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Physiology and Biochemistry

Photosynthesis in Powdery Mildewed Sugar Beet Leaves. T. R. Gordon, Department of Plant Pathology, University of California, Davis 95616; J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 72:718-723. Accepted for publication 20 July 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-718.

Net photosynthesis, dark respiration, and photorespiration were determined from rates of gas exchange by attached leaves of sugar beet (Beta vulgaris) infected with Erysiphe polygoni. Adaxial leaf surfaces were inoculated with conidia and the abaxial surfaces remained free of visible infection during the experimental period. At 6 days after inoculation, rates of net photosynthesis had measurably diminished and after 13 days had declined to less than half those of healthy leaves. Concomitantly, dark respiration increased and photorespiration decreased. Stomatal conductance was calculated independently for both the adaxial and abaxial leaf surfaces from their respective transpiration rates. For the mildewed adaxial surface, stomatal conductance was also calculated from adaxial CO₂ exchange rates and internal CO₂ concentration. The two methods gave similar values for the stomatal conductance of infected adaxial leaf surfaces, suggesting that the rate of water vapor loss by the fungus was small. Stomatal conductances on both the colonized and uncolonized sides of infected leaves declined during disease development. Mesophyll conductance to CO₂ was always much lower than stomatal conductance and was, therefore, of primary importance in limiting the rates of net photosynthesis by infected leaves. Under the light-saturated conditions that were used, the diminished photosynthetic capabilities of the mesophyll in infected leaves were not due to loss of chlorophyll or increased leaf reflectance.