Partitioning of 14C Labelled Assimilates in Arctium tomentosum

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Abstract
Plants of the biennial Arctium tomentosum were grown from seed to seed-set in an open field under three different treatments: control plants receiving full light intensity, plants with a leaf area reduced by 45 per cent, and shaded plants receiving only 20 per cent of natural illumination. At various stages of development the youngest fully expanded leaf of one plant in each treatment was exposed to 14CO2 for half an hour. Subsequent distribution of labelled assimilates in various plant parts was determined after eight hours. In the first year, the most dominant sink was the tap root irrespective of variation in assimilate supply. During the production of new vegetative growth in the second season, a larger amount of radioactive photosynthate was recovered from above ground parts, especially during formation of lateral branches. Seed filling consumed 80–90 per cent of labelled carbon exported from the exposed leaf. In the second year, the most pronounced difference between treatments was in the degree of apical dominance, being highest in shaded plants and lowest in the plants with cut leaves. Results from 14C experiments agreed fairly well with a ‘partitioning coefficient’ derived from a growth analysis of plants grown independently under the same experimental conditions. Reasons for discrepancies between the 14C results and the partitioning coefficient are discussed.