Plant Growth Conditions
For the phenotyping, 190 Arabidopsis thaliana accessions were grown on soil containing limiting, intermediate and optimal amounts of nitrogen and as described in Pandey et al. 2019. The accessions that were obtained from the Nottingham Arabidopsis stock centre (NASC) and are listed in Table S1. The plants were grown on pots (⌀ = 6 cm, h = 5.5 cm) containing the same mass of adjusted white peat soil (Gramoflor GmbH). The seeds were germinated for one day under short day (8h/16h) condition at 20°C/16°C and 150 µE m-2s-1 of light intensity. To ensure flowering, all accessions were vernalized for eight weeks at 4°C under long days and light intensity of 7 µE m-2 s-1. After this, plants were transferred to new pots. Four plants in two pots (n=4) were grown under long days at 21°C/17°C (day/night) and 150 µE m-2 s-1 of light intensity. Early rosette (ERD) diameter was the rosette diameter ten days after pricking and the final rosette diameter (FRD) at the time of flowering. Rosette diameters were measured from pictures taken at these stages using ImageJ (Schneider, Rasband, & Eliceiri, 2012). Flowering time (FT) was scored at the number of days from pricking of the plants to the day of first open flower. Seed yield (YIE) was quantified from two plants together by dividing the total weight of the seeds with the weight of hundred seeds and multiplying by 100. In all experiments, to minimize stochastic variation the replicates were distributed across different trays and the trays were periodically rotated in the growth chamber.
For the adaptability experiments, 19 accessions were selected based on their ERD and FT plasticity to N (Table S8) and were germinated and vernalized as described above. For the experiments under short days, long days (control condition), and low light, five pots with two plants each were prepared for every accession and condition (n = 10). For the experiments under high light, five plants were transferred to big pots (⌀ = 13 cm, h = 10 cm) and in the polytunnel (natural light and temperature), three plants were transferred to big pots (⌀ = 13 cm, h = 10 cm) in both conditions resulting in n=15. For the experiments under short day (8h/16h) and the long day control (16h/8h), the temperature was 20°C/16°C (day/night) and light intensity 120 µE m-2 s-1. In the low and high light the plants were grown at long-day 16h/8h with temperature 21°C/17°C day/night cycle with light intensity of 20 µE m-2s-1 (low light) and 750 µE m-2s-1 (high light). The experiment in the polytunnel under natural conditions was performed during summer 2020.