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BackgroundThe leaf age for harvesting flue-cured tobacco leaves is closely related to the quality of tobacco leaves, so an appropriate leaf age for harvesting is important for improving yield and quality of flue-cured tobacco, however, at present, there are few studies on effects of leaf age on physiological and biochemical changes during flue-curing and there is no clear standard of proper leaf ages for harvesting in production.ResultsIn the Yunnan tobacco-growing area, an experiment was carried from 2016 to 2017 and different leaf ages were set. The results demonstrate that leaf age has a significant on tissue cell gap, leaf age and flue-curing stages exert significant effects on upper epidermis, palisade and spongy tissue, and leaf thickness of tobacco leaves. The thicknesses of upper and lower epidermis as well as palisade and spongy tissues at different ages show an approximately W-shaped change trend during flue-curing. With the advance of flue-curing stages, contents of starch, chlorophyll, carotenoid, and water in tobacco leaves at different leaf ages decrease, while polyphenol and malondialdehyde (MDA) contents increase. The older the leaf, the faster the chlorophyll, carotenoid, and water contents reduce, while the faster the polyphenol and MDA content rise during flue-curing. The flue-cured tobacco leaves at 116 DAT (days after transplanting) show the highest contents of total nitrogen and nicotine, followed by 123 DAT and those at 130 DAT are the lowest; however, the contents of total sugar and reducing sugar demonstrate a contrary tendency, and the starch content at 116 DAT is much lower than those in the other two treatments. The proportion of superior tobacco, average price, yield, and output value of upper tobacco leaves at different leaf ages are the highest at 123 DAT. The highest sensory evaluation score is found at 123 DAT, while that at 130 DAT is significantly lower in comparison with the other two treatments.ConclusionsTobacco leaves harvested at 123 DAT are mature and exhibit a low degree of membrane lipid peroxidation, moderate chemical compositions, and high economic value. 123 DAT improves availability of tobacco leaves.

Long-term mono-cropping and excessive chemical fertilizer application reduce water-stable soil aggregate stability and soil N and C stocks in tobacco production. This study tested the hypothesis that rotation and fertilizer type affected the proportion of water-stable aggregates and aggregate-associated soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations. Two planting systems (tobacco mono-cropping and tobacco-maize rotation) with five fertilizer treatments (0 and 75 kg N ha −1 , 450 kg oil cake + 75 kg N ha −1 , 15,000 kg farmyard manure + 60 kg N ha −1 , and 3000 kg straw + 75 kg N ha −1) were established in 2007. After 10 years, rotation and fertilizer type significantly affected soil aggregates and associated SOC and TSN at different soil depths. Rotation significantly increased mean weight diameter (MWD) and geometric mean diameter (GMD); GMD increased with increasing soil depth. Rotation and organic fertilizer effectively increased the proportion of large macro-aggregates and MWD and GMD, while significantly reducing the proportion of micro-aggregates. The effect of fertilization type and rotation on the SOC content of large macro-aggregates was more obvious than that on TSN content, but there was no significant effect on SOC and TSN contents of aggregates < 250 μm. Rotation and organic fertilizer significantly increased SOC stock and total nitrogen of soil macro-aggregates (> 250 μm). However, this trend weakened with increasing soil depth and decreasing aggregate size. Rotation and organic fertilizers used in tobacco production stabilized the proportion of macro-aggregates in surface soil and maintained SOC and TSN better. Keywords Flue-cured tobacco. Rotation. Organic fertilizer. Water-stable aggregate. Yunnan Abbreviations SOC Soil organic carbon TSN Total soil nitrogen BD Bulk density MWD Mean weight diameter GMD Geometric mean diameter SOM Soil organic matter SOCS Soil organic carbon stock TSNS Total soil nitrogen stock * Congming Zou

Background Weather change in high-altitude areas subjects mature tobacco (Nicotiana tabacum L.) to cold stress, which damages tobacco leaf yield and quality. A brupt diurnal temperature differences (the daily temperature dropping more than 20 °C) along with rainfall in tobacco-growing areas at an altitude above 2450 m, caused cold stress to field-grown tobacco. Results After the flue-cured tobacco suffered cold stress in the field, the surface color of tobacco leaves changed and obvious large browning areas were appeared, and the curing availability was extremely poor. Further research found the quality of fresh tobacco leaves, the content of key chemical components, and the production quality were greatly reduced by cold stress. We hypothesize that cold stress in high altitude environments destroyed the antioxidant enzyme system of mature flue-cured tobacco. Therefore, the quality of fresh tobacco leaves, the content of key chemical components, and the production quality were greatly reduced by cold stress. Conclusion This study confirmed that cold stress in high-altitude tobacco areas was the main reason for the browning of tobacco leaves during the tobacco curing process. This adverse environment seriously damaged the quality of tobacco leaves, but can be mitigated by pay attention to the weather forecast and pick tobacco leaves in advance.