Transplant crops derived from in vitro produced plantlets of cultivars differing in earliness were grown in three experiments in three years in the Netherlands, during field periods of maximally 12 weeks. Seed tuber crops were included in the first year. Fresh tuber yield was analysed as the result of the radiation intercepted by the crop's canopy (AIR; accumulated intercepted radiation), the efficiency of conversion of intercepted radiation into dry matter (RCC; radiation conversion coefficient), the proportion of dry matter allocated to tubers (HI; harvest index), and the tuber dry-matter concentration (TDMC). Transplant crops had a lower AIR and lower yields than crops from seed tubers. Variables RCC, HI and TDMC were not affected by the type of propagule used. In transplant crops, yields from early cultivars could be extremely low when compared to later cultivars, due to a low AIR. This cultivar effect did not occur in crops from seed tubers. Among transplant crops, the lower AIR was the most consistent reason for lower tuber yields of earlier cultivars. It resulted from a slower increase in soil cover after transplanting and a lower maximum soil cover, both caused by a relatively high allocation of dry matter to tubers immediately after transplanting and resulting first in reduced haulm growth rates and subsequently also in reduced total growth rates. Senescence was not different. A higher HI partly compensated the lower AIR of the earliest cultivars. RCC and TDMC were not consistently affected by cultivar's earliness in the transplant crops.
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