Studies on Grape Variability and Field Sampling

Abstract

A study has been made of the amount of variation of sugar and acid content in maturing grapes and factors affecting it. Variation in composition between vines was found to be greater than that between clusters within vines. The effect of the position of clusters on the vine and the weight of individual clusters was quite small. The mean sugar content per vine was inversely correlated with the yield of individual vines while the mean acid content was directly proportional to yield.

Both the between-vine variance component and the within-vine (i.e. between cluster) variance component were highly correlated with the mean level of acid. The mean sugar content was correlated with the between-cluster variance component but not with the between-vine variance component. Irrigated grapes were more variable than non-irrigated grapes when the sugar content was less than 20 per cent.

The most efficient method of sampling a plot of vines was to take a minimum amount of material per vine, i.e. one cluster if cluster sampling is used. The standard error of the mean sugar content in plots of 200 vines was ± 0.25 per cent sugar when 1 cluster was picked from each of 25 random vines or 2 clusters were picked from each of 21 random vines. Single berry sampling gave comparable estimates of the mean sugar content, and is recommended for field assessment of maturity because of the lesser amount of fruit involved. Sample estimates of the mean sugar content, using cluster Sampling, compared well with the total crop. Thirty-four such comparisons were made. Biassed estimates of the mean sugar and acid contents are obtained when equal number of clusters (or berries) are sampled from each vine, due to the correlation between yield and sugar and acid content. Methods of eliminating this bias are given.