULAR BIOLOGY OF BONE RESORPTION

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged.
1. The resorption of bone is the result of successive steps that can be regulated individually.
2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone.
3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation.
4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells.
5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO, by carbonic anhydrase and secreted by an ATP-driven vacuolar H⁺-K⁺-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone.
6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca²⁺) generated as a result of osteoclastic bone resorption.
7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.     

The effect of the impoundment of Lake Kainji, Nigeria, on the indigenous species of mormyrid fishes

SUMMARY. Changes in the numbers and species composition of mormyrid fishes since the impoundment of the Niger (Lake Kainji) are analysed. An initial decline after impoundment appears to have been reversed and these fish are now increasing. The increase is, however, restricted to a few species only; many forms common in the river are still declining or have disappeared altogether. Marked seasonal variations in abundance are described and possible explanations discussed.     

Oxygen and the hatch of eggs and migration of larvae of Meloidogyne javanica

The percentage hatch of eggs of Meloidogyne javanica increased asymptotically as the oxygen concentration increased from 0·2 to 21%. Hatch was unaffected by suction pressures of less than pF 3·4 at any oxygen concentration but was decreased by higher suctions. Both the development of embryos and the migration of larvae approached a maximum at an oxygen concentration of about 15%. Hatching was not limited by the rate oxygen diffused through the gelatinous matrix of the egg sac nor was the concentration of oxygen at the surface of an egg sac significantly lower than that within an individual egg. The maximum rate of metabolism appeared to need an oxygen concentration within the egg greater than 10%.