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Notes: Datura ferox seeds do not germinate when they are incubated intact and in darkness, even after 24 months of dry storage. If allowed to absorb water vapour for 3–4 weeks at 20°C, they lose dormancy, germinating even when incubated in darkness. The breakage of dormancy is directly related to the vapour pressure of the atmosphere where the seeds are stored, and good aeration is necessary for the process to occur.If seeds absorb liquid water, the toss of dormancy is much less than that of seeds stored in a saturated atmosphere. A similar effect is observed when seeds are buried in a soil at field capacity. The results indicate that a certain degree of imbibition, under good gas-exchange conditions, is important for breaking dormancy.As a hypothesis, it is proposed that most of the population remains dormant when not enough O2 is available for (he imbibed seed. The low rate of O2 entry could be due to poor diffusion through the water-saturated seed coat (seeds which absorb liquid water) or to a low O2 level in the surrounding atmosphere (soil at field capacity). One of the possible roles of O2 could be to oxidize the inhibitor(s) present in these seeds. La rupture de la dormance des semences de Datura ferox considérée comme un effet de l'absorplion de l'eau.Les semences de Datura ferox ne germent pas quand elles sont mises au germoir intactes et à l'obscurité, même aprés 24 mois de conservation au sec. Si elles peuvent absorber de la vapeur d'eau pendant 3 à 4 semaines à 20°C, elles perdent leur dormance et germent meme & à l'obscurité. La rupture de la dormance est en relation directe avec la pression de la vapeurd'eau de l'atmosphére dans laquelle les semences sont conservées; une bonne aération est nécessaire pour que le processus se déclanche.Si les semences absorbent de l'eau à l'état liquide, la rupture de dormance est beaucoup moindre que celle de semences conscrvées dans une atmosphére saturée. Un effet analogue est observé lorsque les semences sont ententes dans un sol ayant atteint sa capacity au champ. Les résultats indiquent qu'un certain degriS d'imbibition, sous de bonnes conditions d'changes gazeux. est important pour la rupture de la dormance.Comme hypothtése, il est proposé que la plus grande partie de la population reste dormante lorsqu'il n'y a pas assez d'oxygene disponible pour la semence imbibeée. Le faible taux d'absorption d'oxygéne pourrait étre du à sa fuible diffusion à travers l'enveloppe saturée d'eau de la semence (pour celles qui absorbent l'eau à I état liquide) ou à un bas niveau d'oxygene dans Tatmosphére environnante (cas d'un sol à la capacityé au champ). Un des rôles possibles de l'oxygéne pourrait être d'oxyder les inhibiteurs presents dans les semences. Das Brechen der Keimruhe in Samen von Datura ferox ah ein Effekt der Wasseraiifnahme Samen von Datura ferox keimen nicht, wenn sie bei Dunkelheit gehalten werden; auch nicht, wenn sie zuvor 24 Monate trocken lagerten. Wenn sie bei 20°C, 3 bis 4 Wochen tang Wasserdampf aufnehmen, verlieren sie die Keimhemmung und keimen sogar im Dunketn. Zwischcn dem Brechen der Keimruhe und dem Damptdruck der Luft in der sie gelagert werden, besteht eine direkte Beziehung. Fur diesen Vorgang ist eine gute BelCiflung Vorausselzung.Wenn die Samen flijssiges Wasser aufnehmen, ist der Verlust der Keimhemmung wesentlich geringer als wenn sie in einer gesättigten Atmosphäre gelagert werden. Eine ähnliche Wirkung wird beobachtet. wenn die Samen im Boden bei Feldkapazität gelagert werden. Die Ergebnisse zeigen, dass für das Brechen der Keimruhe ein bestimmtes Mass der Aufnahme bei ausreichendem Gasaustausch wichtig ist.Als cine Hypothese wird vorgeschlagen, dass die meisten der Population dormant bleiben, wenn für die mil Wasser vollgesaugten Samen nicht ausreichend O2 verfügbar ist. Der geringe Zutritt an O2 konnte auf die mässige Diffusion durch die wassergesättigte Samenschale (Samen die fliissiges Wasser aufnehmen), oder auf einen geringen O2-Gehalt in der umgebenden Luft (Boden bei Feldkapazität) zurückzuführen sein. Eine der möglichen Bedeutungen die dem O2 zukommen, könnte in der Oxidation des bzw. der Hemmstoffe der Samen liegen.

Notes: The aroA gene has been identified as a target in screening for the presence of most Aeromonas species so far described by PCR. Synthetic oligonucleotide primers of 24 and 25 nucleotides were used by PCR assay to amplify a sequence of the aroA gene, which encodes 3-phosphoshikimate-1-carboxyvinyltransferase, a key enzyme of aromatic amino acids and folate biosynthetic pathway. A 1236-bp DNA fragment, representing most of the aroA gene, according to the nucleotide sequence of A. salmonicida, was amplified from all Aeromonas species tested, which represented most of the 14 hybridization groups. HaeII digestion of the 1236-bp fragment generated a restriction fragment length polymorphisms which could be used as a powerful tool for identification of aeromonads to the genus level.

Notes: Abstract We studied the responses in growth and N content of the perennial grass Paspalum dilatatum to the substitution of Lotus tenuis for a whole group of species, the dicotyledons of a natural grassland community, in the Salado lowland Pampas of Argentina. Two kinds of manipulations were performed in the field: removal of alien dicots with herbicide application, and introduction of L. tenuis, resulting in a combination of four treatments, arranged in a 2 × 2 factorial randomized block design. Leaf area per tiller of P. dilatatum was higher when it was growing near L. tenuis; this increase was the result of a greater leaf elongation rate and slower leaf senescence. In the vicinity of L. tenuis, P. dilatatum exhibited an increase in tiller production and a decrease in tiller death. More tillers were functional at the end of the growing season and their aboveground biomass was 5␣times higher than for plants growing in plots where the community dicots were removed. This increment was accompanied by a higher N content. Growth enhancement of P. dilatatum plants when L. tenuis was the␣immediate neighbour is interpreted as the result of facilitation mediated by higher N availability, and not as a consequence of a release from competition exerted by the community dicots. Competition and facilitation did not interact to produce an increase in the vegetative output of Paspalum dilatatum plants growing under these field conditions. It is on these grounds that Lotus tenuis might be considered as a keystone species in the managed grassland.

Notes: Abstract Strong fluctuations are exhibited by populations of the perennial herb Ambrosia tenuifolia in the grasslands of the Salado basin (Province of Buenos Aires, Argentina), an area frequently enduring prolonged floods. Flooding causes the death of most dicotyledon plants of the community, A. tenuifolia among them, opening numerous gaps of various sizes. After the recession of the flood the density of A. tenuifolia seedlings was higher in flooded than in non-flooded plots and it was larger in wider gaps. Canopy removal in non-flooded plots increased field seedling emergence of A. tenuifolia up to the levels found in flooded plots. Responses of the seeds in the soil to gap-associated environmental factors such as light quality and temperature regime were studied both in the field and under controlled were studied both in the field and under controlled conditions. Seedling emergence was significantly enhanced when the red:far-red ratio of natural light reaching the soil surface under the canopy of nonflooded plots was increased by means of copper sulfate filters. The influence of light quality and temperature on germination of the soil seed population was also tested using grassland soil monoliths or mesocosms, transported from the field to the laboratory, in which the canopy was clipped and the soil exposed to either red or far-red light and kept at constant or fluctuating temperatures. Significant seedling emergence was observed only when the soil samples were exposed to red light and incubated at alternating temperatures. No emergence was recorded in samples exposed to far-red light or incubated at a constant 25°C. Seeds stored dry in the laboratory were also stimulated to germinate by red light and alternating temperatures but only after dormancy was sufficiently decreased by low temperature stratification or by low temperature under immersion. The results are consistent with the hypothesis that primary dormancy of A. tenuifolia seeds is decreased by low temperatures in winter even if the seeds are submerged as happens when floods occur. The decrease in dormancy makes the seeds prone to be stimulated to germinate by the Pfr form of phytochrome in combination with alternating temperatures. These conditions are likely to be met in the gaps opened by the flood-caused death of dicotyledon plants.

Notes: Abstract The vegetation in the Coironal arid steppe consists of grasses and shrubs. The objective of this paper was to test Walter's hypothesis that woody vegetation and grasses compete for water in the upper layers of the soil, but woody vegetation has exclusive access to a source of water at deeper levels. Analysis of root profiles and patterns of leaf and soil water potential led us to accept the hypothesis for this arid steppe. Additional information on phenology and on the ability of the major grass species to respond to watering permitted to identify two ecological strategies corresponding to grasses and shrubs. Grasses behave as opportunists having always leaves ready to grow as soon as water becomes available. They have a shallow root system and are able to respond very rapidly to increases in soil water availability. In contrast, woody species have a clear-cut periodic pattern of growth and dormancy. They possess thick horizontal roots running below 35–40 cm and utilized water stored in lower layers of the soil. A diagrammatic model summarizes the role of periodic and opportunistic species upon water circulation in the ecosystem. The effect of changes in the proportion of the two groups upon water dynamics is also discussed.

Notes: Abstract Two patch types were recognized in the Occidental District of the Patagonian arid steppe: i) shrubs encircled by a ring of tussock grasses, and ii) tracts of scattered tussocks. Completeness of the ring of grasses around the three dominant shrubs was a function of shrub size. Average completeness was 62, 71 and 83%, respectively for the three dominant shrubs (Senecio filaginoides, Mulinum spinosum and Adesmia campestris). A model for the cyclic dynamics of the two patch types was proposed. It includes a building phase (grass ring construction), a mature phase (maximum ring completeness) and a degenerate phase. In this last phase, triggered by shrub death, completeness of the ring progressively decreases until remnant grasses become undistinguishable from the scattered tussocks patch type. Ring formation occurred independently of shrub species. Grass species were differentially associated to the two patch types and to rings of different shrub species. Cyclical patch dynamics influenced the pattern of resource utilization, since the shrub-ring patch, with a share of only 18% of cover, contributed 44% of the total primary productivity.