Climatic factors determine the yield and quality of Honghe flue-cured tobacco

Flue-cured tobacco (*Nicotiana tabacum* L.) is a crucial cash crop in Yunnan, China, contributing significantly to the national tobacco leaf yield. Yunnan's unique climate and geography produce high-quality tobacco. However, tobacco is sensitive to environmental conditions, with yield and quality influenced by climate, soil properties, and other factors. This study focuses on the Honghe Tobacco Zone, a region with diverse planting sites at varying altitudes and latitudes, resulting in different climatic conditions. The primary research questions were: (1) Do yield and quality differ among the three cultivars (K326, Yunyan87, and Honghuadajinyuan) across four planting sites? and (2) What are the dominant environmental variables (climatic or soil factors) influencing yield and quality?

Existing literature highlights the substantial impact of ecological conditions on tobacco yield and quality. Optimal light, temperature, and water are essential for tobacco growth. Low-temperature stress can lead to early flowering and gray speckles during curing, reducing economic value. Conversely, high-temperature forced ripening negatively impacts yield and quality. Temperature influences soil carbon mineralization, and tobacco's resistance mechanisms are temperature-dependent. Moisture significantly affects various leaf properties, such as biomass, leaf area, sugar, and nicotine content. Yunnan's rainfall is concentrated between May and September, making tobacco susceptible to water stress. Studies have shown varying responses of different tobacco varieties to water stress at the proteomic level. Climatic factors also influence disease incidence. Soil physicochemical properties also play a vital role; normal growth requires suitable soil pH, fertility, and microbial communities. Nutrient imbalances can hinder growth and metabolism. While previous research explored individual effects of climatic or soil factors, the combined and relative contributions remained unclear.

The experiment was conducted in the Hani-Yi Autonomous Prefecture of Honghe, Yunnan Province, China, which features a plateau subtropical monsoon climate. Three main local cultivars (K326, Yunyan87, and Honghuadajinyuan) were planted at four sites (Jianshui, Luxi, Mile, and Shiping) with diverse soil types and climatic conditions. Standard agricultural practices recommended by the Tobacco Institute of Yunnan were followed, including seedling breeding, transplanting (with a density of 15,000–16,500 plants/ha), irrigation (using various methods), crop rotation, fertilization (with varied N:P₂O₅:K₂O ratios for each cultivar), and disease/pest management. Middle leaves were collected in August for chemical analysis. Tobacco yield, economic value (based on grade classification using national standards), and the number of left leaves were recorded. Soil samples (0–20 cm depth) were collected before transplanting and analyzed for mechanical composition, pH, organic matter, alkali-hydrolyzed nitrogen, available phosphorus, available potassium, and chlorine content. Meteorological data (temperature, rainfall, sunshine hours, relative humidity) were obtained from the Yunnan meteorological observatory and the Honghe branch of the Yunnan Tobacco Company. Data analysis included ANOVA (with Duncan's test), Pearson correlation analysis, and RDA. The normality and homogeneity of residuals were checked, and rank transformations were applied where necessary.

Analysis of yield and economic value showed a cultivar × planting site interaction. K326 had a higher proportion of superior middle tobacco and left leaves than other cultivars but lower yield and economic value. Hongda showed the opposite trend. Yield and economic value were highest at the Mile site for K326 and Yunyan87. ANOVA of plant quality properties revealed significant effects of cultivar × planting site interactions on nicotine, reducing sugar, petroleum ether extract, starch, and the nitrogen/alkaloid ratio. Pearson correlation analysis showed that yield was negatively correlated with rainfall and relative humidity but positively correlated with temperature and sunshine hours. Soil properties and climatic factors influenced seven yield variables and eleven chemical composition variables. RDA revealed that climatic factors were the primary determinants of yield and quality. For K326, rainfall and alkali-hydrolyzed nitrogen were significant. For Honghuadajinyuan, temperature, clay content, and available phosphorus were most important. For Yunyan87, rainfall, sunshine hours, and soil chlorine content were key factors. Specific chemical components (e.g., potassium, chlorine, total sugar) showed relationships with both climatic and soil factors.

The findings demonstrate the significant influence of climatic factors on flue-cured tobacco yield and quality in the Honghe Tobacco Zone. The positive correlation between yield and sunshine hours and temperature aligns with the known requirements of tobacco for sufficient sunlight and warmth. The negative correlation with rainfall and humidity suggests that excessive moisture negatively affects growth and yield. The varied responses of different chemical components to climatic and soil factors highlight the complexity of tobacco's response to environmental conditions. The RDA results provide further support for the primary role of climatic factors, especially moisture and temperature, depending on the cultivar. The differences in responses among cultivars indicate that certain cultivars may be better suited to specific climatic conditions within the Honghe region. This highlights the importance of site-specific management strategies to optimize tobacco production.

This study comprehensively evaluated the influence of climatic and soil factors on the yield and quality of flue-cured tobacco in the Honghe Tobacco Zone. Climatic factors, particularly temperature and moisture, were identified as the primary drivers of yield and the chemical composition of tobacco leaves. Soil properties also played a role, with specific nutrients and soil texture influencing certain quality attributes. The study emphasizes the importance of site-specific management strategies and cultivar selection based on climatic conditions. Future research should focus on the detailed mechanisms of drought responses among different cultivars and optimization of agronomic practices for specific site and cultivar combinations.

The study was conducted in a specific region and may not be fully generalizable to other tobacco-growing areas with different soil types and climates. The study focused on one year's data; long-term data analysis would strengthen the conclusions. While the study considered several key environmental factors, other factors (e.g., specific soil microbes, pest pressure) could have also influenced yield and quality. The study primarily analyzed the middle leaves, and analysis of other leaf positions might reveal additional insights.