Effects of Powdery Mildew Infection on the Efficiency of CO2 Fixation and Light Utilization by Sugar Beet Leaves

Sugar beet leaves (Beta vulgaris L.) infected with powdery mildew (Erysiphe polygoni D.C.) show declining rates of net photosynthesis as the disease develops; relative to healthy controls, reductions of 35, 70, and 75% were observed at 9, 16, and 22 days after inoculation, respectively. A leaf gas exchange procedure in which an air stream flowed through the leaf showed that mesophyll conductance declined in parallel with photosynthesis in mildew-infected leaves. Viscous flow conductance of diseased leaves also declined over the same period suggesting that stomatal aperture was reduced. From the magnitude and time course of disease effects on stomatal aperture and mesophyll conductance, it appears that the effects at the mesophyll level were primarily responsible for mediating the decline in net photosynthesis. Changes in mesophyll conductance were closely correlated with reduced activity of ribulose-1,5-bisphosphate carboxylase on a leaf area basis. This decrease could be attributed to a reduction in the concentration of the enzyme, a reduction which was greater than the reduction in total soluble protein. The quantum efficiency of light use was also decreased by the disease. Mildew-infected leaves had quantum yields that were reduced, relative to healthy leaves, by 17 and 22% at 14 and 18 days after inoculation, respectively.