826. HALESIA MACGREGORII

Halesia macgregorii, a little‐known Chinese member of the familiar American genus, is illustrated. Its phylogeny is discussed, particularly in view of the conflicting DNA and morphological evidence. The life and work of the Revd. Stephen Hales, after whom the genus is named, is briefly considered.     

842. ACER MICRANTHUM

Acer micranthum, a small‐leaved maple from the mountains of Japan, is illustrated. Its position in the genus and cultivation are discussed.     

Species interactions at the level of fine roots in the field: influence of soil nutrient heterogeneity and plant size

Interference at the level of fine roots in the field was studied by detailed examination of fine root distribution in small soil patches. To capture roots as they occur in natural three-dimensional soil space, we used a freezing and slicing technique for microscale root mapping. The location of individual roots intersecting a sliced soil core surface was digitized and the identity of shrub and grass roots was established by a chemical technique. Soil patches were created midway between the shrub, Artemisia tridentata, and one of two tussock grasses, Pseudoroegneria spicata or Agropyron desertorum. Some soil patches were enriched with nutrients and others given only deionized water (control); in addition, patches were located between plants of different size combination (large shrubs with small tussock grasses and small shrubs with large tussock grasses). The abundance of shrub and grass roots sharing soil patches and the inter-root distances of individual fine roots were measured. Total average rooting density in patches varied among these different treatment combinations by only a factor of 2, but the proportion of shrub and grass roots in the patches varied sixfold. For the shrub, the species of grass roots sharing the patches had a pronounced influence on shrub root density; shrub roots were more abundant if the patch was shared with Pseudoroegneria roots than if shared with Agropyron roots. The relative size of plants whose roots shared the soil patches also influenced the proportion of shrub and grass roots; larger plants were able to place more roots in the patches than were the smaller plants. In the nutrient-enriched patches, these influences of grass species and size combination were amplified. At the millimeter- to centimeter-scale within patches, shrub and grass roots tended to segregate, i.e., avoid each other, based on nearest-neighbor distances. At this scale, there was no indication that the species-specific interactions were the result of resource competition, since there were no obvious patterns between the proportion of shrub and grass roots of the two species combinations with microsite nutrient concentrations. Other potential mechanisms are discussed. Interference at the fine-root level, and its species-specific character, is likely an influential component of competitive success, but one that is not easily assessed.     

A series of bisaryl imidazolidin-2-ones has shown to be selective and orally active 5-HT2C receptor antagonists

Bisaryl cyclic ureas have been identified as high affinity 5-HT2C receptor antagonists with selectivity over 5-HT2A and 5-HT2B. Compounds such as 8 and 22 have shown oral activity in a centrally mediated pharmacodynamic model of 5-HT2C function in rodents.     

Phytohormones in Seedlings of Maize Hybrids Differing in Their Tolerance to High Temperatures

The contents of phytohormones (IAA, ABA, cytokinins, and gibberellin-like compounds) were measured in shoots and roots of eight-day-old seedlings of two maize (Zea mays L.) hybrids differing in their tolerance to elevated temperatures. More tolerant seedlings initially contained more ABA and cytokinins, and the contents of these hormones changed less after a temperature increase than in seedlings of the sensitive hybrid. Hyperthermia induced a destruction of chloroplast lamellar structure in the leaf sheath cells of the sensitive but not of the tolerant hybrid.     

A chemically explicit model for the mechanism of proton pumping in heme–copper oxidases

A mechanism for proton pumping is described that is based on chemiosmotic principles and the detailed molecular structures now available for cytochrome oxidases. The importance of conserved water positions and a step-wise gated process of proton translocation is emphasized, where discrete electron transfer events are coupled to proton uptake and expulsion. The trajectory of each pumped proton is the same for all four substrate electrons. An essential role for the His-Tyr cross-linked species is discussed, in gating of the D- and K-channels and as an acceptor/donor of electrons and protons at the binuclear center.     

TROPAEOLUM UMBELLATUM: IS IT EXTINCT IN THE WILD?

The brief history of Tropaeolum umbellatum Hooker in cultivation is described, and the original illustration ( Fig. 1 ) is republished. Only three collections of this very distinct species appear to have been made in the wild, all in Ecuador around 1846, and no further records are noted either in the literature, or on the internet.

Figure 1 Open in figure viewer PowerPoint Tropaeolum tuberosum. From a hand‐coloured lithograph by W. H. Fitch in Curtis's Botanical Magazine 74, t. 4337(1848).     

Inorganic N turnover and availability in annual- and perennial-dominated soils in a northern Utah shrub-steppe ecosystem

The exotic annual grass Bromus tectorum has replaced thousands of hectares of native perennial vegetation in semi-arid ecosystems of the western United States. Inorganic N availability and production were compared in soil from monodominant patches of Bromus tectorum, the perennial bunchgrass Elymus elymoides, and the shrub Artemisia tridentata, in Curlew Valley, a salt-desert shrub site in Northern Utah. Bromus-dominated soil had greater %N in the top 10 cm than Artemisia or Elymus-dominated soils. As determined by spring isotope-dilution assays, gross mineralization and nitrification rates were higher in Bromus-dominated than Artemisia-dominated soils, but gross rates of NH₄ ⁺ and NO₃ ^– consumption were also higher. Litterbags had greater mass loss and N mineralization when buried in Bromus stands than in Artemisia stands, indicating the soil environment under the annual grass promotes decomposition. As determined by nitrification potential assays, nitrifier populations were higher under Bromus than under Artemisia and Elymus. Soil inorganic N concentrations were similar among vegetation types in the spring, but NO₃ ^– accumulated under Bromus once it had senesced. An in situ net mineralization assay conducted in autumn indicated that germinating Bromus seedlings are a strong sink for soil NO₃ ^–, and that net nitrification is inherently low in soils under Artemisia and Elymus. Results of the study suggest that differences in plant uptake and the soil environment promote greater inorganic N availability under Bromus than under perennial species at the site.