Effect of foliar applications of gibberellic acid or daminozide on plant growth, tuberisation, and carbohydrate accumulation in tubers grown from true potato seed

Foliar application of gibberellic acid (GA(3)) increased vegetative growth (fresh and dry weights) and the number of tubers per plant grown from true potato seed (TPS). Tubers from plants that had been treated with GA(3) were smaller in size and had lower sucrose, but higher glucose and fructose contents. Microscopic examination of starch grains indicated that the changes in sugar content coincided with the breakdown of starch and an increase in alpha-glucosidase activity. In contrast, the activities of UDP-glucose pyrophosphorylase and ADP-glucose pyrophosphorylase were apparently not affected by GA(3). Although daminozide (an inhibitor of endogenous gibberellin biosynthesis) inhibited the vegetative growth of plants, it did not affect tuberisation, carbohydrate composition, or the activity of enzymes associated with carbohydrate metabolism. It is apparent, therefore, that tuber formation and carbohydrate composition can be affected by exogenously applied GA(3), but not by a possible reduction in endogenous gibberellin levels due to daminozide. Moreover, since the foliar application of GA(3) induces tuber elongation as well as the formation of stolons and 'chain tubers', it is likely that these processes have a high demand for sugars, thus leading to a decrease in the specific weight of tubers, increased breakdown of starch, and higher glucose and fructose levels in the tuber.