Root Development and Source-Sink Relations in Carrot,Daucus carotaL

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Abstract
Physiological responses to root pruning were investigated by comparing 14CO2 fixation rates, the partitioning of 14C-labelled assimilate, and soluble and insoluble carbohydrate levels in the leaves of carrot plants following the removal of some of the fibrous roots, or fibrous roots and part of the tap root. Root pruning reduced 14CO2 fixation by 28–45% but leaf specific activity (14C assimilation g-1 leaf fresh weight) was unchanged. The proportion of total assimilate exported to the root system increased following root pruning and this was at the expense of the developing leaves. In younger plants (where the tap root received 10% of the assimilate) the supply of 14C to 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 storage organ), reduced its sink capacity and resulted in slightly greater retention of 14C in the mature leaves. Greater concentrations of insoluble carbohydrate were found in the mature leaves following root pruning but soluble sugar content was unaffected. Only small differences were observed in the distribution of 14C between soluble and insoluble carbohydrate fractions when plants were fed 14CO2 several days after the root pruning operations. These physiological responses were mainly associated with the removal of fibrous roots and support the view that the fibrous root system is more important than the developing storage organ in regulating growth in young carrot plants.

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