Stage of development is an important determinant in the effect of nitrate on photoassimilate (13C) partitioning in chicory (Cichorium intybus)

The effect of nitrogen supply to chicory plants on carbon partitioningbetween shoot, root and tuberized root was studied at differentstages of vegetative growth, using long-term ¹³CO₂ labelling-chaseexperiments. This approach was complemented by measurement ofstorage carbohydrates and activities of enzymes involved inroot sucrose metabolism (sucrose-sucrose fructosyl transferase(SST), sucrose synthase, invertase). In both young and matureplants, low resulted in a 30–35% decrease in ¹³C assimilation. However, the partitioningof ¹³C between shoot and root was affected differently at differentstages of development. In young plants, in which carbohydrateswere being used for structural root and shoot growth, neither¹³C shoot/¹³C root ratio nor root activities of the above enzymeswere modified by supply. In contrast, in mature plants storing large amounts of carbohydratesas fructan in the tuberized root, low caused the ratio to decrease from 0.6 to 0.2, despiteunchanged net flux of ¹³C from shoot to root. The extractableactivity of SST was elevated in mature plants, compared to youngplants, at both low and high , consistent with its role in fructan synthesis. However, matureplants grown at low exhibited SST activity double that of plants grown at high . From these results, it is concluded that the observeddecrease in shoot/root dry weight ratio at low supply is caused by increased utilization of carbohydratesfor storage due to elevated root SST activity. Key words: Chicory, nitrate, ¹³C, shoot/root ratio, fructans, SST