The relative importance of some sources of plant-to-plant root-weight variation in a field-grown carrot crop was investigated in two experiments sown in contrasting soil conditions but using the same seed lot and experimental design. In each experiment the following possible causes of root-weight variation were recorded on an individual plant basis: umbel order and size of seed, sowing depth, time taken for seedling emergence, length of cotyledons at emergence, distance to nearest neighbours within a row and row position within the seed bed. The relative importance of these differing sources of root-weight variation was assessed by determining how much of the variation in In (root dry weight) they accounted for at two harvests in each experiment.
In the first experiment, carrot seeds were sown in warm, moist conditions in May. These conditions gave a near-ideal emergence, 82% of seedlings emerging over only 5 days. Thirteen days after sowing, the most important source of root-weight variation, out of the variables measured, was position of the row within the bed. However, approximately two-thirds of the root-weight variation could not be attributed to any of the measured variables.
In the second experiment carrot seeds were sown in hot, drying conditions in July. The subsequent spread of seedling emergence extended over approximately 40 days. The total sum of squares of In (root dry weight) in this experiment was six times greater than the sum of squares in the first experiment. At 105 days, 61% of the rootweight variation in Expt 2 was accounted for by differences in time of seedling emergence. Differences in cotyledon length, measured when fully expanded, accounted for 24% and sowing depth differences accounted for 19% of the root-weight variation. Differences in umbel order, seed size and distance to the nearest neighbouring plants within a row never accounted for more than 8% of the sum of squares of In (root dry weight) in either experiment. Thus these variables were not important sources of rootweight variation. In contrast variables which affected the time of seedling emergence, such as sowing depth and physical conditions around the seed, were major sources of root-weight variation.