The Relationship between Cyanide-Resistant Root Respiration and the Storage of Sugars in the Taproot inDaucus carotaL.

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Abstract
Dry weight increase, root respiration, photosynthesis, and shoot dark respiration were measured prior to and during the development of the taproot of carrot (Daucus carota L., var. Amsterdamse Bak) which stores sucrose and reducing sugars. Before storage of sugars in the taproot started, root respiration was high and up to 50% inhibited by salicyl hydroxamic acid (SHAM), indicating a high activity of the alternative pathway. The onset of sugar storage in the taproot coincided with a sharp decrease in alternative pathway activity. When sugars were stored in the taproot, the alternative pathway in the taproot was not operative, although present. It was concluded that the alternative pathway only consumes sugars that are not used for energy production in growth or maintenance processes, for carbon skeletons, osmoregulation or storage, in accordance with the energy overflow model. A positive quantitative relationship exists between the storage of sugars in the taproot and the efficiency of root respiration. After day 32 the amount of daily produced carbohydrates transported to the root system increased and these sugars were stored in the taproot. In this period less sugars were invested in structural shoot growth. In the present investigation no positive influence from the developing sink is found on photosynthetic rate or net assimilation rate. Both rates decrease with increasing age of the plants, although increasing amounts of sugars were stored in the taproot.