INTERACTIVE EFFECTS OF OZONE AND POWDERY MILDEW ON PEA-SEEDLINGS

We assessed the effects of ozone and powdery mildew infection on plant growth, ozone injury, colonization of leaves by the fungus, and pisat in content of pea (Pisum sativum) seedlings. Seedlings of cultivars Al aska and Bounty grown in the greenhouse were moved to controlled-envir onment growth chambers and exposed to ozone concentrations of 0.00, 0. 06, and 0.12 mul L-1 for 6 h/day (I mul L-1 = 1,960 mug M-3 at standar d temperature and pressure). Fumigations were performed for 5 days bef ore, after, or both before and after inoculation with conidia of Erysi phe polygoni f. sp. pisi, the causal agent of pea powdery mildew. Ozon e at 0.12 mul L-1 suppressed growth in both cultivars only in the prei noculation fumigation treatment, indicating a protective effect of pow derY mildew infection, which by itself did not affect dry weight. Boun ty was more resistant to infection by E. p. pisi than Alaska and was a lso less sensitive to foliar ozone injury. Colonization of leaves by t he fungus resulted in suppressed foliar ozone injury in both cultivars . Both preinoculation and postinoculation exposure to ozone at 0.12 mu l L-1 significantly suppressed fungal leaf colonization. The apparent increase in resistance indicated a plant defense mechanism induced by the ozone exposure. Pisatin, a pea phytoalexin, was not detected in le af tissue of noninoculated plants that were exposed to ozone, however, indicating that pisatin probably is not involved in this increased re sistance.