Basic studies on the production and performance of potato minitubers.

Minitubers are small seed potato tubers that can be produced year-round in glasshouses on in vitro propagated plantlets planted at high density. The research reported in this thesis studied the agronomical and physiological principles of the production of minitubers and their performance under Dutch field conditions. The minitubers had fresh weights between 0.125 and 4.000 g.More than 3000 minitubers per m <sup><font size="-1">2</font></SUP>were produced in 10 weeks (average fresh weights 1 - 2 g), when tubers were harvested 4, 7 and 10 weeks after planting, using a non-destructive harvesting technique in the first two harvests. Removing tubers in the first harvest resulted in initiation of new tubers because more potential tuber sites became available that were not subjected to the dominance of rapidly growing tubers. Part of the newly initiated tubers grew to a harvestable size within three weeks, but the number of tubers in harvestable sizes did not increase thereafter, whereas part of the undersized tubers was resorbed. The second harvest stimulated growth of tubers that otherwise would have been resorbed or would have remained too small.Almost all minitubers ≥0.5 g survived storage at 2 °C for 1.5 years. After 6 months of storage, growth of plants from minitubers was still poor. Largest leaf areas were achieved after 10 - 11 months of storage, highest stem numbers, progeny tuber weights and harvest indices after 14 - 15 months of storage for cv. Agria and after 18 - 19 months for cv. Liseta.The performance of minitubers was affected considerably by their weight. Lighter tubers had a longer dormant period, partly because of a slower sprout growth up to 2 mm (used to assess the end of dormancy). Plants from lighter tubers took longer to emerge and at emergence had thinner stems, lower root weights, and higher shoot:root ratios. Crops from lighter minitubers produced lower yields because of less radiation intercepted (slower ground cover) and a lower harvest index. Multiplication factors per planted tuber were lower in crops from lighter minitubers because fewer plants emerged or survived, and fewer progeny tubers and lower weights were produced per plant. Yield variation within a crop was higher in crops from lighter minitubers, but - when properly nursed - variation in yield over years was not affected by the weight. Effects of minituber weight generally became less clear in the higher weight ranges. Differences in performance between minitubers and conventional tubers were attributed to weight and age of seed tubers, presprouting method and crop husbandry.Minitubers can be used in the first year of potato seed production programmes to speed up multiplication and to increase the quantity of seed from new cultivars