USING EXOGENOUS HORMONE APPLICATION TO SUPPRESS AXILLARY SHOOT DEVELOPMENT IN TOBACCO

The variability in the number of basal axillary shoots (ground suckers) among all tobacco (Nicotiana tabacum L.) varieties, has increased since the hydroponic transplant production system became the standard. The larger root ball of hydroponically produced transplants compared to traditionally produced transplants potentially generates a difference in the ratio of auxin (inhibits axillary shoot formation) to cytokinin (promotes lateral branching), that induces basal axillary shoot development. Starting in 2014, studies were conducted to investigate whether the addition of synthetic auxins or cytokinins to hydroponic transplant production could prevent ground sucker formation. Different tobacco cultivars, with high or low ground sucker potential, were evaluated in extensive dilution trials using the synthetic auxin 1-Naphthalene acetic acid (NAA) or cytokinin 6-Benzylaminopurine (BA). Initial results indicated that a 2nM concentration of NAA significantly reduced axillary shoots in the known ground sucker producer, compared to the control. From these results, NAA and BA was added to the hydroponic transplant solution at concentrations in the range of 2-50 nM for the 2015 field trials; however, these studies failed to confirm the preliminary greenhouse findings. Nonetheless, it was clear from the 2015 field results that the varieties used are genetically different from one another in ground sucker potential. Following the 2015 field study, an additional greenhouse experiment using higher concentrations of NAA showed that a 1µM NAA hydroponic solution reduced ground sucker number. From this, a second set of field studies were conducted in 2016 using the higher rates of NAA. Burley variety TN86 and dark variety KTD6 (known ground sucker producers) were used in the 2016 studies. Five hormone treatments (NAA 500-5000 nM and an untreated control) and five tray (128, 200, 242, 288, and 338 cells per tray) were evaluated. Tray size was added to determine if increasing transplant root ball size was correlated with an increase in ground suckers. Although statistically significant differences were found among treatments applied to both TN86 and KTD6, there was no treatment that consistently reduced ground sucker numbers. No notable trend in ground sucker number indicates that an increase in the root ball size of tobacco transplants produced in the float bed system compared to traditionally produced transplants is not the sole cause of increased ground sucker number. In conclusion, from the results of the entire study, it is apparent that an exogenous auxin application (within the conditions used in this study) will not consistently or predictably suppress ground sucker development in tobacco; perhaps the only consistency in the data is how inconsistent ground sucker formation is from environment to environment, and variety to variety