Detection of hormesis effect in longevity: simulation approach for heterogeneous population.

Manifestation of hormesis in longevity was modelled by modification of the mortality rate during and after the period of a stress factor action. In heterogeneous population this can lead to observation of unchanged mortality during action of the stress and decrease in mortality after stress period. Stochastic simulations were made to investigate the possibility of detecting the hormesis effect on the basis of the stress-control longitudinal data. The goal of the stochastic simulation was to investigate the role in the hormesis detection of control and stressed group size, of population heterogeneity variance value, of stress and hormesis attributable risks as well as the role of a prior information about the survival in the control group. It was demonstrated that if the attributable risks for stress and hormesis effects are approximately equal, then in both 'high' and 'low' heterogeneous populations the hormesis phenomenon is detected with probability higher than 75% even in relatively 'small' groups of 50 subjects. In case of 'weak' effect the hormesis phenomenon is not detected in a 'highly heterogeneous' population even in a group composed of 1000 subjects. In a 'low heterogeneous' population the hormesis phenomenon is detected with probability higher than 70% when the group size is not less than 200 subjects. Information about the survival in control group did not play a critical role in all experiments and exact survival curve may be replaced by the traditional Kaplan-Meier estimate.     

Reduction of the filtration rate of Daphnia galeata by dissolved photosynthetic products of edible phytoplankton

The filtration rate of Daphnia galeata was determined in in situ experiments in Bautzen Reservoir and in laboratory experiments, where daphnids were exposed to filtrates that previously contained either natural phytoplankton or cultured eukaryotic algae (Scenedesmus obliquus or Asterionella formosa), respectively. Individual filtration rate (FR) was measured using fluorescent beads, taking into account ingested beads in the gut only. Compared to heated control treatments (100 °C), dissolved compounds released by the nutritious cultured algae during the preconditioning phase or by the natural phytoplankton assemblages from Bautzen Reservoir strongly reduced the filtration rate of D. galeata (down to 60%). Heating deactivated these dissolved compounds. A significant correlation was found between primary production measured in situ and the reduction of FR in the filtrate of reservoir water, indicating that extra-cellular products released during photosynthesis triggered the reduction of the filtration rate. The ratio of ingested to collected beads was used to quantify the proportion of food, which was not only collected but passed the mouth of D. galeata. The ratio of ingestion to collection was compared between filtered and unfiltered reservoir water both media identical with respect to the concentration of dissolved compounds, whereas other factors (e.g. food concentration, temperature, filtration rate) were different. The changes in this ratio between filtered and unfiltered reservoir water suggest that D. galeata is capable of a chemosensory control of the ingestion behaviour by detecting external metabolites.     

The impact of intralacustrine introductions with regard to space utilization and competition for territories to a cichlid fish community in Lake Malawi,Africa

At Thumbi Island West in Lake Malawi, a number of cichlid species was introduced from elsewhere in the lake some 20 years ago. These are now, in part, well established and could have an effect on the indigenous fish fauna. Repeated removals of the males of the introduced species from their territories were undertaken and artificial miniature reefs were established to find out whether the emptied existing territories or the newly established potential territories would be occupied by indigenous species. The results show that the emptied territories were almost invariably re-occupied by either a male of the introduced species conspecific to the previous territory holder or they remained empty. The newly established miniature reefs were not occupied by territorial males of either introduced or indigenous species but by young fish of a variety of species.     

Modulation of shoot formation in tobacco callus by tissue transfer between permissive and inhibitory media

Summary In an attempt to understand events involved in the cellular regulation of in vitro plant organogenesis, experiments were performed in which tobacco (Nicotiana tabacum L.) callus was transferred at different days in culture from a shoot-forming medium to a non-shoot-forming medium and vice versa. The transfers were made at key histologic stages of the shoot-forming process and known biochemical and biophysical correlates were examined. The changes in starch accumulation and disappearance supported the previously assigned functions, and could be correlated with the histologic changes that occurred in the callus after transfer at the different culture times. In contrast, the changes in respiration could not be correlated with these events. The changes in osmotic and turgor potentials after transfer showed that osmotic adjustment preceded both shoot initiation and development. This suggests that osmotic adjustment might play an important role in in vitro organogenesis. This research was supported by the Natural Sciences and Engineering Research Council of Canada grant A-6467 to T. A. T.     

Potassium content in soil,uptake in plants and the potassium balance in three European long-term field experiments

This study quantitatively assesses the fate of K derived from mineral fertilizers and organic manures and the effective K balance in three long-term field experiments at Rothamsted (UK), Bad Lauchstaedt (Germany) and Skierniewice (Poland). Plant availability, uptake and the overall utilization of K over the last 30 years (1965–1996) are discussed and related to soil K Availability Indices determined by the standard methods used in each of the three countries. In addition, to provide a standard comparison of the three sites, Exchangeable K (1 M NH₄OAc) and Non-exchangeable K (K extracted by boiling with 1 M HNO₃) were measured on one recent (1995) set of soil samples. Plant availability and utilization of K was partly related to clay content, but more closely to the cation exchange surfaces associated with both mineral and organic constituents and also, at Rothamsted, to the capacity of clay minerals to fix K. The recovery rate of K from mineral fertilizer by crops did not exceed 62%. Fertilizers were least effective in the most strongly K fixing soil at Rothamsted (44% maximum) and most effective in the soil with the highest cation exchange capacity (CEC) at Bad Lauchstaedt (62%), where the greater quantity of exchange sites appear to be associated with humic material. Recoveries of K from farmyard manure (FYM) varied from 22–117% (values of >100% indicating subsoil uptake or the release of reserves). Deficiencies of N, P and Mg in some treatments decreased the effectiveness of applied K and may have caused increased leaching of K from the plough layer. FYM was generally more effective than mineral fertilizer where mineral N and P were not applied because these nutrients were effectively supplied in the manure. But the effectiveness of mineral K fertilizer decreased when applied in combination with FYM because FYM was the preferred source of K. Where FYM application increased the CEC of soils, this also improved K utilization but only where K was not extensively leached or fixed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nonconsumptive predator effects modify crayfish‐induced bioturbation as mediated by limb loss: Field and mesocosm experiments

1. We addressed the implications of limb loss and regeneration for multispecies interactions and their impacts on ecosystem engineering in freshwater stream environments.
2. We included regenerative and nonregenerative crayfish as well as fish predators in a 2 × 2 factorial design to assess the effects on water turbidity of interactions between crayfish ecosystem engineers differing in regenerative status and their fish predators.
3. We demonstrated that crayfish limb loss and predation risks lead to more turbidity in field and mesocosm conditions. Moreover, ongoing regeneration of crayfish increased turbidity, while fish presence seemed to hinder crayfish turbidity‐inducing behaviors (such as tail‐flipping and burrowing) in the mesocosm experiment.
4. We confirmed that greater numbers of crayfish produce a greater amount of turbidity in situ in streams.
5. Although mechanical burrowing crayfish capacities may depend on crayfish burrowing classification (primary, secondary, or tertiary), our work emphasizes the implication for turbidity levels of crayfish autotomy in freshwater streams.