Tobacco germplasm screening for genetic variability, alkaloid accumulation and their relationships
Nicotine is the predominant alkaloid in commercially grown tobacco ( Nicotiana tabacum L.) and accounts for greater than 90% of the total alkaloid fraction. Nicotine conversion results in formation of nornicotine, which is the amine precursor of N'nitrosonornicotine (NNN), a major tobacco specific nitrosamine (TSNA). Field experiments were conducted to determine the distribution of alkaloids and frequency of nicotine conversion in different tobacco types. A set of 140 tobacco entries representing a wide array of genetic background were grown in a summer field nursery on 10 plant plots at Blackstone, Virginia Polytechnic Research Station. At flowering, one mature leaf per plant and three plants per entry were harvested and yellowed using the established ethephon nicotine conversion screening protocol. Alkaloid and secondary alkaloids data were obtained on a total of 420 plants. Percent total alkaloid conversion to nornicotine was obtained to identify plants that had potentially mutated at the nicotine conversion locus. Plants that had a percent conversion ratio greater than 5% were considered converter and removed from the data analysis. Correlation data analysis of the remaining 293 plants was performed on total alkaloids, secondary alkaloids, nicotine, and nornicotine. Positive and highly significant correlations were observed between percent total alkaloids and percent nornicotine (0.75) as well as between percent total alkaloids and percent anatabine (0.78). Also, it seemed that percent nicotine conversion to nornicotine increased as total alkaloids decreased. The data suggests that lowering nicotine level in tobacco leaf would ultimately reduce levels of secondary alkaloids, especially nornicotine and anatabine, and consequently the level of TSNA. However, lowering nicotine could potentially increase the ratio of nicotine conversion to nornicotine. This paper will discuss how critical it is to consider percent nicotine conversion and total alkaloids when screening for nicotine conversion in breeding programs.