The role of nitrogen concentration in determining the outcome of the interaction between Dactylorhiza incarnata (L.) Soó and Rhizoctonia sp.

Abstract

Protocorms of Dactylorhiza incarnata (L.) Soó grown in shaking cultures were used for synthesis of mycorrhizas on agar plates in the presence of a Rhizoctonia strain. At low nitrogen (N) concentration [0.36 m++++m Ca (NO3)2.4H2O] normal mycorrhizas were formed, whereas the same concentration of carbon a higher concentration of N (3.6 mm) led to parasitism with low survival of protocorms. Carbon source did not influence the symbiotic interactions. Growth of mycorrhizal protocorms at low N concentrations increased with sucrose concentrations up to 58 mm, but in the absence of the fungus there was practically no growth. Increases in nitrogen concentration can thus lead to change from a mycorrhizal to a parasitic interaction. The effects of nitrogen on the interaction between fungus and orchid provided an experimental system which was subsequently used to investigate hormone production in the different types of symbiosis by enzyme immunoassays. Phytohormone levels remained low under asymbiotic conditions. The presence of the fungus led to a large increase in phytohormone concentration, which was correlated with a growth stimulation in mycorrhizal protocorms. In parasitized protocorms auxin and to a lesser extent cytokinins remained at higher concentrations for longer than in mycorrhizal protocorms. It is thought that the phytohormones are derived mainly from the mycorrhizal fungus. Copyright © 1991, Wiley Blackwell. All rights reserved