| Summary: | This study evaluates the effect of end-of-day blue (EOD B) light on the physiological response of lettuce (<i>Lactuca sativa</i>, Lobjoits Green Cos) at different phenological development stages. Plants were grown in a controlled environment growth chamber (day/night temperature 21 ± 2 °C; relative air humidity 60 ± 5%) under the light of light-emitting diodes (LEDs) consisting of 5% blue (B; 450 nm), 85% red (R; 660 nm), and 10% green (G; 530 nm) photosynthetic photon flux density (PPFD) at 200 µmol m<sup>−2</sup> s<sup>−1</sup> for 16 h d<sup>−1</sup> (BRG, control) for 8, 15, and 25 days (BBCH 12, BBCH 14, and BBCH 18, respectively). For the EOD B treatments, lettuce plants were additionally illuminated with 100% of B light at 30 and 60 µmol m<sup>−2</sup> s<sup>−1</sup> PPFD for 4 h d<sup>−1</sup> (B<sub>30</sub> and B<sub>60</sub>, respectively). The results show that EOD B light caused the elevated shoot elongation of lettuce plants regardless of their growth stages. However, leaf width increased only in more developed lettuce plants (BBCH 18). EOD B light negatively affected the development of new leaves and fresh weight, except for seedlings (BBCH 12). Most photosynthetic and spectral leaf indices also decreased when lettuce was treated with EOD B light, especially under the PPFD level of 60 µmol m<sup>−2</sup> s<sup>−1</sup>. Moreover, the changes in metabolic parameters such as DPPH free radical activity, free proline content, and H<sup>+</sup>-ATPase activity in lettuce showed a plant response to unfavorable conditions to EOD B light.
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