A molecular genetic assessment of herbicide-resistant Sinapis arvensis

The acquisition of resistance to both the auxinic herbicide dicamba and the sulfonylurea herbicide chlorsulfuron has been recorded in Canadian populations of the weed species Sinapis arvensis L. (charlock, wild mustard) To study the effect of this selection for herbicide resistance on levels of genetic variation, polymerase chain reaction-based DNA fingerprinting techniques were used to characterize two herbicide-resistant and one susceptible population of S. arvensis . Analysis of the resultant DNA marker profiles revealed extensive polymorphism between individuals. However, segregation of the three biotypes was detectable despite high levels of intrabiotype polymorphism. No reduction in the levels of heterozygosity within the resistant populations were found compared with the susceptible population.     

An assessment of genetic diversity within and between populations of Phalaris minor using ISSR markers

Intra- and inter-sample similarities for four populations of the annual grass weed Phalaris minor from Haryana state, India, were examined using inter-simple sequence repeat (ISSR) DNA markers. Levels of polymorphism within and between populations were low in comparison with values reported from other grassy weed species. Analysis of inter-population similarities allowed a partial differentiation of the four populations and of pairs of populations classified by cropping system. Analysis of the intra-population similarity data showed a weak but consistent and statistically significant negative correlation between the molecular similarity of seedlings and the physical distance between their mother plants over distances up to 40 m (the maximum separation tested) in all four populations. The consistency of the observed relationship between molecular similarity and physical separation, and the differences in cultivation practices at the four sites, suggested that the relationship may be a result of localized out-crossing, rather than an effect of localized seed rain. The results of the analyses are discussed in relation to the potential for evolution of multiple traits in the weed in response to changes in the wheat production system in the region.     

Pathogen behaviour and plant cell reactions in interactions between Alternaria species and leaves of host and nonhost plants

The growth behaviour of several Alternaria species on leaf discs of host and nonhost plants was examined by u.v. microscopy. Specific patterns of development were found for the pathogens. Differences in development which correlated to host specificity, or which differentiated specialized pathogens from weak or opportunistic pathogens, were obvious only after attempted penetration. Thus, little variation was observed amongst different plants for individual Alternaria species in the rate of germination, in the extent of germ-tube growth or in the frequency of appressorium production. However, plant responses to attempted penetration, which included the formation of callose-containing papillae, callose deposition in the walls of attacked cells and their neighbours and cell necrosis, varied with specific pathogen–plant interactions. Callose deposition occurred at sites of both successful and unsuccessful penetrations and may, therefore, not be a determining factor in the plant–pathogen interactions examined. A biplot technique is used to illustrate the different degrees of host specificity apparent at pre- and post-penetration stages of fungal development.