| Summary: | Low temperature combined with low light (LL stress) is a typical environmental stress that limits peppers’ productivity, yield, and quality in northwestern China. Glycine betaine (GB), an osmoregulatory substance, has increasingly valuable effects on plant stress resistance. In this study, pepper seedlings were treated with different concentrations of GB under LL stress, and 20 mM of GB was the best treatment. To further explore the mechanism of GB in response to LL stress, four treatments, including CK (normal temperature and light, 28/18 °C, 300 μmol m<sup>−2</sup> s<sup>−1</sup>), CB (normal temperature and light + 20 mM GB), LL (10/5 °C, 100 μmol m<sup>−2</sup> s<sup>−1</sup>), and LB (10/5 °C, 100 μmol m<sup>−2</sup> s<sup>−1</sup> + 20 mM GB), were investigated in terms of pepper growth, biomass accumulation, photosynthetic capacity, expression levels of encoded proteins <i>Capsb</i>, cell membrane permeability, antioxidant enzyme gene expression and activity, and subcellular localization. The results showed that the pre-spraying of GB under LL stress significantly alleviated the growth inhibition of pepper seedlings; increased plant height by 4.64%; increased root activity by 63.53%; and decreased photoinhibition by increasing the chlorophyll content; upregulating the expression levels of encoded proteins <i>Capsb A</i>, <i>Capsb B</i>, <i>Capsb C</i>, <i>Capsb D</i>, <i>Capsb S</i>, <i>Capsb P1</i>, and <i>Capsb P2</i> by 30.29%, 36.69%, 18.81%, 30.05%, 9.01%, 6.21%, and 16.45%, respectively; enhancing the fluorescence intensity (OJIP curves), the photochemical efficiency (Fv/Fm, Fv′/Fm′), qP, and NPQ; improving the light energy distribution of PSΠ (Y(II), Y(NPQ), and Y(NO)); and increasing the photochemical reaction fraction and reduced heat dissipation, thereby increasing plant height by 4.64% and shoot bioaccumulation by 13.55%. The pre-spraying of GB under LL stress also upregulated the gene expression of <i>CaSOD</i>, <i>CaPOD</i>, and <i>CaCAT</i>; increased the activity of the ROS-scavenging ability in the pepper leaves; and coordinately increased the SOD activity in the mitochondria, the POD activity in the mitochondria, chloroplasts, and cytosol, and the CAT activity in the cytosol, which improved the LL resistance of the pepper plants by reducing excess H<sub>2</sub>O<sub>2</sub>, O<sub>2</sub><sup>−</sup>, MDA, and soluble protein levels in the leaf cells, leading to reduced biological membrane damage. Overall, pre-spraying with GB effectively alleviated the negative effects of LL stress in pepper seedlings.
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