Inhibition of Photosynthesis and Transpiration in Soybean Infected by Microsphaera diffusa

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Abstract
Net photosynthesis, transpiration and dark respiration were determined by gas exchange or attached leaves of soybean (Glycine max ''Harosoy 63'') infected by M. diffusa, which causes powdery mildew. In this variety, the presence of mycelia was not accompanied by visible host symptoms. By the time 82% of the leaf area was covered by M. diffusa, rates of photosynthesis and transpiration were reduced to less than half those of healthy controls. The diffusive conductivity of the leaf for CO2 was reduced significantly, but neither internal CO2 nor leaf porosity was altered by infection. Photosynthesis reduction was not therefore attributed to availability of internal CO2 nor to reduced stomatal aperture. A decrease in the O2 concentration to 1% increased net photosynthesis by 1.4 times relative to the rate in normal air (21% O2), regardless of disease levels. Increases in rates of light-induced respiration did not account for decreased net photosynthesis. Change in rates of dark respiration were insufficient to affect net photosynthesis. Also, removal of surface mycelia did not increase photosynthesis or transpiration. There was no loss of photosynthetic tissue due to necrosis or early senescence (chlorosis). The lack of respiratory effects, stomatal effects and changes in internal CO2 concentration suggest that M. diffusa inhibited photosynthesis at the chloroplast level. Because the fungus was restricted to the epidermis, a translocatable factor produced by the fungus or host may have caused the chloroplast response.