Low Light Alters the Photosynthesis Process in Cigar Tobacco via Modulation of the Chlorophyll Content, Chlorophyll Fluorescence, and Gene Expression

Shading or low light (LL) conditions are a key and necessary cultivation technique in cigar wrapper tobacco production. However, the effect of low light on the photosynthesis in cigar tobacco is not clear. Therefore, this study is designed to know the photosynthesis of cigar tobacco under different light intensities (T200, T100, and T50 μmol m⁻² s⁻¹). The results reveal that under low light, T50 especially improved the light interception and increased carbon utilization, as witnessed by a higher specific leaf area and lower specific leaf weight. Low light intensity caused better light interception and carbon utilization in cigar tobacco leaves, and thus thinner leaves are more able to use low light efficiently. The chlorophyll content is related to the photosynthesis process; thus, LL affected the photosynthesis process by lowering the chlorophyll content. Similarly, LL also altered the photosynthetic efficiency by lowering the QY_Lss, qP_Lss, and Rfd_Lss. Additionally, higher expression of <i>Lhcb4.2</i>, <i>Lhcb6</i>, <i>PsbA</i>, <i>PsbB</i>, and <i>PsbD</i> under low light, especially T50, shows that the PSII and antenna proteins complex efficiently utilized the absorbed energy for photosynthesis. Finally, the lower photosynthesis, particularly in T50, is attributed to the downregulation of genes related to NADPH production (<i>petH</i>) and the rubisco enzyme synthesis-related gene (<i>rbcs</i>) for CO₂ fixation in the Calvin cycle. Overall, the results show that the photosynthesis is decreased under LL intensities which might be related to lower chlorophyll content and downregulation of <i>petH</i> and <i>rbcs</i> genes.