Experimental design and sampling
The experimental design is schematized in Fig. S1a. After emergence (1 week after sowing), plants were cultivated for 2 weeks under specific nutrient conditions (described above). Three weeks after emergence, plants had one true leaf pair, and after four weeks, there were two leaf pairs. These two pairs are referred to as “young” and “old” (Fig. S1b). There were two sampling campaigns: sampling 1 two weeks after emergence (3 weeks old plants), and sampling 2 three weeks after emergence (4 weeks old plants). Two weeks and four days after emergence, half of the plants were labelled with 15N-nitrate (nutrient solution of exactly the same composition, with just nitrate replaced to its 15N form) while the other half was kept at natural abundance. The day before sampling, photosynthesis and dark respiration (reported in Fig. 2) were measured on the same plants. Upon sampling, plants were measured for size, fresh weight, leaf surface, and dissected and kept in liquid nitrogen for analyses. Roots were sampled after having removed sand and washed with water. In practice, roots were washed, rapidly dried with absorbing paper and quenched in liquid nitrogen within 2-3 min. Extractions for metabolomics and proteomics analyses were performed on fresh material. Isotope and ionomics analyses were performed on freeze-dried material.