INFLUENCE OF OZONE ON CARBON PARTITIONING IN TOMATO: POTENTIAL ROLE OF CARBON FLOW IN REGULATION OF THE MYCORRHIZAL SYMBIOSIS UNDER CONDITIONS OF STRESS

Abstract

SUMMARYTomato seedlings inoculated with the vesicular‐arbuscular mycorrhizal fungus, Glomus fasciculatus, were exposed for 3 h to 15 parts 10−8 (294 μg m−3) ozone twice weekly or to 30 parts 10−8 (589 μg m−3) ozone once per week, for 9 weeks. Mycorrhizal infection was reduced by 46 and 63 % in the 15 and 30 parts 10−8 ozone treatments, respectively. The total dry weight of mycorrhizal plants was significantly reduced in the 30 parts 10−8 ozone treatment but dry weights of non‐mycorrhizal plants were not affected by any ozone treatment. In a second experiment, 3–week‐old seedlings of tomato inoculated with G. fasciculatus were twice exposed to 30 parts 1O−8 ozone or filtered air for 3 h with a 7‐day interval. Soluble amino acids in root exudates and root extracts were significantly reduced in mycorrhizal plants exposed to ozone. The amino acid content of non‐mycorrhizal exudates was not reduced by ozone but was significantly increased in root extracts. Levels of reducing sugars also declined in root exudates and in root extracts of mycorrhizal and non‐mycorrhizal plants exposed to ozone. A greater amount of photoassimilated 14CO2 was retained in leaves and less translocated to roots 4 h after fumigation with ozone. Labelled leaves exhibited no leaf injury until 8 h after exposure to ozone and showed only 10 to 20% chlorosis after 24 h.