Root Development and Source-Sink Relations in Carrot, Daucus carota L: II. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated bycomparing ¹⁴CO₂ fixation rates, the partitioning of ¹⁴C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced ¹⁴CO₂ fixation by 28–45% but leafspecific activity (¹⁴C assimilation g-¹ leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of ¹⁴Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of ¹⁴C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of ¹⁴C betweensoluble and insoluble carbohydrate fractions when plants werefed ¹⁴CO₂ several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants.