The Influence of Carbohydrate and Mineral Nutrient Supply on the Growth of Potato Tubers

The uptake and distribution of nitrogen, phosphorus, and potassium has been studied throughout the life of potato plants. There appears to be a net loss of all three elements from the plant during emergence growth even though uptake occurs. When daughter tubers are formed they very quickly become the dominant sink for mineral nutrients, the concentrations of N, P, and K remaining steady for a long period. These concentrations are maintained in spite of decreasing rates of uptake, indicating the transference of mobile ions from the haulm to the growing tubers. 14C tracer experiments have shown that after tuberization there is a greater export of recently incorporated photosynthate from the leaves than takes place before tuberization. There is no good correlation between the size of individual tubers and the amount of photosynthate transported into them. This is thought to be because the largest tubers are not necessarily growing faster than the smaller tubers. The most active sinks are also most active in converting the mobile 14C into storage compounds. The mother tuber continues to import 14C until it is detached from the plant, but over much of this period there is no change in the tuber dry-weight, indicating that there is an equivalent export from the tuber. The similarities between these distribution patterns and those found in tubers showing second-growth are described and the implications with respect to the control of tuber growth discussed.