Planting climate causes of the style features for the chemical quality of flue-cured tobacco in Yunnan Province

Yunnan Province has a complex geographical environment with large altitude differences and significant climate variations. The province has the largest tobacco leaf output in China, with flue-cured tobacco-growing areas distributed at altitudes between 1 000–2 000 meters. To understand the relationships between the chemical quality of flue-cured tobacco and planting climates, we analyzed field experiments and climate data from 10 counties (districts) from 2017 to 2020 in Yunnan. Systematic cluster and principal component analyses were used to analyze the effects of the planting climate on the contents of nicotine, total nitrogen, and protein in tobacco leaves. The results showed that the climates of tobacco-growing areas of Yunnan Province during the tobacco field growth period (from mid-April to late August) were divided into six types, indicating climate diversity in planting flue-cured tobacco. Most planting climate types exhibited the main climatic characteristic of Yunnan’s low latitude plateau region, which is marked by lower average air temperature during the tobacco field growth period, with the average values of the hottest month being lower than 22.5 ℃. Simultaneously, during the vigorous growth and maturation period of flue-cured tobacco (mainly in summer, i.e., from June to August), the region experienced higher average relative humidity (exceeding 78%), increased rainfall (being greater than 5.5 mm per day on average), and reduced sunshine duration (less than 5.0 hours per day on average). In contrast, during the transplanting and root growth period (from mid-April to mid-May), the region experienced lower average relative humidity (below 66%), limited rainfall (less than 1.8 mm per day on average), and increased sunshine duration (more than 7.0 hours per day). In other words, in most tobacco-growing areas of the Yunnan Province, the characteristics of phased climate change during the flue-cured tobacco field growth season were similar. Higher average relative humidity and more rainfall during the tobacco field growth period, especially during the maturation period from July to August, were conducive to increasing the total nitrogen and protein contents in the tobacco leaves. Lower temperature conditions during the field growth period, particularly in the vigorous growth and mature periods, were associated with nicotine accumulation in tobacco leaves. In summary, the temperate, humid climate with inadequate sunlight during the vigorous growth and maturation period of flue-cured tobacco was the main reason for the relatively higher contents of the aforementioned three chemical components inside tobacco leaves in Yunnan. The climatic characteristics of the tobacco field growth season (temperate, humid climate with inadequate sunlight in summer, and relatively dry, sunny, warm climate in spring) had significant influences on the contents of the main chemical components (including total sugar, reducing sugar, potassium, petroleum ether extract, and the three aforementioned components) in tobacco leaves. These characteristics led to the development of distinctive chemical quality traits of tobacco leaves produced in most of the tobacco-growing areas in Yunnan. These results can be applied to the development of characteristic tobacco leaves under different climatic conditions. They would also help predict and evaluate changes in the chemical quality of tobacco leaves under varying climatic conditions, thereby providing a scientific basis for the tobacco industry to rationally allocate raw tobacco leaves from different regions.