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Effects of ozone on host-parasite relations Resh, Howard Martin


Investigations of the effects of the air pollutant ozone on the growth of bush bean (Phaseolus vulgaris L. cv Pure Gold Wax) were undertaken to determine their nature and the modifications in growth responses caused by interactions of the pollutant with concurrent development of two plant pathogens, rust (Uromyces phaseoli [Rebent.] Wint.) and halo blight (Pseudomonas phaseolicola [Burkh.] Dowson). Two dosages of ozone (0.04 to 0.05 ppm = "low" and 0.09 to 0.10 ppm = "high"), administered daily during the photoperiod for 6 or 12 days, were used in most experiments. Neither dosage is sufficient to cause symptoms of acute injury. When compared with growth in filtered, ozone-free air, treatment, "low" ozone frequently resulted in enhanced growth of the host plant. "High" ozone treatment invariably resulted in impaired growth and the appearance of chronic injury symptoms after 10-12 days of treatment. A reflectance spectrophotometry method was developed to permit quantitative assessment of such injury. Growth responses to ozone in the presence of rust infection varied with infection level. In "high" ozone, the reduced dry weight gains of uninfected plants were reversed by heavy infection, leading to significant increases in total dry weight of infected leaves. Lower levels of infection, however, had little effect in modifying the nature of the growth responses of bean to ozone. The effects of heavy infection in counteracting ozone-induced growth suppression appear to be due to the ability of the fungus to maintain vigorous growth in the presence of ozone, rather than to an effect oh host susceptibility to ozone injury. Halo blight infection offered no protection against ozone stress, but on the contrary, increased the overall retardation of plant growth. The presence of rust infection offered little protection against ozone-induced premature senescence except in the "green islands" immediately surrounding the rust pustules. Higher levels of cytokinins have been associated with green islands, and both kinetin and 6-benzyladenine were found to offer some protection against ozone-induced, accelerated senescence, but not against injury to the leaf palisade cells, as revealed by reflectance measurements. Exposures of the two host-parasite systems to ozone levels used in this study revealed interactions rather than additive effects. "High" ozone in the presence or absence of the pathogen impaired plant growth with little protection offered by the pathogen to the host against the pollutant. Fungal development (before sporulation) was retarded by ozone but greater pustule frequency and the formation of secondary pustules maintained inoculum potential under ozone stress.

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