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

Abstract

The effect of nitrogen supply to chicory plants on carbon partitioning between shoot, root and tuberized root was studied at different stages of vegetative growth, using long-term 13CO2 labelling-chase experiments. This approach was complemented by measurement of storage carbohydrates and activities of enzymes involved in root sucrose metabolism (sucrose-sucrose fructosyl transferase (SST), sucrose synthase, invertase). In both young and mature plants, low NO3- resulted in a 30–35% decrease in 13C assimilation. However, the partitioning of 13C between shoot and root was affected differently at different stages of development. In young plants, in which carbohydrates were being used for structural root and shoot growth, neither 13C shoot/13C root ratio nor root activities of the above enzymes were modified by NO3- supply. In contrast, in mature plants storing large amounts of carbohydrates as fructan in the tuberized root, low NO3- caused the ratio to decrease from 0.6 to 0.2, despite unchanged net flux of 13C from shoot to root. The extractable activity of SST was elevated in mature plants, compared to young plants, at both low and high NO3-⁠, consistent with its role in fructan synthesis. However, mature plants grown at low NO3- exhibited SST activity double that of plants grown at high NO3-⁠. From these results, it is concluded that the observed decrease in shoot/root dry weight ratio at low NO3- supply is caused by increased utilization of carbohydrates for storage due to elevated root SST activity.