Carbon and nitrogen partitioning in walnut seedlings during the acquisition of autotrophy through simultaneous13CO2and15NO3long-term labelling

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Abstract
Carbon (C) and nitrogen (N) autotrophy was studied in walnut seedlings (Juglans regia L.) grown in controlled conditions (22°C, 12 h, 90% RH) during the first two months of development. Seedlings were submitted to simultaneous 13CO2 (1.3 13C atom %) and 15NO3 (2.0 13N atom %) long-term labelling, to quantify the assimilation and partitioning of recent C and N in growing organs compared to the mobilization and use of C and N derived from the seed reserves. The first organ to develop, at day 8, was the taproot, which built Its biomass essentially during the first month with the help of C and N reserves of the seed. N autotrophy in the taproot occurred very early on, at day 13, and in growing lateral roots from day 20. The stem bearing the first leaves emerged at day 13. Until day 25 it derived N mainly from seed reserves while up to day 32 its C content was entirely heterotrophic in origin. The accumulation of labelled N in the leaves increased markedly from day 27, but C was almost entirely heterotrophic until day 29 despite the beginnings of photosynthetic activity. C export from leaves towards the stem began at day 32 and towards roots at day 43. During the first month the chief sink organs for labelled N were the roots. Thereafter, root growth decreased and at day 54 the actively growing shoot was the main sink for autotrophic C (80% of total C) and N (88% of total N).

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