Root tuber yield, weight of individual tubers, root tuber/stem weight ratio at harvest, mean bulking rate, portion of total dry matter diverted into the root tubers, and net assimilation and relative growth rates were greater in single-shoot than in multi-shoot plants. Multi-shoot plants, however, had greater leaf area and leaf area duration. Early planting encouraged the production of greater leaf area, root tuber number per plant, root tuber yield, individual tuber size and dry matter. Bulking rate was positively related to net assimilation rate. In single-shoot plants differences in leaf area duration accounted for 95% of the variation in yield between treatments, and in both single-and multi-shoot plants differences in the product of leaf area duration and mean net assimilation rate accounted for 75% of the total variation in yield between treatments.