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.