Some effects of differing times of seedling emergence, population density and seed size on root-size variation in carrot populations

Abstract

SUMMARYIn three experiments the effects of population density, spread in time of seedling emergence and variation in seed size on carrot plant-to-plant root-size variation were investigated.In the first experiment, coefficient of variation, c.v., of root dry weight was only 27–33% when using uniform growing conditions, graded seed, low population density and synchronous seedling emergence (i.e. over 3–11 days). Allowing asynchronous emergence (i.e. over 8–21 days) increased the c.v. to 32–37%, whereas increasing population density from 25 to 400 plants/m2, but maintaining synchronous seedling emergence, produced a c.v. of 57–73%. Asynchronous emergence combined with high population density increased the c.v. even further to 79–97%. Spread in time of emergence was associated with 5–48% of the plant-to-plant root-size variation. Thus asynchrony of seedling emergence was a source of plant-to-plant root-size variation and this effect was magnified by high population density, but other sources of variation were also important. Statistical analysis showed that the effects of these other sources of root-size variation were no greater in populations where emergence had occurred over only 3 or 4 days than in those where emergence had occurred over 3 weeks.There was no effect on root-size frequency distribution in a second experiment in which the spread of seedling emergence was truncated from 19 to 14 days by spraying paraquat onto the first seedlings to emerge from field plots.In a third experiment seeds 1·6–2·0 or 1·0–1·25 mm in diameter were grown in pure or mixed populations under conditions similar to those of the synchronous emergence high-density treatment of Expt 1. Storage roots derived from the large seeds weighed 38% more than those derived from the small seeds. Thus differences in seed size caused some of the root-size variation not already linked with different seedling emergence times. However, the c.v. of root dry weight from all the plants in mixed seed population was not greater than the c.v.s of root dry weight of either of the pure seed populations.