Seasonal variations of symbiotic ultrastructure and relationships of two natural ectomycorrhizae of beech (Fagus sylvatica/Lactarius blennius var. viridis and Fagus sylvatica/Lactarius subdulcis)

Abstract

This study concerned two perennial ectomycorrhizae of beech: Fagus sylvatica L./Lactarius blennius var. viridis and Fagus sylvatica L./Lactarius subdulcis, collected in a natural forest site. Nutritional exchanges existing between beech roots and fungi were evaluated over a seasonal cycle by studying the variations of ultrastructure, polyphosphate and nitrogenous granules and glycogen content (periodic acid thiocarbohydrazide silver test). The acid phosphatase activity, which is an indicator of the intensity of the physiological activity of symbiosis partners, was also revealed by transmission electron microscopy. The occurrence of the two ectomycorrhizae in the soil was not related to the climatic conditions. In winter, the number of polyphosphate and nitrogenous granules was high in the fungi, and glycogen was abundant in the cytoplasm of hyphae. In late winter, an intense acid phosphatase activity was revealed on plasmalemmas of live hyphae. It could indicate the beginning of fungal reserve mobilization, which progressively disappeared as spring progressed. In summer, glycogen was quasi non-existent and only few polyphosphate and nitrogenous granules were observed. At this period, the acid phosphatase activity was weak. Finally, in autumn and early winter, glycogen and granules were progressively restored in fungal structures. Relationships between changes of granules and glycogen content and the physiological states of each symbiont throughout the year are discussed.