Dehiscence of Fruit in Oilseed Rape (Brassica napus L.): I. ANATOMY OF POD DEHISCENCE

The development sequence of anatomical changes taking placewithin the pericarp tissues of Brassica napus siliquae havebeen studied at a fine- and ultra-structural level. Tissue differentiationoccurred during the initial 20 d after anthesis (DAA) and allowedthe identification of dehiscence zone cells. This descrete tissuewas subsequently further delineated by extensive lignificationof adjacent valve edge and replar vascular cells. Concomitantwith the onset of pericarp lignification, cytoplasmic contentsof the thin-walled dehiscence zone cells exhibited progressivesenescence and degradation. Wall breakdown, initially evidentin pods by 60 DAA, exclusively affected cells within the dehiscencezone, and eventually extended throughout this tissue from theepidermal suture to the locule, thus precipitating valve detachment.Ultrastructural examination confirmed that this loss of cellularcohesion was primarily attributable to middle lamella degradationand, furthermore, the dissolution of wall material was apparentlydependent on rupture of the dehiscence zone protoplast. Thesignificance of dehiscence zone cell modifications in relationto autolytic cell wall breakdown, together with possible implicationsfor the regulation of pod shatter, are discussed. Key words: Oilseed rape, Brassica napus, pod shatter, dehiscence zone, cell wall breakdown     

Responses of periphyton to changes in current velocity, suspended sediment and phosphorus concentration

SUMMARY.

• 1 Research was performed in laboratory streams to evaluate periphytic biomass accrual, export, and community composition over a range of limiting nutrient (phosphorus) concentrations with variable velocity, and suspended sediment addition, in comparison to constant velocity and no suspended sediment. In fixed-velocity treatments, velocity increase to 60 cm s^?1 significantly enhanced biomass accrual, but further increase resulted in substantial biomass reduction. Average biomass loss rates did not change significantly over a velocity range of 10–80 cm s^?1. Diatoms were favoured at relatively high velocities and low phosphorus concentrations, whereas the blue-green Phormidium tended to dominate at higher SRP concentrations and the green Mougeotia seemed to prefer lower velocities.
• 2 Sudden increases in velocity raised instantaneous loss rates by an order of magnitude or more, but these high rates persisted only briefly. As a result, marked biomass reductions were not apparent a day after the velocity change. Dominance change from filamentous green or blue-green to diatoms immediately after the increase was reversed within 2 days. Loss rate increases due to solids addition were much smaller than those accompanying velocity increase, but simultaneous velocity elevation and solids addition produced instantaneous loss rates approximately double those with velocity increase alone.
• 3 The experiments demonstrated that an elevation in velocity, above that to which algae were accustomed, led to increased loss rates and temporarily reduced biomass. However, recolonization and growth after biomass reduction were apparently rapid. Substantial export of periphyton following solids addition required erosion of the protective boundary layer accompanied by a velocity increase. These results arc applicable to understanding the response of lotic periphytic algae to elevated, turbid storm discharges and similar runoff or high-flow events.
• 4 Areal uptake rates of P by algae growing in the laboratory streams increased with soluble reactive phosphorus (SRP) concentration, up to approximately 15 μg I^?1 in overlying water. They also increased above 35 cm s ^?1. Overall, uptake rate seemed to vary inversely with biomass. The ralio of areal uptake rate/biomass was significantly less where mean biomass was 411±6 mg chl a m^?2 compared to 223±17 mg chl a m^?2.
• 5 The results suggested that although nutrient uptake is primarily a surface phenomenon, diffusion to interior cells can also determine the responses of attached communities. Both diffusion and uptake rate were stimulated by increasing nutrient concentration and velocity up to certain levels, but became limited by biofilm thickness and scouring.

     

Probable predatory borings in late Cretaceous bryozoans

Borings are described in zooids of Cenomanian to Campanian melicerititids, an aberrant group of cyclostome bryozoans with calcified opercula. The borings are circular or elliptical, straight-sided and have a diameter of 40–90 μn. Autozooids were drilled in preference to heterozooids and most borings penetrate the operculum. The shape and distribution of boreholes suggests that they were made by a predator attacking one zooid at a time. The predator responsible may have been a gastropod, most probably a nudibranch or a micromorphic muricid. Bryozoa, Gastropoda, predation, borings, Upper Cretaceous.     

The toxicity of tributyltin oxide to the wood-boring beetles Lyctus brunneus Steph. and Anobium punctatum (Deg.)

When tributyltin oxide (TBTO) was tested by British Standard 3653 against Lyctus brunneus, 0·5 % solution, equivalent to 0·72 g/m₂ surface application, gave almost complete protection. Tests to BS 3651, using larvae of Anobium punctatum transferred into fully impregnated wood gave toxic limits of 1·39–2·93 kg/m³ TBTO when benzene was used as diluent. The use of white spirit as diluent gave lower toxic limits, probably due to combined toxic action of TBTO and white spirit residues. TBTO appears to have no contact action against the insects tested. Probit analysis of the test with A. punctatum gave an LD 50 of 0·254 kg/m³ and an LD99 of 3 kg/m³. The advantage of treating results by probit analysis rather than by deriving ‘toxic limits’ is discussed.     

Production and fitness of Fusarium pseudograminearum inoculum at elevated carbon dioxide in FACE

Rising atmospheric carbon dioxide (CO₂) concentration is increasingly affecting food production but how plant diseases will influence production and quality of food under rising CO₂ is not well understood. With increased plant biomass at high CO₂ the stubble‐borne fungal pathogen Fusarium pseudograminearum causing crown rot (CR) of wheat may become more severe. We have studied inoculum production by Fusarium using fungal biomass per unit wheat stubble, stem browning from CR and the saprophytic fitness of Fusarium strains isolated from two wheat varieties grown in 2007 and 2008 at ambient and elevated CO₂ in free‐air CO₂ enrichment (FACE) with or without irrigation and once in a controlled environment. Fungal biomass, determined using primers for fungal ribosomal 18s and the TRI5 gene, increased significantly at elevated CO₂ in two of the three studies. Stem browning increased significantly at elevated CO₂ in the 2007 FACE study. At elevated CO₂ increased stem browning was not influenced by irrigation in a susceptible variety but in a resistant variety stem browning increased by 68% without irrigation. Wheat variety was significant in regression models explaining stem browning and Fusarium biomass but pathogen biomass at the two CO₂ levels was not significantly linked to stem browning. Fusarium isolates from ambient and elevated CO₂ did not differ significantly in their saprophytic fitness measured by the rate of colonization of wheat straw. We show that under elevated CO₂Fusarium inoculum in stubbles will be amplified from increased crop and pathogen biomass while unimpeded saprophytic fitness will retain its effectiveness. If resistant varieties cannot completely stop infection, Fusarium will rapidly colonize stubble to further increase inoculum once the crop is harvested. Research should move beyond documenting the influence of elevated CO₂ to developing disease management strategies from improved knowledge of pathogen biology and host resistance under rising CO₂.     

Forage Enhancement Plots as a Management Tool for Sonoran Pronghorn Recovery

ABSTRACT We studied the efficacy of forage enhancement plots for Sonoran pronghorn (Antilocapra americana sonoriensis) during a drought in a large (130-ha) enclosure. As drought conditions continued, Sonoran pronghorn increased percent of time foraging in forage enhancement plots from 5.3±2.2% (SE) to 47.8±2.8% but showed signs of poor body condition and one died of starvation. Our results confirm the utility of forage enhancement plots for Sonoran pronghorn recovery, but suggest future research be conducted to determine the optimal size and watering regime of plots to increase survival and recruitment during prolonged droughts.