Relationships between Leaf Age, Peroxidase and β-1,3-GIucanase Activity, and Resistance to Downy Mildew in Grapevines

The susceptibi lity of leaves or various ages of fie ld and greenhouse-grown grapevi nes (Vilis Iʹillifera), to downy mildew caused by Plaslllopara Lʹilico/a was evaluated. Older leaves from the bottoms of the shoots were more resistant to natural infection in the field or (0 artificial in oculation in the growth chamber with P. llilico/a. than were younger leaves from the tops of the shoots. This age-related resistance was confirmed by the use ofa quantitali ve leaf di sc assay. The areas of lesions and sporangia prod uced on leaf discs obtained from leave ʹ of various ages decreased significa ntly wi th increasing leaf age. This resistance was related to increases in P-1.3-glucanase and peroxidase activities, as determined on the same leaves from which leaf d iscs were obtained. Rema rkable increases of up to 10-30 times and two- to sevenfold in peroxidase were found in the soluble and ionically bound fractions, respecti vely. of older leaves, as compared with yo unger leaves. Lower increase of 87 and 65% in /1-1,3- glueanase was found in the so luble and ionically bound fra ctions, respectively. of older leaves, as compared with younger lea ves. The da ta suggest the presence of defence mechanisms against P. v;t;co/a in grapevine lea ves. that appear to be dependent on leaf age and which may in volve these enzymes