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The effect of Bordeaux mixture on the physiology of leaves of Vitis vinifera L., cv. Touriga Nacional, growing under field conditions in the Douro Wine Region, was evaluated. Especially in late summer, this fungicide modifies the light microclimate and leaf physiological characteristics, namely stomatal aperture and photosynthesis. Leaves treated with Bordeaux mixture showed higher reflectance, whereas transmitted photon flux density and temperature were lower compared to control leaves. Photosynthetic rates of treated leaves increased due to a lowering of both, stomatal and non-stomatal limitation. In addition, transpiration rates were higher, but neither the intrinsic efficiency of water use nor leaf water potentials were affected. Delay of leaf senescence of grapevines sprayed with Bordeaux mixture inhibited scorching of clusters and, consequently, led to higher yields per plant.