Experiments on soil drenching with fungicides against take-all in wheat

In short term pot experiments benomyl, iprodione and KWG 0599 applied as soil drenches in several types of soil significantly suppressed take-all symptoms from inoculum placed just below wheat seeds planted 1×5 cm deep, and in sand but not other soils when seeds were 5 cm deep. Benomyl was, however, effective against inoculum below seed planted 5 cm deep in a loam-sand mixture when the drench contained an alcohol ethoxylate surfactant. Computer simulations of fungicide distributions in the soils correlated well with disease control observations. In long term outdoor pot experiments two drenches with benomyl (without surfactant) controlled disease significantly for at least 3 months against inoculum placed 15 cm deep. The significance of these results for the practical control of take-all by fungicides is discussed.     

Nuclear Receptors in Mosquito Vitellogenesis

Vitellogenesis in insects involves the coordinated activityof the fat body, which produces large amounts of yolk proteinprecursors (YP), and oocytes, which specifically accumulatethese proteins. The expression of YP genes is achieved throughstrict sex-, tissue-, and hormone-specific control in the femalefat body. In mosquitoes, expression of YP genes is controlledby 20-hydroxyecdysone (20E). To elucidate the role of 20E inmosquito vitellogenesis, we cloned cDNAs encoding the Aedesaegypti ecdysteroid receptor (AaEcR) and two isoforms of itsheterodimeric partner, the Ultraspiracle homologue (AaUSP).The two AaUSP isoforms differ in their A/B domains and havedistinct expression patterns. The ecdysone regulation of YPgenes likely involves products of early genes. We cloned thegene of the mosquito homologue to the Drosophila early geneE75 (AaE75) belonging to the nuclear receptor superfamily. Kineticsof AaE75 expression correlate with the expression of YP genes,suggesting that AaE75 may have a regulatory role in YP geneexpression. A second nuclear receptor superfamily member, theNGFI-B homologue AaHR38 is implicated in repression of the ecdysone-signalingpathway in the fat body of the previtellogenic female mosquitoat the state-of-arrest. Finally, three isoforms of the hepatocytenuclear factor 4 (HNF-4) homologue AaHNF-4 are differentiallyexpressed in the mosquito fat body during vitellogenesis, suggestingtheir involvement in regulating vitellogenic events in thistissue.     

A comparison between two models for plant competition

The differential equation of de Wit (1970) which predicts the growth of plant species in mixtures from their yield–density response in monocultures is compared with a simpler equation. This model is also based on the yield–density response of species grown in monocultures, but is static and predicts only the net result of plant competition during discrete time periods. Data from successive harvests in yield–density experiments in monocultures of one grass and two legume species were fitted to both models and the relative reproductive rate and the relative replacement rate were calculated from model predictions. Predictions from our model agreed more closely with observations than predictions from the de Wit model. It is concluded that our model deserves further attention in the studies of population dynamics in mixtures when the final outcome of competition is the main interest.     

Gene flow of Acanthaster planci (L.) in relation to ocean currents revealed by microsatellite analysis

Population outbreaks of the coral-eating starfish, Acanthaster planci , are hypothesized to spread to many localities in the Indo-Pacific Ocean through dispersal of planktonic larvae. To elucidate the gene flow of A. planci across the Indo-Pacific in relation to ocean currents and to test the larval dispersal hypothesis, the genetic structure among 23 samples over the Indo-Pacific was analysed using seven highly polymorphic microsatellite loci. The F -statistics and genetic admixture analysis detected genetically distinct groups in accordance with ocean current systems, that is, the Southeast African group (Kenya and Mayotte), the Northwestern Pacific group (the Philippines and Japan), Palau, the North Central Pacific group (Majuro and Pohnpei), the Great Barrier Reef, Fiji, and French Polynesia, with a large genetic break between the Indian and Pacific Oceans. A pattern of significant isolation by distance was observed among all samples ( P =  0.001, r  = 0.88, n  = 253, Mantel test), indicating restricted gene flow among the samples in accordance with geographical distances. The data also indicated strong gene flow within the Southeast African, Northwestern Pacific, and Great Barrier Reef groups. These results suggest that the western boundary currents have strong influence on gene flow of this species and may trigger secondary outbreaks.     

Carbon Dioxide Effects on Carbohydrate Status and Partitioning in Rice

The atmospheric carbon dioxide (CO₂) concentration has beenrising and is predicted to reach double the present concentrationsometime during the next century. The objective of this investigationwas to determine the long-term effects of different CO₂ concentrationson carbohydrate status and partitioning in rice (Oryza sativaL cv. IR-30). Rice plants were grown season-long in outdoor,naturally sunlit, environmentally controlled growth chamberswith CO₂ concentrations of 160, 250, 330, 500, 660, and 900µmolCO₂ mol¹ air. In leaf blades, the priority between the partitioningof carbon into storage carbohydrates or into export changedwith developmental stage and CO₂ concentration. During vegetativegrowth, leaf sucrose and starch concentrations increased withincreasing CO₂ concentration but tended to level off above 500µmolmol^–1 CO₂. Similarly, photosynthesis also increased withCO2 concentrations up to 500µmol mol^–1 and thenreached a plateau at higher concentrations. The ratio of starchto sucrose concentration was positively correlated with theCO₂ concentration. At maturity, increasing CO₂ concentrationresulted in an increase in total non-structural carbohydrate(TNC) concentration in leaf blades, leaf sheaths and culms.Carbohydrates that were stored in vegetative plant parts beforeheading made a smaller contribution to grain dry weight at CO₂concentrations below 330µmol mol^–1 than for treatmentsat concentrations above ambient Increasing CO₂ concentrationhad no effect on the carbohydrate concentration in the grainat maturity Key words: CO₂ enrichment, starch, sucrose