Nicotine conversion results in abnormally high levels of nornicotine which lead to high nitrosonornicotine (NNN) content and off-taste of smoke. Nicotine conversion is a worldwide problem in burley tobacco production and has been the focus in burley research in recent years. Nicotine conversion level in Sichuan burley tobacco was the highest with the percent nicotine conversion being 62.4%, followed by Chongqing, Hubei and Yunnan. The leading variety Xuanhan-5NL had serious problem of nicotine conversion, with the proportion of converters being 96% of the total plants. In the populations of hybrid Dabai 1, Dabai 2 and Eyan 1, a substantial amount of converters were also found. Newly developed hybrids Dabai 3 and Dasuo 24 had serious problems of nicotine conversion with almost 100% of the plants being converters. Further study using ethylene early identification method was conducted to investigate the contribution of different parental lines to nicotine conversion in Dabai series hybrids. Both parents of Dabai 1 contributed to the conversion level in the hybrid. The male parent of Dabai 2 was the main contributor of conversion gene to the hybrid. The male parent of Dabai 3, had extremely high converter proportion and conversion level, and was almost the exclusive contributor of conversion gene to Dabai 3. Improved Dabai series hybrids developed by selecting and crossing non-converter parent plants, showed significantly reduced converters in next populations. Genetic improvement for low conversion trait by early non-converter parent selections was very effective.
Key words: Burley, tobacco, nicotine to nornicotine conversion, improvement, hybrid.
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