Background: The 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. Results: In 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. Conclusions: Tobacco 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.

Background: Manganese (Mn) stress is an important reason for forming tobacco ( Nicotiana tobacum L. ) leaf browning, leading to low yield and quality of tobacco leaves. The mechanism of flue-cured tobacco varieties, Honghuadajinyuan (Hongda) and K326 tobacco leaves browning by Mn were explored. The two tobacco varieties were planted in red soil and sandy soil and Mn concentration of 0, 250, 500, 750 and 1000 mg/kg were applied. The research investigate the traits of agronomic, physiological and economic of flue-cured tobacco. Results: As Mn application rate increased, browning scale increased while plant height, root, stem and total weight, content of chlorophyll decreased. Activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) showed a rising parabolic formation. As Mn application rate increased, the content of malondialdehyde (MDA) and relative conductivity increased. Peroxidase (POD) activity continually increased with Mn application rate in K326 variety, while in Hongda variety, POD activity first increased and then decreased in sandy soil, while it continuously rose in red soil. Mn concentration and browning scale were significantly and negatively correlated with height, soil and plant analysis development (SPAD), CAT, total weight, average priceand yield, but significantly positively correlated with total plant Mn content, POD and MDA. Plant Mn concentration was significantly and positively correlated with browning scale. This study founded that SOD, CAT, APX, POD enzyme activity and MDA content in tobacco leaves affected by Mn concentration in soils, cause the change of chlorophyll content, agronomic and economic traits of flue-cured tobaccos. Conclusion: The mechanism of Mn stress to the physio-biochemical of tobacco varieties would be useful for controlling browning tobacco leaves by agronomic strategies and regarding as markers for Mn tolerance in breeding.