Yield-density relationships of above- and belowground organs in Allium cepa var. aggregatum populations

The quantitative response of yield to density in plant populations has been an important focus of both theoretical research and empirical research. Most studies on yield-density effects have focused mainly on aboveground plant parts, and rarely on various plant organs and belowground parts. We tested the hypothesis that yield-density effects of belowground parts are different from those for aboveground parts. Bulbs of Allium cepa var. aggregatum were sown at five densities at the Pasture Ecology Research Station, western Jilin Province, China. We harvested populations at four different points in time and analyzed yield-density relationships of above- versus belowground parts and component organs. A hyperbolic model provided a very good fit to above- and belowground biomass, as well as the biomass of specific organs throughout the experiment. Aboveground and leaf biomass achieved constant final yield, but stand stem and root biomass increased monotonically with increasing sowing density. Belowground and specifically bulb yield was highest at intermediate densities at the later harvests. Constant final yield may be widely applicable to total biomass production by a population, but it does not apply to specific organs, such as stems, roots, or bulbs. Asymptotic leaf biomass reached its asymptote earlier than that of other aboveground parts. The effect of density on A. cepa var. aggregatum organs is a consequence of allocation of photosynthate to different organs in response to competition. Yield-density effects are different above- and belowground as a result of the different mechanisms of competition, constrained by the functional relationship between above- and belowground organs.