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We performed manipulative field experiments to investigate the effects of soil disturbance and exposure to a fungal plant pathogen, Puccinia coronata (Corda), on the establishment and spread of two introduced, cultivated genotypes of perennial ryegrass, Lolium perenne (L.). The two cultivars of L. perenne with different levels of susceptibility to P. coronata were introduced to grassland sites to investigate whether a plant cultivar selected to resist a pathogen shows better establishment in semi-natural plant communities than a susceptible cultivar. At two sites where L. perenne was already present, the addition of L. perenne seeds had no significant effect on the shoot biomass of the species, indicating that these populations were not seed limited. Exposure to the pathogen resulted in disease, and infected L. perenne populations showed increased shoot biomass over the course of the 3 year experiment and at harvest the final year, but no effect on seed production. Reproductive allocation was not affected by disease exposure in disturbed plots, but decreased in the presence of disease in undisturbed plots. The increased biomass observed in the semi-natural plant communities when exposed to the pathogen contrasts with the reduced biomass observed in garden experiments when the two cultivars of L. perenne were exposed to pathogen attack. The surprising positive effect of P. coronata on biomass in semi-natural communities indicates that processes here are more complex than in more intensively managed production systems.
Soluble carbohydrates, particularly oligosaccharides, can take part in defense responses preventing and restricting fungal pathogen invasion. Morphological changes were observed and β-glucosidase activity was studied in yellow lupin embryo-axes infected and uninfected with F. oxysporum and cultured for 24 lto 96 hours under conditions of a varying carbohydrate supply. The first disease, symptoms - necrotic pathological changes were observed 48 hours after the .inoculation. A higher intensity of the disease changes as well as growth restriction Cle found in 72- and 96-h embryo axes growing under carbohydrate deficiency. The activity of ậ-glucosidase increased during 24 - 96 hours after inoculation. An especially high increase of the activity was noted in 72- and 96-h embryo axes cultured under carbohydrate deficiency. We suggest that sugars are involved in mechanisms of yellow lupin embryo axis tissues resistance, since they constitute source of precursors for the synthesis of antimicrobial factors of plant defense response.
The review focuses on the progress in recent decades on various aspects of molecular responses taking place in plants infested by herbivorous arthropods. The diversity of herbivoryinitiated molecular events was presented with special emphasis on: (1) elicitors and a general recognition process, (2) signal perception, transduction, integration and defence gene expression, (3) cross-talk between salicylate- and jasmonate-dependent defence pathways, and (4) regulators of plant defence responses.
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