Effect of Light Intensity on Photosynthetic and Chlorophyll Fluorescence Characteristics of Cigar Tobacco Leaves

To elucidate the effects of light intensity on photosynthetic and chlorophyll fluorescence characteristics of cigar leaves, a study was conducted using the cigar variety “Guyin No.4” as the experimental material. Two treatments were applied: high light intensity (300±12) µmol/(m²·s), T300 and low light intensity (100±12) µmol/(m²·s), T100. Measurements of photosynthetic and chlorophyll fluorescence characteristics were measured at 12, 20 and 28 days after the 10th leaf position became visible, and the biomass was determined when the leaves reached full length. At 12 days post-leaf emergence, T300 exhibited significantly higher net photosynthetic rate (P_n), stomatal conductance, and transpiration rate compared to T100, with increases of 86.3%, 69.3%, 60.2%, respectively. T300 showed significantly higher chlorophyll a, chlorophyll b, total chlorophyll content, and Chl a/b ratio than T100, with increases of 29.8%, 16.8%, 25.3%, 15.7%, respectively. Under photosynthetic active radiation (PAR) of 200-1800 µmol/(m²·s), the P_n of T300 was significantly higher than T100. The maximum net photosynthetic rate (P_nmax) and light saturation point (LSP) of T300 were 87.7% and 20.3% higher than T100. The actual photochemical quantum yield of PSII (ΦPSII) in T300 was 51.3% higher than T100, while the non-photochemical quenching coefficient (NPQ) was 55.8% lower than T100, indicated superior overall photosynthetic capacity in T300. At 20 days post-leaf emergence, with leaf maturation, T100 showed substantial increases in P_n, chlorophyll content, P_nmax, and ΦPSII, narrowing the gap with T300, with no significant differences between treatments. However, T300 maintained 24.2% higher LSP and 28.6% lower light compensation point than T100. At 28 days post-leaf emergence, no significant differences were observed between T300 and T100 in P_n, P_nmax, and chlorophyll content. T300 displayed 13.4% higher NPQ and 23.8% higher photochemical quenching coefficient than T100. The photosynthetic light-response curve revealed that P_n of T300 was significantly lower than T100 under PAR of 1500-1800 µmol/(m²·s). T300 had 36.4% higher fresh mass and 115.4% higher dry mass than T100 at leaf length fixed. High light intensity (T300) significantly enhanced photosynthetic capacity at early growth stage (12 days), the difference was diminished at mid-growth stage (20 days), and T300 exhibited reduced efficiency under extremly high light intensity at late growth stage (28 days). Overall, T300 maintained higher net photosynthetic rates, which was beneficial for biomass accumulation in cigar tobacco leaves.