The potato minitubers have been widely used for the elite seed propagation to improve the seed potato system in China. However, little information is available for an efficient production of the minitubers with high plant density in the protected growing conditions like plastic houses. In present research, the minitubers of a wide-grown potato variety, Favorita, were produced with the microtubers from tissue culture and the tip-cuttings of the microtuber plants. Three plant densities, 200, 400 and 600 microtubers or plants m-2 were set up with the randomized block design of 3 replications and the experiment was repeated in 4 seasons in 2009-2010. The canopy development, light interception, dry weight production and partitioning, tuberization and tuber weight were investigated to elucidate the mechanism by which the plant density affects the formation and growth of the minitubers. The results showed that the number of the tubers formed per unite area was in line with the increase in plant density. The difference in leaf area index (LAI) between the plant densities, especially in early stage of the plant growth, resulted in more radiation interception and dry weight producing in higher plant density than in lower one. However, our analysis demonstrated that the conversion coefficient of the cumulative intercepted radiation to plant weight and the dry weight partition rate to the tubers were constant between plant densities, suggesting that less amount of the photoassimilates partitioned to individual tubers is causal for more small tubers in high plant density. A negative exponential curve model, determined by total number of tubers produced per unit area and the mean tuber weight, fitted well to the tuber size distribution pattern. The optimum plant density could be estimated from this model for a maximum production of the minitubers with desired size.

Cultivated potato with high level of horizontal resistance against late blight is one of the most important goals of potatobreeding. The recurrent selection has been adopted to increase the level of potato horizontal resistance and a B3C1population without R1-R11 dominant genes has been released by the International Potato Center at the short-day conditionof Peru. The present research was carried out to further improve the resistance and the agronomic traits of B3C1population under long-day condition of Hubei, China, with maximized retention of its genetic diversity. Twenty-sevenindividual clones of B3C1 were used to generate population B3C2 by in-population crossing with the bulk pollens aimingto elevate the frequency of late blight resistance genotypes and to improve the adaptation to local long-day conditions.The late blight resistance and the main agronomic traits including the maturity, the plant characters and the tuber traitswere evaluated for the foreground selection in three years, by which 130 pedigrees were maintained as the basic populationof B3C2 for further selection. A total of 312 polymorphic loci detected by 9 AFLP marker combinations were used tomonitor the genetic diversity of the populations for the background selection. The B3C2 population of 51 clones wasfinally selected, of which the frequency of resistant genotypes increased by 23.8% points and the genetic diversity wasmaintained by about 96% as referred to B3C1. Our results strongly suggested that combination of the foregroundselection for target traits and the background selection for the genetic diversity is an efficient strategy in the recurrentselection of tetraploid potato to improve quantitative traits.