EFFECT OF PHOTON IRRADIANCE ON THE DEVELOPMENT AND ACTIVITY OF VA MYCORRHIZAL INFECTION IN ALLIUM-PORRUM

The effects of reduced photon irradiances on the development of Glomus mosseae infection in Allium porrum were investigated. The percentage and intensity of root length infected and the frequencies of arbuscules and vesicles were determined at regular intervals in two experiments. Reduced photon irradiances and inoculation decreased the root/shoot ratio and increased the fresh weight/dry weight ratio, indicating a response by the plant to increase its photosynthetic capacity. Reduced photon irradiances did not significantly affect the percentage or intensity of the root length infected, but significantly reduced the frequency of arbuscules and vesicles within the infection. Numbers of vesicles remained low throughout both experiments, whilst the arbuscules increased to the same levels as in plants grown at higher irradiances by 4-6 weeks, depending upon the severity of the light stress. The addition of P had no effect on any aspect of infection. The activity of the infection was unaffected by all treatments. Implications for the delayed arbuscule development under reduced irradiances are discussed.