Floral transition mutants in Arabidopsis

An inventory of genetic differences in flowering time in Arabidopsis is presented and discussed. Many genes influence the transition to flowering in a quantitative way. Two groups of mutants and natural variants can be distinguished: those that are responsive to environmental factors and those that are less responsive or unresponsive. It is possible that all late/early-flowering mutants isolated to date carry a mutation with an effect, either promotive or repressive, on a floral repressor. The interaction between light perception and flowering has been studied by analysis of phytochrome- and cryptochrome-deficient mutants, which showed that phyA and probably also cryptochrome have a promotive role in flowering, whereas phyB and other stable phytochromes have an inhibitory role. A circadian rhythm is important in establishing daylength sensitivity, as was shown by the phenotype of the elf 3 mutants.     

Pattern of Nucleolar Growth in Differentiating Cotton Fibres (Gossypium hirsutum L.)

The nucleolar size in cotton fibres has been examined at earlystages of growth by light microscopy. The volumes of the nucleoliwere computed from the perimeters measured by means of a projectionmicroscope. The frequency distributions were normal at eachstage. The mean nucleolar volume increased rapidly, reacheda maximum around 10 d after anthesis and declined to a constantvalue around 20 d after anthesis. The nucleolar growth patternsof some prediction models have been compared with the in situobservations. This comparison suggests a simultaneous increasein nucleolar size in all fibres within a population. Therefore,the hypothesis that the size of the fibres is correlated withthe size of resident nucleoli, is favoured. Gossypium hirsutum, cotton fibre, nucleolus, cell differentiation     

Climate-related differences in the dominance of submerged macrophytes in shallow lakes

It has been suggested that shallow lakes in warm climates have a higher probability of being turbid, rather than macrophyte dominated, compared with lakes in cooler climates, but little field evidence exists to evaluate this hypothesis. We analyzed data from 782 lake years in different climate zones in North America, South America, and Europe. We tested if systematic differences exist in the relationship between the abundance of submerged macrophytes and environmental factors such as lake depth and nutrient levels. In the pooled dataset the proportion of lakes with substantial submerged macrophyte coverage (> 30% of the lake area) decreased in a sigmoidal way with increasing total phosphorus (TP) concentration, falling most steeply between 0.05 and 0.2 mg L⁻¹. Substantial submerged macrophyte coverage was also rare in lakes with total nitrogen (TN) concentrations above 1–2 mg L⁻¹, except for lakes with very low TP concentrations where macrophytes remain abundant until higher TN concentrations. The deviance reduction of logistic regression models predicting macrophyte coverage from nutrients and water depth was generally low, and notably lowest in tropical and subtropical regions (Brazil, Uruguay, and Florida), suggesting that macrophyte coverage was strongly influenced by other factors. The maximum TP concentration allowing substantial submerged macrophyte coverage was clearly higher in cold regions with more frost days. This is in agreement with other studies which found a large influence of ice cover duration on shallow lakes' ecology through partial fish kills that may improve light conditions for submerged macrophytes by cascading effects on periphyton and phytoplankton. Our findings suggest that, in regions where climatic warming is projected to lead to fewer frost days, macrophyte cover will decrease unless the nutrient levels are lowered.     

Nucleolar Activity in Differentiating Cotton Fibres is Related to the Position of the Boll on the Plant

Significant differences in nucleolar size at early stages ofcotton fibre development have been observed between bolls ofdifferent fruiting branches. Cutting off the apex or removalof the leaf subtending the cotton boll 6 d before floweringor emasculation followed by an addition of GA₃ resulted in asignificant decrease of the nucleolar size. Excising half ofthe roots had no influence on the nucleolar activity. The differencesin nucleolar size between bolls of the first and fifth fruitingbranches remained significant except in the case where the leafsubtending the boll was removed. Key words: Cotton fibres, nucleolus, gibberellin     

Both female castes contribute to colony emigration in the polygynous ant Mystrium oberthueri

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Growth of shredders on leaf litter biofilms: the effect of light intensity

1. The effect of light intensity on the decomposition of poplar (Populus nigra) leaves and growth of the shredders, Asellus aquaticus and Gammarus pulex, was studied in a laboratory experiment. The response was studied along a gradient of six light intensities of 0, 5, 23, 54, 97 and 156 μmol m^?2 s^?1. It was hypothesised that an increase in light intensity would increase growth of shredders, because of an increase of algae (i.e. food quality) in the leaf‐biofilm. 2. Light intensity affected both leaf‐biofilm quality and consumer behaviour and affected several aspects of the decomposition‐consumer interaction. In the absence of invertebrates, leaf mass loss was lower in the dark, while light intensity had no significant effect on mass loss of poplar leaf in the presence of invertebrates. Light intensity affected algal biomass, density and composition, and had a significant positive effect on the growth of both shredders. 3. Our results suggest that algae can be an important component of the nutritional value of the leaf‐biofilm for benthic invertebrates, directly as an additional food source and indirectly through a link with bacteria and/or fungi. 4. The River Continuum Concept mainly emphasises allochthonous inputs to headwater streams and autochthonous production further downstream. Our results suggest that light, by its effect on the biofilms on leaf surfaces, might be a more important factor in headwaters than is usually assumed.