NITRATE ((NO3)-N-15) LIMITATION AFFECTS NITROGEN PARTITIONING BETWEENMETABOLIC AND STORAGE SINKS AND NITROGEN RESERVE ACCUMULATION IN CHICORY (CICHORIUM-INTYBUS L)

In chicory, we examined how NO3- supply affected NO3- uptake, N partit ioning between shoot and root and N accumulation in the tuberized root throughout the vegetative period. Plants were grown at two NO3- conce ntrations: 0.6 and 3 mM. We used N-15-labelling/chase experiments for the quantification of N fluxes between shoot and root and for determin ing whether N stored in the tuberized root originates from N remobiliz ed from the shoot or from recently absorbed NO3-. The rate of (NO3-)-N -15 uptake was decreased by low NO3- availability at all stages of gro wth. In young plants (10-55 days after sowing; DAS), in both NO3- trea tments the leaves were the strongest sink for N-15. In mature (tuberiz ing) plants, (55-115 DAS), the rate of (NO3-)-N-15 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 rece nt N absorption, while in N-replete plants, N remobilized from the sho ot contributed more to N-reserve accumulation in the root. In senescin g plants (115-170 DAS) the rate of (NO3-)-N-15 uptake decreased mainly in N-replete plants whereas it remained almost unchanged in N-limited plants. In both NO3- 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, wh ereas it increased slightly in N-replete plants. As a consequence, acc umulation of the N-storage compounds vegetative storage protein (VSP) and arginine was delayed until later in the vegetative period in N-lim ited plants. Our results show that although the dynamics of N storage was affected by NO3- supply, the final content of total N, VSP and arg inine 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 i n tuberized roots there is a maximal capacity for N accumulation, whic h was reached earlier (soon after 100 DAS) in N-replete plants. This h ypothesis is supported by the fact that in N-replete plants despite NO 3- availability, N accumulation ceased and significant amounts of N we re lost due to N efflux.