Progesterone and 20 alpha-dihydroprogesterone in sheep: a model of their distribution and metabolism.

The rates of metabolism of progesterone and 20alpha-dihydroprogesterone (20alpha-diHP) in sheep have been measured during and after the infusion of tracer amounts of [3H]progesterone. There were significant differences in the blood concentration of [3H]progesterone between experiments, but these were not attributable to the stage of gestation or to the difference between pregnant and non-pregnant animals. The mean (+/- S.E.M.) metabolic clearance rate of progesterone was 3-277 +/- 0-239 litres blood/min. The simplest model of the distribution of progesterone was one containing two pools, V1[P] and V2[P], where [p] is the blood concentration of progesterone, and in 23 experiments on 7 sheep the mean pool dimensions were 7-8[P] mug and 70[P] mug, respectively. This model was developed to include the formation of 20alpha-diHP from progesterone. Progesterone appeared to be the major source of 20alpha-diHP, though this did not seem to be an obligatory metabolite. The parameters obtained gave comparably low residual deviations for both labelled steroids and were consistent with other observations made on progesterone clearance.     

Establishment of an in vitro sciarid fly larvae assay to study plant resistance

Mechanisms underlying natural plant resistance to herbivorous invertebrates are still poorly understood in comparison with bacterial or fungal interactions. One reason is the difficulty in reliably and reproducibly assessing the effects under controlled conditions. This article describes a newly developed in vitro biological assay system that enables the interactions between sciarid larvae and plants, whose roots they feed on, to be studied under highly controlled conditions. The bioassay eliminates the problems created by the often variable environmental factors by providing an aseptic arena where experimental plants can be germinated and grown on agar within a Petri dish. Sciarid fly eggs are then collected, sterilised and added to the Petri dish. The system allows the eggs to hatch and the larvae to feed on the plant roots. A range of developmental parameters can then be recorded over time which can then be correlated with the experimental plant type. This assay system also allows a simultaneous comparison or 'choice chamber' between two (or more) different genotypes. The assay should greatly help to facilitate the identification of new components involved in insect resistance mediated pathway via the characterisation of mutant plants.     

The Art Collector

Abstract

The initiating point of this paper was an article in AEPR by Peter J. Smith in March/April 2003. At the end of the article, Peter J. Smith provides a short answer.     

Nutrient limitation and botanical diversity in wetlands: can fertilisation raise species richness?

The 'resource balance hypothesis' proposes that the species richness of grassland vegetation is potentially highest when the N:P ratio of plant tissues is 10–15 (co-limitation), so that species richness could be raised by fertilisation with N or P at sites with lower or higher N:P ratios, respectively. Here we use data from field surveys in Swiss, Dutch and American fens or wet grasslands to analyse what changes in N:P ratios might produce noticeable changes in species richness. Plant species numbers, above-ground biomass, tissue N and P concentrations and soil pH were recorded in plots of 0.06–4 m². In each data set, plots with intermediate tissue N:P ratios (6–20) were on average most species-rich, but N:P ratios explained only 5–37% of the variation in species richness. Moreover, these effects were partially confounded with those of vegetation biomass and/or soil pH. The unique effects of N:P ratios (excluding those shared with biomass and pH) explained 11–17% of variation in species richness. The relationship between species richness and N:P ratios was asymmetric: plots with high N:P ratios were more species-poor than those with low N:P ratios. This was paralleled by a smaller species pool size at high N:P ratios (estimated from species numbers in multiple records), suggesting that fewer species are adapted to P-limited conditions than to N-limited conditions. According to these data, species richness in wetlands may possibly be raised by P-fertilisation when the initial N:P ratio of the vegetation is well above 20, but this option is not recommended for nature conservation as it might promote common species at the expense of rare ones.