COMPARISON OF SEASONAL DEVELOPMENT OF TUBER SPECIFIC-GRAVITY AMONG 7 POTATO CLONES

Specific gravity of potato tubers provides an estimate of starch conte nt of which uniformly high levels are important for product quality, T he objectives of this study were to document the relationship between the level of specific gravity and the variability among tubers and to model the seasonal development of specific gravity for seven potato cl ones. Individual determinations were made on twenty tubers from each p lot using the weight-in-air, weight-in-water method on each of eight h arvest dates spanning the tuber growth period, For each plot, the mean and standard deviation of specific gravity were calculated and used t o define the correlation between specific gravity level and tuber-to-t uber variability and to construct a predictive model of the seasonal s pecific gravity developmental pattern. As the season progressed, tuber -to-tuber variability of specific gravity increased for some clones bu t not for others. A positive correlation existed between tuber specifi c gravity mean and its standard deviation for four of the seven clones in 1991 and five in 1992. Correlation coefficients were generally low , suggesting only a weak association between specific gravity and its standard deviation. Tuber specific gravity developed in a similar seas onal pattern for the seven clones. It generally started low, reached a maximum approximately seven to eleven weeks after tuber set, at which time some clones showed a tendency to decline through the remainder o f the season. Quadratic regression equations provided an adequate mode l to describe specific gravity development for all seven clones over t he tuber growth period. Statistical contrasts were used to test for di fferences in equation parameters, including Line coincidence, Line slo pe, and intercept. The model showed that initial specific gravity leve l at tuber set, the rate of early season increase, as well as the leng th of time from tuber set to specific gravity maximum, varied by clone and determined the level of specific gravity at season's end.