**2. Materials and Methods**

### *2.1. Plant Materials, Growth Conditions and Experimental Treatments*

Tomato (*Solanum lycopersicum* L.) cv 'Micro-Tom' seeds were purchased from the Ball Horticulture Company (West Chicago, IL, USA). Healthy seeds were selected and sown on a petri dish with distilled water. The germinated seedlings were transferred to 72-well trays and cultured under artificial climate chamber conditions: temperature 28 ◦C/22 ◦C (14 h day/10 h night), humidity 80% and light intensity 600 μmol m−2s−1. When the plants had four fully expanded leaves, uniformly grown tomato seedlings were planted in a hydroponic tank filled with half-strength Japanese Yamazaki tomato formula nutrient solution [22]. After five days of seedling culture in hydroponics, the following treatments were applied: (1) CK (control), Yamazaki formula nutrient solution (Fe concentration was 100 μM); (2) LF, low-Fe nutrient solution (Fe concentration was 10 μM); (3) Spd, Yamazaki formula nutrient solution (100 μM Fe) + 0.25 mM Spd foliar spray; (4) LF + Spd, low-Fe (10 μM) nutrient solution + 0.25 mM Spd foliar spray. The Spd was purchased from the Beijing Solarbio Technology Company. Both sides of the tomato leaves were sprayed with freshly prepared Spd solution (approximately 10 mL per plant). Low-Fe stress was imposed 1d after the Spd treatment. Foliar-spraying of Spd was repeated every two days. The control tomato plants were foliar-sprayed with an equal volume of distilled water. The nutrient solution was changed every three days, the pH value was adjusted to 6.0 ± 0.2 and an intermittent supply of oxygen was provided using an aeration pump. On the 10th day of treatments, unless otherwise stated, samples were collected/used for various analyses such as photosynthetic fluorescence indicators, osmoregulatory substance content, organic acid and polyamine contents and RNA sequencing. Biomass measurements were performed on day 15 of treatment.
