SummaryThe study of the evolutionary and population genetics of quantitative traits requires the assessment of within-and among-population patterns of variation.We carried out experiments including eight Iberian Arabidopsis thaliana populations (10 individuals per population) in glasshouse and field conditions. We quantified among-and within-population variation for flowering time and for several field life-history traits. Individuals were genotyped with microsatellites, single nucleotide polymorphisms and four wellknown flowering genes (FRI, FLC, CRY2 and PHYC). Phenotypic and genotypic data were used to conduct Q ST -F ST comparisons.Life-history traits varied significantly among-and within-populations. Flowering time also showed substantial within-and among-population variation as well as significant genotype 9 environment interactions among the various conditions. Individuals bearing FRI truncations exhibited reduced recruitment in field conditions and differential flowering time behavior across experimental conditions, suggesting that FRI contributes to the observed significant genotype 9 environment interactions. Flowering time estimated in field conditions was the only trait showing significantly higher quantitative genetic differentiation than neutral genetic differentiation values.Overall, our results show that these A. thaliana populations are genetically more differentiated for flowering time than for neutral markers, suggesting that flowering time is likely to be under divergent selection.

Currently, there exists a limited knowledge on the extent of temporal variation in population genetic parameters of natural populations. Here, we study the extent of temporal variation in population genetics by genotyping 151 genome-wide SNP markers polymorphic in 466 individuals collected from nine populations of the annual plant Arabidopsis thaliana during 4 years. Populations are located along an altitudinal climatic gradient from Mediterranean to subalpine environments in NE Spain, which has been shown to influence key demographic attributes and life cycle adaptations. Genetically, A. thaliana populations were more variable across space than over time. Common multilocus genotypes were detected several years in the same population, whereas low-frequency multilocus genotypes appeared only 1 year. High-elevation populations were genetically poorer and more variable over time than low-elevation populations, which might be caused by a higher overall demographic instability at higher altitudes. Estimated effective population sizes were low but also showed a significant decreasing trend with increasing altitude, suggesting a deeper impact of genetic drift at high-elevation populations. In comparison with single-year samplings, repeated genotyping over time captured substantially higher amount of genetic variation contained in A. thaliana populations. Furthermore, repeated genotyping of populations provided novel information on the genetic properties of A. thaliana populations and allowed hypothesizing on their underlying mechanisms. Therefore, including temporal genotyping programmes into traditional population genetic studies can significantly increase our understanding of the dynamics of natural populations.

We assessed the allelopathic effects of the aqueous extract of Sonchus oleraceus dry shoots on the germination and seedling growth of Trifolium alexandrinum, three weed species (Brassica nigra, Chenopodium murale and Melilotus indicus) and S. oleraceus itself. We assayed four different concentrations of the aqueous extract (w v −1 ): 1%, 2%, 3% and 4%. To determine whether the effects of the extract were attributable to the presence of allelopathic compounds, its osmotic potential or both, we prepared concentrations of polyethylene glycol (PEG) with osmotic potentials equivalent to those of the aqueous extract. All concentrations of the plant extract completely inhibited the germination and seedling growth of C. murale. The lowest concentration of the plant extract partially inhibited germination and seedling growth of B. nigra, M. indicus and S. oleraceus, whereas the higher concentrations inhibited those parameters completely. The germination of T. alexandrinum was not affected by the aqueous extract at 1% or 2%. In general, the aqueous extracts were more effective in inhibiting seed germination and seedling growth than were the PEG solutions. Phytochemical analyses revealed that phenols and alkaloids were the most abundant compounds in S. oleraceus dry matter. Our results suggest that the aqueous extract of S. oleraceus has an allelopathic effect on some weeds, and its usefulness as a bioherbicide therefore merits further study.

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The aim of this study was to identify the main weed communities in Al-Jouf province in northern Saudi Arabia. Moreover, the composition and diversity of these communities were studied in relation to soil variables and crop type. Some 54 stands representing olive orchards, date palm orchards, wheat crop and watermelon crop were studied, using ten quadrats (1 × 1 m) per stand. A total of 71 species belonging to 22 families and 61 genera were observed. The classification of vegetation using the Two Way Indicator Species Analysis (TWINSPAN) resulted in the recognition of four vegetation groups representing wheat crop, orchards in winter season, orchards in summer season and watermelon crop. These results suggested the importance of both crop and season for the formation of weed community. Detrended Correspondence Analysis (DCA) showed that these groups are clearly distinguished by the first two DCA axes. The species richness was higher in both olive and date palm orchards than in wheat and watermelon crops. This pattern of species richness could be related to farm management practices and habitat micro-heterogeneity. Soil electrical conductivity, organic carbon and soil texture showed significant correlations with species richness and the cover values of some dominant species, suggesting the significant role of soil characteristics in weed community structure and diversity.

-Laboratory and greenhouse experiments were conducted to evaluate the phytotoxic effect of black mustard extracts and root exudates on two crops: Trifolium alexandrinum and Triticum aestivum, and two weeds: Phalaris paradoxa and Sisymbrium irio. The seeds were treated with aqueous and ethanolic extracts and chloroform for eight days, or subjected to root exudates of just harvested mustard in a greenhouse for five weeks. High-performance liquid chromatography (HPLC) was used to quantify phytotoxins from plant tissues. Seed germination of P. paradoxa was reduced with the lowest concentration of the different extracts. However, the aqueous extract at 4% completely curtailed the germination of all the target species. In general, plant extracts had a concentration-dependent reduction of seedling growth of the target species. However, the ethanolic extract, at the lowest concentration, has stimulated the shoot length of both T. alexandrinum and T. aestivum, and the root length of the former. Mustard root exudates inhibited emergence and growth of the target species throughout the experiment. Ferulic and syringic acids were the dominant allelochemicals found when HPLC was used.Keywords: allelopathy, Brassica nigra, plant extracts, isothiocyanates (ITCs), phenolic acids.RESUMO -Experimentos de laboratório e estufa foram realizados para avaliar o efeito fitotóxico dos extratos de mostarda-preta e exsudatos de raiz de duas culturas: Trifolium alexandrinum e Triticum aestivum, bem como de duas plantas daninhas: Phalaris paradoxa e Sisymbrium irio. As sementes foram tratadas com extratos aquosos, etanólicos e clorofórmio por oito dias, ou submetidas a exsudatos de raiz de mostarda recém-colhidaem estufa durante cinco semanas. A cromatografia líquida de alto desempenho (HPLC) foi usada para a quantificação de fitotoxinas a partir de tecidos de plantas. Sementes de P. paradoxa apresentam germinação reduzida com a menor concentração dos diferentes extratos. No entanto, o extrato aquoso a 4% restringiu completamente a germinação de todas as espécies-alvo. Em geral, os extratos de plantas tiveram uma redução, dependendo da concentração do crescimento de mudas das espécies-alvo. No entanto, o extrato etanólico, na concentração mais baixa, tem estimulado o comprimento dos ramos de T. alexandrinum e T. aestivum e o comprimento da raiz da primeira. Exsudatos de raiz de mostarda inibiram o surgimento e crescimento das espécies-alvo durante todo o experimento. Os ácidos ferúlico e siringico foram os aleloquímicos dominantes encontrados utilizando HPLC.Palavras-chave: alelopatia, Brassica nigra, extratos vegetais, isotiocianatos (ITCs), ácidos fenólicos.

The floristic composition and species diversity of the germinable soil seed bank were studied in three different habitats (desert salinized land, desert wadi, and reclaimed land) in the Eastern Desert of Egypt. Moreover, the degree of similarity between the seed bank and the above-ground vegetation was determined. The seed bank was studied in 40 stands representing the three habitats. Ten soil samples (each 25 × 20 cm and 5 cm depth) were randomly taken per stand. The seed bank was investigated by the seedling emergence method. Some 61 species belonging to 21 families and 54 genera were identified in the germinable seed bank. The recorded species include 43 annuals and 18 perennials. Ordination of stands by Detrended Correspondence Analysis (DCA) indicates that the stands of the three habitats are markedly distinguishable and show a clear pattern of segregation on the ordination planes. This indicates variations in the species composition among habitats. The results also demonstrate significant associations between the floristic composition of the seed bank and edaphic factors such as CaCO3, electrical conductivity, organic carbon and soil texture. The reclaimed land has the highest values of species richness, Shannon-index of diversity and the density of the germinable seed bank followed by the habitats of desert wadi and desert salinized land. Motyka's similarity index between the seed bank and the above-ground vegetation is significantly higher in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%).

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