Effect of Light Intensity on Photosynthetic and Chlorophyll Fluorescence Characteristics of Cigar 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 (T300) and low light intensity (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 (Pn), stomatal conductance (Gs), and transpiration rate (Tr) compared toT100 (p<0.05), with increases of 86.3%, 69.3%, and 60.2%, respectively. T300 showed significantly higher chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll content, and Chl a/b ratio than T100, with increases of 29.8%, 16.8%, 25.3%, and 15.7%, respectively. Under photosynthetic active radiation (PAR) of 200-1800 µmol/(m²·s), the Pn of T300 was significantly higher than T100. The maximum net photosynthetic rate (Pnmax) and light saturation point (LSP) of T300 were 87.7% and 20.3% higher than T100 (p<0.05). The actual photochemical quantum yield of PSII (ΦPSII) in T300 was 51.3% higher than T100 (p<0.05), while the non-photochemical quenching coefficient (NPQ) was 55.8% lower, indicating superior overall photosynthetic capacity in T300. At 20 days post-leaf emergence, with leaf maturation, T100 showed substantial increases in Pn, chlorophyll content, Pnmax, and ΦPSII, narrowing the gap with T300, with no significant differences between treatments. However, T300 maintained a 24.2% higher LSP and 28.6% lower light compensation point (LCP) than T100 (p<0.05). At 28 days post-leaf emergence, no significant differences were observed between T300 and T100 in Pn, Pnmax, and chlorophyll content. T300 displayed 13.4% higher NPQ and 23.8% higher photochemical quenching coefficient (qP) than T100 (p<0.05). The photosynthetic light-response curve revealed that Pn of T300 was significantly lower than T100 under PAR of 1500-1800 µmol/(m²·s). T300 had 36.4% higher fresh weight and 115.4% higher dry weight than T100 (p<0.05) at full leaf length. High light intensity (T300) significantly enhanced photosynthetic capacity at early growth stage (12 days), but the difference was diminished at mid-growth stage (20 days), and T300 exhibited reduced efficiency under extreme high light intensity at late growth stag e(28 days). but overall, T300 maintained higher net photosynthetic rates, favoring dry matter accumulation in cigar tobacco leaves.