PLANTING TIME AND SEED DENSITY EFFECTS ON POTATO IN SUBTROPICAL CHINA

Planting times of potato in subtropical China vary and are often not o ptimal; their effects were studied in association with those of plant density. The research programme included 10 trials at three elevations (500, 750, 1200 m asl) in the spring and autumn seasons of 2 years wi th several contrasting varieties and different seed origins. Earlier p lanting in spring increased yield at 500 m asl. This effect was associ ated with better light use efficiency, higher rates of photosynthesis and more tubers per plant. At 750 m asl the effects of planting time i n spring were the opposite: later planting increased yield. At 1200 m asl effects on yield were absent. Effects of planting time in autumn o n yield were generally absent at all elevations, although plant stands at early planting were consistently and often severely reduced. Dense r planting produced yields that were higher per unit area because of l arger accumulated canopy cover but were lower per plant. Higher densit y consistently reduced plant stand. Density did not interact with plan ting time, not even in those experiments in which planting time also a ffected plant stands. Optimal planting time depended on the amount of rainfall prior to planting especially in the autumn season, but also o n cultivar and dormancy of the seed tubers used. The earlier variety y ielded more than the later variety at low elevation, whereas the later variety yielded more at higher elevation. It is necessary to use cult ivars that tuberize early to make better use of the limited growing pe riod at lower elevation. The large variation of dormancy that affects plant growth and yield is caused primarily by differences in accumulat ed temperature sum (as affected by planting time, elevation of growth and storage, and season), genetics (variety), and their interaction wi th cultivation techniques.