Relationship between the population of viable sclerotia ofSclerotium rolfsii in soil to cropping sequence in the Nigerian savanna

Populations of viable sclerotia ofSclerotium rolfsii were highest in soil in a field in which tomato was planted for three successive years before sampling and in one in which tomato followed groundnut in the 2 years prior to this study. The lowest sclerotial numbers were recorded in fields in which groundnut followed maize or in which maize or sorghum was the last crop before sampling.     

Nitrogen assimilation in field-grown winter wheat: Direct measurements of nitrate reduction in roots using15N

Summary Nitrate fertiliser labelled with¹⁵N was applied to a field grown crop of winter wheat. Uptake and assimilation of fertiliser nitrate was studied by monitoring the appearance of labelled nitrate and labelled amino acids in the xylem sap. Shortly after applying¹⁵N-nitrate to the soil about 30 per cent of recently absorbed¹⁵N was in the reduced form, indicating that roots of cereal crops can make a substantial contribution in reducing nitrate. Seasonal changes in crop growth andin vivo NRA are also described.     

The genetic effective and adult census size of an Australian population of tiger prawns (Penaeus esculentus)

This study compares estimates of the census size of the spawning population with genetic estimates of effective current and long-term population size for an abundant and commercially important marine invertebrate, the brown tiger prawn (Penaeus esculentus). Our aim was to focus on the relationship between genetic effective and census size that may provide a source of information for viability analyses of naturally occurring populations. Samples were taken in 2001, 2002 and 2003 from a population on the east coast of Australia and temporal allelic variation was measured at eight polymorphic microsatellite loci. Moments-based and maximum-likelihood estimates of current genetic effective population size ranged from 797 to 1304. The mean long-term genetic effective population size was 9968. Although small for a large population, the effective population size estimates were above the threshold where genetic diversity is lost at neutral alleles through drift or inbreeding. Simulation studies correctly predicted that under these experimental conditions the genetic estimates would have non-infinite upper confidence limits and revealed they might be overestimates of the true size. We also show that estimates of mortality and variance in family size may be derived from data on average fecundity, current genetic effective and census spawning population size, assuming effective population size is equivalent to the number of breeders. This work confirms that it is feasible to obtain accurate estimates of current genetic effective population size for abundant Type III species using existing genetic marker technology.     

Cryopreservation by encapsulation-dehydration of ‘Christmas bush’ (<Emphasis Type="Italic">Ceratopetalum gummiferum</Emphasis>) shoot tips

Summary Christmas bush (Ceratopetalum gummiferum Sm) is a shrubby tree species of the east coast of New South Wales in Australia. It is much prized as a cut flower crop because of its bright, pinky red floral calyces. New varieties are being developed, the storage of which is an important issue. In this study, it was shown that shoot tips sampled from in vitro plantlets withstood cryopreservation using the encapsulation-dehydration technique. The protocol leading to optimal regrowth was the following: excised shoot tips were pretreated for 1 d in the dark on hormone-free Murashige and Skoog (MS) medium with 0.3 M sucrose, then encapsulated in 3% calcium alginate and precultured in liquid MS medium with 0.5 M sucrose for 3 d. Precultured beads were dehydrated for 6 h in the air current of the laminar flow cabinet to 24.3% moisture content (fresh weight basis) before rapid immersion in liquid nitrogen. Under these conditions, regrowth of shoot tips after cryopreservation reached 61.4%. Regrowth of cryopreserved shoot tips was not affected by the period of cold acclimation of in vitro mother plants.     

Risk assessment for engineered bacteria used in biocontrol of fungal disease in agricultural crops

A plant growth promoting rhizobacterium (PGPR)Pseudomonas fluorescens SBW25 (WT) protects a number of crop plant species from damping-off caused by Pythium ultimum. A genetically modified, phenazine-1-carboxylic acid (PCA) producing variant, 23.10, carries on its chromosome a single copy of phzABCDEFG, under the control of the P tac constitutive promoter. The genetically modified biological control agent (GM-BCA), 23.10, has improved biocontrol activity when compared to wild type SBW25, and can effectively suppress Pythium spp. present at up to 100 times normal field infestations. GM-BCA inocula establish high population densities which persist well in the phytosphere of several crop plants including pea, wheat and sugar beet, effectively suppressed infection and promoted increase in total plant biomass. It also has an improved spectrum of activity over other plant phytopathogens such as Fusarium spp. Gaeumannomyces graminis var. tritici, Phytophtora cinnamomi and Rhizoctonia solani. However in developing BCAs and in particular GMBCAs it is important to determine whether their use has any adverse effect in the environment. Any observed changes following inoculation with wild type BCA or GM BCA in microbial diversity (bacteria and fungi) were negligible when assessed by either quantitive selective plate count methods (CFU/g) or culture independent molecular assays (SSU rRNA based PCR-DGGE). Rhizosphere community diversity profiles (DGGE) in infected plants in the presence of inocula were highly similar to disease free systems. Histological assessment of the impact of inocula on established functional mycorrhizae associations were conducted on cores collected from an established field margin grassland pasture. No adverse impact on mycorrhizal colonization and root infection were recorded after addition of WT or GM-BCA bacterial inocula as a soil drench. This approach and the related culturable and culture independent methods have recorded only a minor, transient perturbation to microbial communities, but as far as we are aware this is the first direct demonstration that a functional, AFC producing GMM also has only a transient impact on mycorrhizal associations in established plant communities. In all instances studied the plant species, plant stage of development and disease, damping-off, had a greater impact on changes in rhizosphere diversity than the presence of an introduced GM bacterial inocula.