RELATIONSHIPS BETWEEN LATERAL ROOT ORDER, ARBUSCULAR MYCORRHIZA DEVELOPMENT, AND THE PHYSIOLOGICAL-STATE OF THE SYMBIOTIC FUNGUS IN PLATANUS-ACERIFOLIA

The rapid development of an efficient root system resulting from arbus cular mycorrhiza formation is essential to the successful establishmen t of many plant species. We have analysed root system development and used histochemical staining to define relationships between lateral ro ot order dynamics, arbuscular mycorrhiza development, and the physiolo gical state of the symbiotic fungus Glomus fasciculatum (Thaxter sensu Gerdeman) Gerd & Trappe amend. Walker and Koske, in a woody plant spe cies Platanus acerifolia Willd. Arbuscular mycorrhiza induced modifica tions in root system development in P. acerifolia, compared with nonmy corrhizal root systems. Third-order lateral roots dominated in arbuscu lar mycorrhizal plants, while second-order laterals were most numerous in nonmycorrhizal systems. Arbuscular mycorrhiza colonization was clo sely related to the appearance of different root orders; the most acti ve mycelium (characterized by fungal succinate dehydrogenase and alkal ine phosphatase activities) was mainly localized in newly formed later al roots. Nine weeks after inoculation with G. fasciculatum the propor tion of alkaline phosphatase-active mycelium strongly decreased in all root orders, and this was related to an increased phosphorus content of the host plant. The dynamics of development of the arbuscular mycor rhizal fungus and the possible regulation of its activity by the host plant are discussed.