Influence of organic and conventional management on yield and composition of grape cv. 'Grignolino'

Organic management frequently involves a reduction of nitrogen input and it is claimed to increase grape quality. Excess nitrogen application reduces anthocyanin concentration, thus decreasing the antioxidative potential of grapes and wines. In order to compare the effects of organic and conventional fertilization management on vine performance and the antioxidant properties of wine grapes, a field trial was carried out in 20-year-old 'Grignolino' vineyards located in the Piedmont region (North-Western Italy). The vineyards were managed either conventionally (40 kg/ha N) or under one of the following organic fertilization regimes: i) organic fertilizer (300 kg/ha; 11% organic N); ii) a mixture of mycorrhiza and plant growth promoting bacteria (100 kg/ha; 0.034% organic N); iii) a combination of both (i) and (ii) (250 + 100 kg/ha). The following variables were measured: yield, vine growth, leaf N content, total soluble solids content and acidity of must, flavonoid and anthocyanin concentration, and the antioxidant potential of berry skin extracts. The latter was estimated by measuring the decrease of the EPR signal associated with the semiquinone radical (generated by enzymatic oxidation of catechol) induced by the presence of the berry skin extract. Yields in the organic vineyards were lower than under conventional management. No differences were observed in grape yield and growth amongst the organic fertilization treatments. The overall polyphenol concentration was higher in organic grapes, resulting in a higher protection from oxidation. Moreover, the three organic treatments enhanced the polyphenol content in the berry skin extracts, thus increasing its antioxidant potential. Treatment (iii) was the richest in polyphenols, while the antioxidant potential was the highest in the extracts from treatment (ii). In conclusion, environmental factors seem to prevail over genotype in determining the polyphenol concentration and antioxidant properties of grapes.