Relationships between leaf age, peroxidase and β-1,3-glucanase activity, and resistance to downy mildew in grapevines

The susceptibility of leaves of various ages of field and greenhouse-grown grapevines (Vitis vinifera), to downy mildew caused by Plasmopara viticola was evaluated. Older leaves from the bottoms of the shoots were more resistant to natural infection in the field or to artificial inoculation in the growth chamber with P. viticola, than were younger leaves from the tops of the shoots. This age-related resistance was confirmed by the use of a quantitative leaf disc assay. The areas of lesions and sporangia produced on leaf discs obtained from leaves of various ages decreased significantly with increasing leaf age. This resistance was related to increases in β-1,3-glucanase and peroxidase activities, as determined on the same leaves from which leaf discs were obtained. Remarkable increases of up to 10-30 times and two-to sevenfold in peroxidase were found in the soluble and ionically bound fractions, respectively, of older leaves, as compared with younger leaves. Lower increase of 87 and 65% in β-1,3-glucanase was found in the soluble and ionically bound fractions, respectively, of older leaves, as compared with younger leaves. The data suggest the presence of defence mechanisms against P. viticola in grapevine leaves, that appear to be dependent on leaf age and which may involve these enzymes.