Output and dispersal of propagules of Carduus acanthoides L

Propagules of Carduus acanthoides are pappus-bearing achenes. Flower production, potential achene output, seedbank and wind dispersal of propagules of Carduus acanthoides were analysed. Flower production was measured in four cohorts in 1986–87 and three cohorts the following reproductive season, when the potential achene output was estimated. Achene output was very large in all cases but early cohorts had a longer fruiting period and produced more flowers and fruits per plant. The seedbank was small and results from autumn and spring samples showed a low seed persistence in soil. In laboratory experiments greater landing and dispersion distances were reached at higher wind speeds, the major wind effect being that on the movement of propagules after landing. Barriers simulating vegetation almost completely stopped this movement after landing and the final distances reached were almost the same at all wind speeds. In field experiments achenes recovered in seed traps and seedling emergence show the same pattern of distribution around the parent plant. The implication of these results for the behaviour of thistle populations are discussed.     

Gap colonization by weeds in a wheat crop grown under different cultivation regimes

The effects of tillage systems (mouldboard plough, chisel, disk and non-tillage, over 3 years) and gap openings during August and October of 0, 0.30 m × 0.30 m and 0.45 m × 0.45 m on the weed community were investigated in a wheat (Triticumaestivum L.) crop. Weed flora composition and species abundance were recorded in August, September and November and the data analysed with MULVA programs and non-parametric tests. The tillage system was more important than the timing or gap size on the weed floristic composition, and the changes in weed communities were evident within a 3-year period in spite of herbicide use. Less disturbing tillage systems (non-tillage and chisel plough) allowed the build-up of a more diverse community, whereas the most disturbing one (mouldboard plough) prevented high diversity in the weed community.     

Oxygen isotope fractionation in reactions catalyzed by enzymes

A study has been made of some of the enzymatic mechanisms responsible for the previously reported fractionation of oxygen isotopes by whole organisms. The data indicate that the fractionation occurs through the activity of metalloenzymes.     

Specific immunologic unresponsiveness to synthetic polypeptide antigens

The injection of two synthetic polypeptide antigens into adult rabbits treated with 6-mercaptopurine, or into newborn rabbits, resulted in immunological "unresponsiveness" to subsequent immunizations with these antigens. The "tolerant" animals were shown to be reactive toward the non-related antigen ovalbumin.     

The effect of different tillage systems on the composition of the seedbank

The soil seedbank of a wheat crop grown with four tillage systems (mouldboard plough, disk, chisel and no-tillage) for 3 years was analysed. Density and composition of seedbanks varied according to tillage system and depth. The mouidboard plough crop had seedbanks with the lowest seed density and there was no difference in density or composition at 0-5 cm and 5-10 cm depth whereas no—tillage had a more dense seed bank, especially in the upper part of the soil profile. Diversity of seedbanks also increased from mouidboard plough, to disk, to chisel, to no-titlage, which had the most diverse bank. Therefore, the data strongly support the hypothesis that the systems causing less disturbance allow the build-up of a larger and more diverse soil seed bank.     

Survival and establishment of Carduus acanthoides L.

The fate and establishment of propagules of Carduus acanthoides L. (plumeless thistle) under different conditions were analysed. Placing a gauze (mesh width 0.5 × 0.3 cm) greatly reduced predation, suggesting that rodents and insects are the main predators, which drastically reduce the population of this species in open habitats. Litter and fine mesh diminished predation risk. Although propagules germinated under low light conditions, they failed to estabhsh under a closed canopy. Establishment and growth of thistles in gaps were affected by vegetation cover, presence of litter and gap size. Presence of litter had a positive effect on establishment and survival. In high vegetation cover sites, small gaps were rapidly colonized by surrounding vegetation, which hindered thistle germination and survival. The thistle regeneration niche is characterized and suggestions are given for pasture management in order to prevent its establishment. Survie et établissement de Carduus acanthoides L. Le devenir et l'établissement de propagules de Carduus acanthoides (chardon acanthin) ont étéétudiés sous différentes conditions. Une gaze de maille 0,5 × 0,3 cm réduisait beaucoup la prédation, suggérant qu'à l'exterieur, les rongeurs et les insectes sont les principaux responsables de la limitation des populations de cette espéce. Une litière ou une maille fine réduisaient la prédation. Bien que les propagules germaient sous faible luminosité, elles ne réussissaient pas à s'établir sous un couvert végétal ferme. L'établissement et la croissance des chardons dans des interruptions du couvert dépendaient de ce dernier, de la présence de litière et de la dimension des interruptions. Lorsque le couvert était important, les interruptions de faible dimension étaient rapidement colonisées par la végétation environnante, ce qui perturbait la germination et la survie des chardons. La niche de regeneration du chardon a été caractérisée et des suggestions sont données pour la gestion du pâturage, de manière à réduire son établissement. Fortpflanzung und Bestandesbildung bei Carduus acanthoides L. Das überleben und die Keimung von Diasporen von Carduus acanthoides L. wurden unter verschiedenen Bedingungen untersucht. Abdeckung mit einem dünnen Gewebe (Maschenweite 0.5 × 0.3 cm) senkte die Verluste erheblich, worin sich zeigt, daß Nager und Insekten hauptsächlich für die starke Verkleinerung von Beständen dieser Art verantwortlich sind. Obwohl die Samen bei geringem Licht keimten, konnten sie sich in einer geschlossenen Pflanzendecke nicht entwickeln. Ob sich die Distel in Vegetationslücken etablieren konnte, hing von der Pflanzennarbe, der Streu und der Lückenbröße ab. Streulagen waren für die Keimung und die Entwicklung der Pflanzen günstig. In hochwüchsigen Pflanzenbeständen wurden kleine Vegetationslücken von den vorhandenen Pflanzen schnell geschlossen, so daß die Distelsamen nicht keimen und die Keimpflanzen sich nicht entwickeln konnten. Das Regenerationsverhalten der Distel wird dargestellt, woraus Empfehlungen für die Pflege von Weiden abgeleitet werden, um die Ansiedlung dieser Pflanzenart zu verhindern.     

Mammalian cytochrome b

Digestion of a preparation of cytochromes b and c(1) with pancreatic protease followed by ammonium sulfate precipitation resulted in a soluble cytochrome b uncontaminated by cytochrome C(l). This preparation, which was free of succinic dehydrogenase and cytochrome oxidase activity, had an estimated deltaE(1 cm)(1g/ml) of 102 for its alpha-peak. In the reduced form absorption maxima were found at 560 to 562, 530 to 532, and 427 to 428 mmicro, and in the oxidized form, at 413 mmicro.