*2.2. Light Treatments*

Mixed LEDs (W1R2B1) (Custom made, Sung Kwang LED Co., Ltd., Incheon, Korea), which were selected as the best artificial light source in precedent research [15], were used as the sole light source. The grafted seedlings were cultivated for an additional 10 d in a glasshouse with a daily light integral (DLI) of 11.59 mol·m−2·d−<sup>1</sup> from the sun and supplemental lighting for 16 h/d. The supplementary

light intensities were set at 50, 100, or 150 <sup>μ</sup>mol·m−2·s−<sup>1</sup> PPFD (henceforth shortened as *50*, *100*, and *<sup>150</sup>*). The DLIs of the three supplementary light treatments were 2.88, 5.76, and 8.64 mol·m−2·d<sup>−</sup>1, respectively. The distance between the LEDs and seedlings was 50 cm. Light intensity was measured at the horizontal position at the seedling stem. The culture environment had 30/25 ◦C day/night temperatures, 70% ± 5% relative humidity (RH), with a natural photoperiod of 14 h. A portable photo/radiometer (Type specification: HD-2102.2, Delta, OHM, Padova, Italy) was used to measure the photon flux density. Different light intensities were controlled by adjustable electric currents.

#### *2.3. Stomatal Conductance*

Mature and expanded leaves were selected for the measurement of stomatal conductance. Measurements were carried out during the day (9:00–10:00) on a porometer (Type specification: Sc-1 Porometer, Decagon Devices Inc., Pullman, WA, USA).

#### *2.4. Quantitative Real-Time PCR Analysis*

Tomato leaves were ground under RNase-free conditions after being frozen in liquid nitrogen. An Easy-Spin total RNA extraction kit (Type Specification: iNtRON Biotechnology, Seoul, Republic of Korea) and a GoScript Reverse Transcription System (Specification: Promega, Madison, WI, USA) were used for RNA extraction and cDNA synthesis, respectively, following the manufacturer's instructions. The Rotor-Gene Q detection system (Qiagen, Hilden, Germany) was applied in the examination of gene expression. Reaction volumes (20 μL) contained 1 μL cDNA, 0.5 μL primer (10 μM), 10 μL of 2× AMPIGENE qPCR Green Mix Lo-ROX (Enzo life Sciences Inc., Farmingdale, NY, USA), and 8 μL ddH2O. The 2−Ct method was applied to determine the relative gene expressions, and the *18S* gene was used as housekeeping gene. All primers used are shown in Supplementary Table S1.
