Effect of solar radiation levels on grapevine leaf polyphenolic content and interaction with plasmopara viticola

In the experimental parcel two light environments were applied to induce differences in total polyphenolic content in grapevine leaves (Vitis vinifera, cv Sangiovese) and to assess if the resistance to downy mildew was affected. Shaded environment was obtained using 70% shading nets placed 3 m above ground. The canopy microclimate, evaluated as air and leaf temperature, relative humidity leaf wetness and different wavelenght of solar radiation was measured. Leaf epidermal polyphenols, mainly flavonoids, under the two light regimes were non-destructively measured during the growing season by the dualex optical device. Epidermal polyphenols were found to be considerably higher (80%) in sunny leaves with respect to shaded ones independently from leaf age. Both full developed shade and sun leaves were then inoculated with a sporangial suspension of downy midew (Plasmopara viticola) and infection severity was assessed as percentage of leaf infected area. Downy mildew development showed to be inversely correlated with epidermal polyphenolic content with 15% and 5% of severity value on shaded and sunny leaves respectively.