**1. Introduction**

Light, temperature, humidity, concentration of CO2, and nutrition supply are vital factors that influence photosynthesis of plants. Light intensity, quality, photoperiod, and direction influence plant growth, development, photomorphogenesis, and anatomical structure as energy sources and regulatory signals [1–3]. Previous studies have proven that light intensity can adjust formation of the chloroplast protein complex, electron transport, and quantum yield between photosystems I (PSI) and II (PSII) [4,5].

Grafting as an efficient technique can be applied to overcome the obstacles of continuous cropping and improve yield, quality, and stress resistance in various plant species including fruit trees, vegetables, and ornamental flowers [6]. In addition to rootstock, affinity of the scion, and grafted methods, the surviving rates, formation of new vascular bundles, and biological properties of grafted seedlings are also related to environmental conditions and management in healing and acclimatization processes [7]. Light-emitting diodes (LEDs) are widely used in facility horticulture as an artificial light source for production of high-quality seedlings. LEDs have shown promising foreground applications because of their advantages of being light-weight, having a small volume, having a specific wavelength, they are easy to integrate, and they have low heat dissipation. Previous studies have shown that the LEDs play a vital role in toughening grafted seedlings in healing and acclimatization processes [8,9].

Generally, stocky plug seedlings are preferred by crop growers for the convenience of transportation and to insure a higher survival ratio and enhanced development after transplanting. Each country has different standards for evaluating the quality of seedlings. In South Korea, it is generally agreed that high-quality seedlings are those without overgrowth, with short and not lodging stems and leaves, and with well-developed roots that hold the medium in the plug tray. Light intensity has been proven to affect plant height and biomass growth [10,11]. Leaf area also changes with light intensity as plants use leaves to absorb light energy [12,13]. Moreover, higher light intensities may result in more root biomass so the absorption of water and mineral elements could be promoted [14]. Therefore, seedlings with good quality could be obtained by supplementing them with optimal light intensity.

This study was carried out aimed at finding an optimal light intensity to improve the quality of two kinds of grafted tomato seedlings. After grafting, well-healed grafted seedlings were moved to and cultivated for an additional 10 days in a glasshouse, which had natural lighting and supplemental lighting. The supplementary light intensities were set at 50, 100, or 150 <sup>μ</sup>mol·m−2·s−<sup>1</sup> photosynthetic photon flux density (PPFD) supplied from mixed LEDs (W1R2B1) to further improve the quality of seedlings. After treatment, expressions of two photosynthesis-related genes (*PsaA* and *PsbA*) and the corresponding proteins were analyzed to evaluate the capacity of photosynthesis.

#### **2. Materials and Methods**

#### *2.1. Plant Materials*

Two cultivars of tomato *(Solanum lycopersicum* L.) 'Super Sunload (SS)' and 'Super Dotaerang (SD)' were selected as the scions and tomato 'B-Blocking' as a common rootstock. Cultivars 'SS' and 'SD', widely used for grafting, were purchased from Sakata Seed Korea Co., Ltd. (Seoul, Korea) and Koregon Co., Ltd. (Anseong, Korea), respectively. The 'SS' has high fruit firmness with an average fruit weight of 220–240 g. The primary advantages of 'SS' are a low ratio of fruit deformity and resistance to high temperatures. The 'SD' is a cultivar with disease resistance and has a high sugar content, firm flesh, and a low deformity ratio. The seeds of these cultivars were sown into 40 square cell plug trays at the same time they were filled with commercial growing medium (Super Mix, NongKyung Co., Jincheon, Korea). The scion and rootstock were cleft-grafted at 20 d after sowing when both scion cultivars had two same sizes of compound leaves. After grafting, the grafted seedlings were immediately transported to a healing chamber, which had a temperature of 23 ◦C and relative humidity of 95% to 100% throughout the healing period. Five days later, the well-healed grafted seedlings were moved from the healing chamber to the greenhouse for light treatment when the average plant height was about 8 ± 0.7 cm and each seedling had two and a half true leaves.
