Effects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating “islands” of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.     

Statistics of RNA melting kinetics

We present and study the behavior of a simple kinetic model for the melting of RNA secondary structures, given that those structures are known. The model is then used as a map that. assigns structure dependent overall rate constants of melting (or refolding) to a sequence. This induces a landscape of reaction rates, or activation energies, over the space of sequences with fixed length. We study the distribution and the correlation structure of these activation energies. Correspondence to: P. Schuster     

Strange limits of stability in host-parasitoid systems

The classical Nicholson-Bailey model for a two species host-parasitoid system with discrete generations assumes random distributions of both hosts and parasitoids, randomly searching parasitoids, and random encounters between the individuals of the two species. Although unstable, this model induced many investigations into more complex host-parasitoid systems. Local linearized stability analysis shows that equilibria of host parasitoid systems within the framework of a generalized Nicholson-Bailey model are generally unstable. Stability is only possible if host fertility does not exceede ⁴=54.5982 and if superparasitism is unsuccessful. This special situation has already been discovered by Hassell et al. (1983) in their study of the effects of variable sex ratios on host parasitoid dynamics. We discuss global behaviour of the Hassell-Waage-May model using KAM-theory and illustrate its sensitivity to small perturbations, which can give rise to radically different patterns of the population dynamics of interacting hosts and parasitoids.     

The unfolded-protein-response pathway in yeast

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers the increased production of several ER-resident proteins. This signalling pathway exists in organisms as divergent as mammals and yeast, and is the only known example of an intracellular signalling system that links the ER and the nucleus. Recently, a transmembrane kinase similar in structure to growth-factor receptor kinases has been identified as a key component of the unfolded-protein-response pathway in yeast.     

A potent specific inhibitor of 6-phosphogluconate dehydrogenase ofCryptococcus neoformans and of certain other fungal enzymes

A particular lot of the zwitterionic buffer, 2(N-morpholino) ethane sulfonic acid (MES), contained a contaminant that inhibited a number of fungal NADP-dependent dehydrogenases. Enzymes that were particularly sensitive include 6-phosphogluconate dehydrogenases fromCryptococcus neoformans andSchizophyllum commune and glucose-6-phosphate dehydrogenase fromSchizophyllum commune. A number of NADP-dependent dehydrogenases of animal origin were tested and all were completely insensitive to inhibition except for rat liver 6-phosphogluconate dehydrogenase, which was 10-fold less sensitive than theCryptococcal enzyme. The pattern of inhibition in all cases was linear competitive versus NADP. The inhibitor has been purified and identified as an ethylenesulfonic acid oligomer. This inhibitor holds promise as a model compound for the development of a specific antifungal agent.     

Zur Kenntnis des Monokotyledonen-Perigons II. Die Perigonblätter vonOrnithoglossum

Zusammenfassung Die Untersuchung der Perigonblätter der südafrikanischen Liliaceen-Melanthioideen-GattungOrnithoglossum an Hand von Herbarmaterial hat ergeben, daß sie, und zwar in beiden Kreisen, gemäß ihrer äußeren Form, Bündelversorgung und Ontogenese als peltat-schlauchförmige Blätter zu bezeichnen sind. Damit ist die morphologisch bedeutsame peltate Bauweise im Perianth einer weiteren Monokotyledonen-Gattung nachgewiesen worden.Mit ihrer sitzenden Spreite und den an ihrem Schlauchteil herablaufenden akzessorischen Randkanten gehören die Perigonblätter vonOrnithoglossum jener Modifikation des Schlauchblattes an, die auch die Nektarblätter vonRanunculus verwirklichen. Bemerkenswerterweise wiederholen sich die verschiedenen Ausbildungsmöglichkeiten der Ventralspreite derRanunculus-Nektarblätter an den Perigonblättern vonOrnithoglossum: die taschenartige Ausbildung der Ventralspreite beiO. viride, das Auswachsen des Randes der Ventralspreite zur freien Nektarschuppe beiO. calcicolum und schließlich der Zusammenschluß des Ventralspreitenrandes durch einen akzessorischen apikalen Wulst zu einer in sich geschlossenen Kurve bei gewissen Formen vonO. viride und beiO. glaucum.     

Medianstipulierte Staubblätter

Zusammenfassung Es werden die sog. Ligulen an der Basis der Staubblätter vonReinwardtia, Simaruba, Zygophyllum und einigen Cruciferen-Gattungen (Alyssum, Aubrietia, Draba) beschrieben und auf ihren Stipularcharakter wird neuerlich hingewiesen. Der morphologische Vergleich mit einem diplophyllen und mit einer Medianstipel versehenen Stengelblatt vonBergenia zeigt deutlich, daß diese fraglichen Bildungen Medianstipeln darstellen. An verbildeten Kronblättern vonReinwardtia trigyna konnte überdies aufgezeigt werden, daß diese Bildung tatsächlich Ausgliederungen des echten Blattrandes darstellen.Es lassen sich demnach an günstig gebauten Staubblättern alle Teile eines typischen Blattes nachweisen: das Qberblatt, Anthere und Filament umfassend, und das Unter blatt, das gewöhnlich als Filamentbasis bezeichnet wird und deutlich in Blattgrund und Nebenblätter gegliedert ist. Diese Nebenblätter treten nun normalerweise in Gestalt einer Medianstipel auf, welche ihrerseits einen weiteren Beweis für den unifazialen Bau des darüber befindlichen Filamentabschnittes darstellt. Durch Hemmung des Querzonenwachstums kann die Medianstipel in laterale Stipeln zerteilt werden, die uns beiZygophyllum als freie Blättchen und bei den in Frage kommenden Cruciferen vielfach als Flügelungen des Filamentes entgegentreten. Das Vorhandensein und die Ausbildung dieser Nebenblätter läßt darüber hinaus erkennen, daß unter Umständen wesentliche Teile des Filamentes dem Unterblatt angehören.