Influence of leaf damage by the horse chestnut leafminer (Cameraria ohridella Deschka & Dimic) on mycorrhiza of Aesculus hippocastanum L.

In many parts of Europe, the white horse chestnut (Aesculus hippocastanum L.) has been attacked by the horse chestnut leafminer (Cameraria ohridella Deschka & Dimic), which causes premature leaf dieback. A. hippocastanum L. establishes mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi. This study involved a comparison of mature A. hippocastanum individuals susceptible to C. ohridella and individuals resistant to this insect after a one-time treatment with a chemical preparation injected into the tree trunks 7 years before the investigation began. Concentration of macronutrients in soil and the activity of soil nonspecific dehydrogenase did not differ between soils under canopies of the treated and untreated trees. Concentrations of C and N were significantly higher in leaves of the treated than those of the untreated trees. The infestation by C. ohridella and defoliation of leaves of the untreated trees did not significantly influence the frequency and intensity of AM colonization compared to the chemically treated trees, although a tendency towards higher average AM colonization of roots of the untreated trees, infested by the herbivores, than roots of the non-infested trees was observed. The results also indicated a tendency for higher biomass of fine roots per soil volume under the trees treated against C. ohridella than under the trees invaded by the insect.

Mycorrhizal formation by Paxillus involutus strains in relation to their IAA-synthesizing activity.

IAA production by the ectomycorrhizal fungus Paxillus invotutus grown in pure culture was investigated by testing IAA accumulation or IAA-synthesizing activity. All the Paxillus involutus strains studied released indole compounds either when incubated with 0.5 mM tryptophan or in its absence. HPLC analyses of indole compounds all yielded two main peaks identified by reference to controls as being IAA and indole-3-carboxylic acid and a small peak of indole-3-aldehyde. Mass spectrometry unequivocally confirmed IAA production in the culture filtrates and in the incubation solution used to study in vitro IAA-synthesizing activity. The IAA-synthesizing activity was found to vary widely, from 7.6 to 137.4 nmol IAA h-1 g-1 f. wt, between strains. The mycorrhizal activity of these strains with seedlings of Pinus sylvestris cultured under aseptic conditions was tested. Fungal strains that were characterized in pure culture conditions by high LAA-synthesizing activity induced more fine roots and significantly higher numbers of mycorrhizas than strains which showed a low activity of IAA synthesizing enzymes. The increased ectomycarrhiza formation was generally accompanied by an increased growth of seedlings.

Gibberellin-induced hydrolysis of endosperm cell walls in gibberellin-deficient tomato seeds prior to radicle protrusion.

The weakening of the mechanical restraint of the endosperm layer in tomato (Lycopersicon esculentum Mill.) seeds, a prerequisite for germination, has been studied with the use of seeds of the gibberellin (GA)-deficientgib-1 mutant. Incubation ofgib-1 endosperms, including part of the testa, in 10 µM GA4+7, resulted within 12 h in the release of fructose, glucose, galactose and mannose into the incubation medium. Only small amounts of sugars diffused out of thegib-1 endosperms during incubation in water. Chemical hydrolysis of endosperm cell walls ofgib-1 seeds showed that they are mainly composed of mannose, and smaller quantities of glucose and galactose. Treatment with GA4+7 induced in the endosperms the production of endo-ß-mannanase activity that was not detectable during incubation in water, and also increased the activities of mannohydrolase and α-galactosidase as compared with the water controls. No cellulase activity was found. It is concluded that in tomato seeds the weakening of endosperms prior to radicle protrusion is mediated by a GA-induced enzymatic degradation of the mannan-rich cell walls.