Nitrate ( 15 NO 3 ) limitation affects nitrogen partitioning between metabolic and storage sinks and nitrogen reserve accumulation in chicory (Cichorium intybus L.)

In chicory, we examined how $\mathrm{N}{\mathrm{O}}_{3}^{-}$ supply affected $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake, N partitioning between shoot and root and N accumulation in the tuberized root throughout the vegetative period. Plants were grown at two $\mathrm{N}{\mathrm{O}}_{3}^{-}$ concentrations: 0.6 and 3 mM. We used 15N-labelling/chase experiments for the quantification of N fluxes between shoot and root and for determining whether N stored in the tuberized root originates from N remobilized from the shoot or from recently absorbed $\mathrm{N}{\mathrm{O}}_{3}^{-}$. The rate of ^{15}\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake was decreased by low $\mathrm{N}{\mathrm{O}}_{3}^{-}$ availability at all stages of growth. In young plants (10—55 days after sowing; DAS), in both $\mathrm{N}{\mathrm{O}}_{3}^{-}$ treatments the leaves were the strongest sink for 15N. In mature (tuberizing) plants, (55—115 DAS), the rate of ^{15}\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake increased as well as the amount of exogenous N allocated to the root. In N-limited plants, N allocation to the tuberized root relied essentially on recent N absorption, while in N-replete plants, N remobilized from the shoot contributed more to N-reverse accumulation in the root. In senescing plants (115—170 DAS) the rate of ^{15}\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake decreased mainly in N-replete plants whereas it remained almost unchanged in N-limited plants. In both $\mathrm{N}{\mathrm{O}}_{3}^{-}$ treatments the tuberized root was the strongest sink for recently absorbed N. Remobilization of previously absorbed N from shoot to tuberized root increased greatly in N-limited plants, whereas it increased slightly in N-replete plants. As a consequence, accumulation of the N-storage compounds vegetative storage protein (VSP) and arginine was delayed until later in the vegetative period in N-limited plants. Our results show that although the dynamics of N storage was affected by $\mathrm{N}{\mathrm{O}}_{3}^{-}$ supply, the final content of total N, VSP and arginine in roots was almost the same in N-limited and N-replete plants. This indicates that chicory is able to build up a store of available N-reserves, even when plants are grown on low N. We also suggest that in tuberized roots there is a maximal capacity for N accumulation, which was reached earlier (soon after 100 DAS) in N-replete plants. This hypothesis is supported by the fact that in N-replete plants despite $\mathrm{N}{\mathrm{O}}_{3}^{-}$ av