Early Forest Soils and Their Role in Devonian Global Change

A paleosol in the Middle Devonian Aztec Siltstone of Victoria Land, Antarctica, is the most ancient known soil of well-drained forest ecosystems. Clay enrichment and chemical weathering of subsurface horizons in this and other Devonian forested paleosols culminate a long-term increase initiated during the Silurian. From Silurian into Devonian time, red clayey calcareous paleosols show a greater volume of roots and a concomitant decline in the density of animal burrows. These trends parallel the decline in atmospheric carbon dioxide determined from isotopic records of pedogenic carbonate in these same paleosols. The drawdown of carbon dioxide began well before the Devonian appearance of coals, large logs, and diverse terrestrial plants and animals, and it did not correlate with temporal variation in volcanic or metamorphic activity. The early Paleozoic greenhouse may have been curbed by the evolution of rhizospheres with an increased ratio of primary to secondary production and by more effective silicate weathering during Silurian time.     

The derivation of compensating translocations involving homoeologous group 3 chromosomes of wheat and rye

Summary The Sr27 translocation in WRT238 was found to consist of chromosome arms 3RS of rye and 3AS of common wheat. An attempt was made to purposely produce compensating translocations having 3RS and a wheat homoeologous group 3L arm. To achieve this, plants, double monosomic for 3R and a wheat homoeologous group 3 chromosome, were irradiated (7.5 Gy gamma rays) or left untreated before being used to pollinate stem rust susceptible testers. Segregation for stem rust resistance was studied to identify F₂ families with Sr27-carrying translocated chromosomes, these were confirmed by means of C-banding. Compensating translocations 3RS3AL and 3RS3BL) were obtained readily and at similar frequencies from untreated and irradiated plants (respectively, 7.2% and 9.3%). Both translocation types have impaired transmission and segregate approximately 3: 2 (present: absent) in the F₂.     

The Transfer of Resistance to the Russian Wheat Aphid from Triticum monococcum L. to Common Wheat

Two Triticum monococcum accessions were found to be highly resistant to the Russian wheat aphid. An attempt was made to transfer the resistance to common wheat through direct hybridization or by using bridge species. In each of the four crossing strategies tried, a gradual loss of resistance occurred as hybrids with higher ploidy levels were obtained. It appeared that the level of resistance observed was directly proportional to the ratio of the Triticum monococcum genome relative to other genomes. This would indicate suppression of the resistance gene(s) by the added genomes or dilution of its product(s) by those of homoeoloci. The degree of protection afforded by the gene(s) at the hexaploid level may prove to be small. Plants suspected to be homozygous for the resistance gene(s) were identified; however, further backcrossing to common wheat will be required to improve their agronomic types and meiotic regularities.     

Identification of toxigenic Pasteurella multocida in atrophic rhinitis of pigs by in vitro characterisation

Toxigenic strains of Pasteurella multocida were readily differentiated from non-toxigenic strains by an agarose overlay method using bovine turbinate cells or bovine lung cells. Cells which were young and densely confluent were best suited to this assay. The incubation period required to distinguish toxigenic strains was dependent on the confluence of the monolayers, which was affected by the seeding rate, cell passage level and growth time prior to overlay. The agarose overlay method correctly identified 11 of 11 reference strains of Pasteurella multocida, and visible cytotoxic changes were present in the monolayers after 48 to 65 h. Outbreaks of the enzootic form of atrophic rhinitis in 2 New South Wales piggeries were associated with the isolation of toxigenic type D strains of P. multocida.     

A comparison, under controlled environmental conditions, of a Lolium multiflorum selection bred for high dry-matter content and non-structural carbohydrate concentration with a commercial cultivar

Abstract The nutritional value of Italian ryegrass (Lolium multiflorum) selection 121, bred for a high dry‐matter content and a high concentration of total non‐structural carbohydrate (TNC), was compared with that of Westerwolds ryegrass (L. multiflorum ssp. Westerwoldicum) cv. Midmar in a controlled environment. The concentration of neutral‐detergent fibre (NDF), acid‐detergent fibre (ADF), acid‐detergent lignin (ADL), nitrogenous compounds, minerals and in vitro digestibility were investigated as characteristics of nutritive value. The anatomical features of selection 121 and the Midmar cultivar were studied to determine possible structural differences. Thirty pots each of selection 121 and Midmar containing four plants per pot were arranged in a randomized block design in a controlled environment chamber. There were two temperature regimes during the study, the first being a warm regime (30 °C/20 °C) for 7 weeks followed by a cold regime(20 °C/7 °C) of a further 7 weeks. In the warm regime, the dry‐matter (DM) content and the TNC concentration of selection 121 were 0·17 and 0·16 higher, respectively, than Midmar. The NDF concentration was significantly (P < 0·01) higher in Midmar than in selection 121. When grown under warm conditions, Midmar had significantly (P < 0·001) higher concentrations of Mg, K, Na and Mn than selection 121. In the cold regime, the DM content and TNC concentration of selection 121 were 0·25 and 0·22 higher, respectively, than Midmar. No significant differences in the anti‐quality factors investigated were found between the two ryegrasses. In the cold regime, Midmar had significantly (P < 0·001 and P < 0·01) higher Ca, Mg, K, Na, Zn, Mn and P concentrations than selection 121. The results from this controlled environment study suggest that selection 121 is superior to Midmar in terms of the quality characteristics DM and TNC, and that these characteristics are not positively linked to anti‐quality factors associated with forage species.     

Influence of temperature and growth stage on the diurnal fluctuation in the non‐structural carbohydrate content of kikuyu (Pennisetum clandestinum Hochst)

The non‐structural carbohydrates of kikuyu (Pennisetum clandestinum Hochst) seem to consist mainly of sucrose. Total non‐structural carbohydrate content increased rapidly during the morning and reached maximum concentration in the afternoon. After sunset, the concentration decreased rapidly, reaching a minimum level by midnight. Low temperatures resulted in a significantly higher non‐structural carbohydrate content in all plant fractions than at warmer growth temperatures. Non‐structural carbohydrate concentrations were higher in stem tissue than in leaf material and the values were higher in 3‐week than in 6‐week regrowth.     

Feeding ecology of major carnivorous fish from four eastern Cape estuaries

Stomach content analyses were performed on eight carnivorous fish from four eastern Cape estuaries. Dietary importance was assessed by using three unrelated methods and by combining them to determine the index of relative importance of each food item. It was attempted to relate the percentage of fish with food in the stomach and the feeding intensity to the abundance of the fish in a particular estuary. In general fish consumed more food in estuaries where they were more abundant. The few exceptions that occurred could indicate that a particular estuary was selected for purposes other than food availability eg. Tachysurus feliceps in the Krom estuary for spawning grounds or because of inter- or intraspecific competition for food eg. T. feliceps and Monodactylus falciform is in the Krom and Argyrosomus hololepidotus in the Gamtoos and Sundays estuaries. Similarity analyses showed resource partitioning. Predators such as A. hololepidotus, Elops machnata, Lichia amia and Platycephalus indicus preferred teleost fishes such as mullets and Gilchristella aestuarius, whereas the stomach content of Pomadasys commersonnl, T. feliceps and M. falciform is were dominated by Crustacea, particularly Upogebia africana. Most estuarine predators showed a diverse prey selection, but the food web was characterized by a high energy flow per pathway.     

The threat of root‐knot nematodes (Meloidogyne spp.) in Africa: a review

Meloidogyne species pose a significant threat to crop production in Africa due to the losses they cause in a wide range of agricultural crops. The direct and indirect damage caused by various Meloidogyne species results in delayed maturity, toppling, reduced yields and quality of crop produce, high costs of production and therefore loss of income. In addition, emergence of resistance‐breaking Meloidogyne species has partly rendered various pest management programmes already in place ineffective, therefore putting food security of the continent at risk. It is likely that more losses may be experienced in the future due to the on‐going withdrawal of nematicides. To adequately address the threat of Meloidogyne species in Africa, an accurate assessment and understanding of the species present, genetic diversity, population structure, parasitism mechanisms and how each of these factors contribute to the overall threat posed by Meloidogyne species is important. Thus, the ability to accurately characterize and identify Meloidogyne species is crucial if the threat of Meloidogyne species to crop production in Africa is to be effectively tackled. This review discusses the use of traditional versus molecular‐based identification methods of Meloidogyne species and how accurate identification using a polyphasic approach can negate the eminent threat of root knot nematodes in crop production. The potential threat to Africa posed by highly damaging and resistance‐breaking populations of ‘emerging’ Meloidogyne species is also examined.