Crossing abilities of ALS target-site resistant black-grass (Alopecurus myosuroides Huds.)

Black-grass crossings were conducted outdoor for two successive years in a way to determine the percentage of resistant black-grass that can be engendered in one generation, after the introduction of one target-site resistant individual. After germination, plants were genotyped and couples of resistant and susceptible individuals were formed, then planted outdoor and enclosed with a pollen-proof cloth to avoid any external pollen contamination. Seeds were collected and tested in glasshouses with herbicide spraying. DNA analyses were also performed on surviving plants to distinguish hetero- from homozygotes. The resistance level of the progeny seems to be fairly high and Mendelean distribution is generally confirmed. The offspring of self-crossings was also observed to assess the facultative autogamy ability, according to the genotype and the presence of mutant ALS alleles (position W-574).     

Seasonal Changes in the Cambium of Trees. I. Sucrose Content in Thuja occidentalis

Cambium samples of Thuja occidentalis L. were collected at five different times, covering spring reactivation and early and late resting period, and used for sucrose determinations. Fragments of the different cell types - xylem ray, cambial initials, sieve-elements including phloem parenchyma cells, phloem ray - were dissected from freeze-dried radial sections and analyzed individually. Results show large differences in sucrose concentrations in the different cell types of the cambial layer. In addition, each cell type also shows seasonal fluctuations in sucrose content, whose amplitudes and patterns of variation appear specific for the particular cell type.     

Diversity of Bacterial Communities in a Profile of a Winter Wheat Field: Known and Unknown Members

In soils, bacteria are very abundant and diverse. They are involved in various agro-ecosystem processes such as the nitrogen cycle, organic matter degradation, and soil formation. Yet, little is known about the distribution and composition of bacterial communities through the soil profile, particularly in agricultural soils, as most studies have focused only on topsoils or forest and grassland soils. In the present work, we have used bar-coded pyrosequencing analysis of the V3 region of the 16S rRNA gene to analyze bacterial diversity in a profile (depths 10, 25, and 45 cm) of a well-characterized field of winter wheat. Taxonomic assignment was carried out with the Ribosomal Database Project (RDP) Classifier program with three bootstrap scores: a main run at 0.80, a confirmation run at 0.99, and a run at 0 to gain information on the unknown bacteria. Our results show that biomass and bacterial quantity and diversity decreased greatly with depth. Depth also had an impact, in terms of relative sequence abundance, on 81 % of the most represented taxonomic ranks, notably the ranks Proteobacteria, Bacteroidetes, Actinobacteridae, and Acidobacteria. Bacterial community composition differed more strongly between the topsoil (10 and 25 cm) and subsoil (45 cm) than between levels in the topsoil, mainly because of shifts in the carbon, nitrogen, and potassium contents. The subsoil also contained more unknown bacteria, 53.96 % on the average, than did the topsoil, with 42.06 % at 10 cm and 45.59 % at 25 cm. Most of these unknown bacteria seem to belong to Deltaproteobacteria, Actinobacteria, Rhizobiales, and Acidobacteria.     

Intraguild interactions of aphidophagous predators in fields: effect of Coccinella septempunctata and Episyrphus balteatus occurrence on aphid infested plants

Intraguild relations between beneficial insects have become a major research topic in biological pest control. In order to understand the intraguild competitions between aphidophagous populations in natural conditions, a field experiment was carried out in the experimental farm of the Gembloux Agricultural University. As biological control of pests involve a community of diverse natural enemies, this experiment firstly aimed to assess the aphidophagous predator diversity and abundance in green pea (Pisum sativum) field and secondly to investigate the impact of the large natural occurrence of C. septempunctata on the aphidophagous beneficial dispersion and efficiency as aphid biological control agents in pea field. Visual observations were weekly performed throughout the 2006 growing season. The pea aphids were attacked by several predatory groups, mainly ladybird beetles and hoverflies. Higher densities of ladybirds and hoverflies were recorded in the beginning of July, associated with an aphid occurrence peak. Using net cage system in the field, the particular intraguild relations between added C. septempunctata or E. balteatus and the natural beneficial arrivals and dispersion were observed. The E. batteatus (eggs and larvae) presence inhibited other aphidophagous predators presence on the aphid infested plants. Lower abundance of E. balteatus was observed on aphid infested plants already colonised by C. septempunctata. To explore more accurately the oviposition and predation behaviours of ladybirds and hoverflies and to determine the chemical factors that could influence these behaviours, current researches are performed in laboratory and will be discussed to promote efficient biological control of aphids by natural enemies.     

Dynamic of black-grass populations depending on the sowing time of winter wheat

Currently, economic, agronomic and environmental concerns, lead to reduce use of herbicides. This reduction can be help by cultural measures like delay of the sowing date. Four sowing dates of winter wheat from 15th of October to 26th of November were tested. Dynamic of black-grass (Alopecurus myosuroides Huds.) populations and their reproduction rate were assessed as well as dynamic of winter wheat for each date. Delay of sowing could significantly reduce reproduction rate of black-grass. It was shown that the emergence rate (pl/m2), but also number of ears per plant and number of seeds per ear of black-grass decreased significantly with the sowing date. This reduction of seeds production already is from sixty per cent for a delay of two weeks sowing.     

Changes in adenine-nucleotide content in the apical bud of Sinapis alba L. during floral transition

The total adenylate pool of the apical buds of vegetative plants of Sinapis alba L. continuously grown in short days fluctuates over a 24-h cycle with the minimum occurring at the end of the dark period. In the buds of plants induced to flower by a single long-day treatment, total adenylate pool increases above the control level 16 h after the start of the long day, resulting mainly from a rise in ATP and ADP contents. This occurs 6 h after the increase in the soluble carbohydrate content previously shown to occur in the apical buds of plants induced to flower (Bodson 1977, Planta 135, 19–23). A transient rise of the energy charge occurs 22 h after the start of the inductive long day.Abbreviations LD long day - SD short day     

Partial floral evocation by exogenous cytokinin in the long-day plant Sinapis alba

A single application of benzyladenine (BA) at 4.3 × 10⁻⁵ M to the shoot apex of vegetative plants of Sinapis alba L. causes an increase of the import of [¹⁴C]-assimilates in the apical bud and of the starch content. BA also produces the breakdown of the vacuoles into smaller vacuolar units and an increase of the mitotic index leading to nearsynchronization of the meristem cell population in G₁. All these changes normally occur in the meristem of Sinapis plants during the transition to flowering (full evocation). Other events normally associated with full evocation are not caused by BA which thus produces only partial evocation. The part of evocation produced by BA is different from that caused by high irradiance.     

Promotion of Flowering in Brassica campestris L. cv Ceres by Sucrose

Flower initiation of the quantitative long-day plant Brassica campestris cv Ceres was earlier and at a lower final leaf number when sucrose was added to the medium in which plants were grown in sterile culture. The optimal concentration of sucrose was 40 to 80 millimolar. This flower-promoting effect of sucrose was not osmotic, as mannitol, sodium chloride, and polyethylene glycol were not effective at equal osmotic potentials.

Seedlings grown heterotrophically after treatment with 4-chloro-5-(dimethylamino)-2-phenyl-3-(2H)-pyridazinone to prevent chlorophyll accumulation were also induced to form flower primordia earlier as the sucrose concentration in the medium was increased up to 80 millimolar. Inclusion of 4 millimolar sodium nitrate in the culture medium of green plants did not reduce the flower-promoting effects of sucrose but delayed initiation in plants grown without added sucrose.

Removal of CO₂ during a single main or supplementary light period, or both, greatly reduced flower initiation. It is concluded that sucrose may be an important controlling factor determining floral initiation in Brassica.