Field experiments were conducted to study the effect of rates and time of nitrogen application on growth, nitrogen and nicotine contents of leaves and stalks of Burley 21. Rates of nitrogen used were 0, 75, 150 and 225 pounds per acre. Nitrogen was applied as preplant and/or as post plant applications. The maximum rate of growth and nitrogen uptake occurred during the month prior to topping. During the period prior to topping 74 percent of the total dry matter, 80 percent of the total nitrogen and 34 percent of the total nicotine accumulated in the shoots. Applying 2/3 of the 75 pounds of nitrogen per acre 21 days after transplanting increased the yield over treatments in which nitrogen was applied earlier or later. However, split application of 150 and 225 pounds of nitrogen per acre did not affect the yield. Split applications in general did not affect average price, nitrogen and nicotine contents of leaves and stalks. Increasing nitrogen rates up to 225 pounds per acre significantly increased the nitrogen and nicotine levels of cured leaves as well as the nitrogen levels in non-cured leaves and stalks. The increase in percent nicotine of non-cured stalks and leaves was not significant beyond applications of 75 and 150 pounds of nitrogen per acre, respectively. Increasing nitrogen rates lowered the ratio of nitrogen incorporated into nicotine in relation to total shoot nitrogen. The average increase in percent nicotine was 0.6 percent for each 1 percent increase in percent nitrogen of cured leaves. Yield and acre-value significantly increased with nitrogen rates up to 225 pounds per acre. Applying 225 pounds of nitrogen per acre significantly decreased the average price and delayed the flowering. / M.S.

Mammoth cultivars of flue-cured tobacco flower under short-day photoperiods, thereby increasing the potential number of leaves per plant. Field experiments were conducted in 1991 and 1992 at the Virginia Tech Southern Piedmont Agricultural Experiment Station near Blackstone, Virginia to determine the influence of nitrogen rate on nitrogen use efficiency and its components on two mammoth cultivars and a conventional cultivar. Experiments were also conducted to determine the effects of leaf number, time of topping, and nitrogen rate on yield, quality, and other agronomic characteristics. Nitrogen uptake, utilization, and use efficiencies decreased while nitrogen accumulation increased as nitrogen rates increased from 67 to 112 kg ha⁻¹. Mammoth cultivars were more efficient than the conventional cultivar for nitrogen uptake, utilization, and use. Only NC 27 NF benefits from increasing nitrogen above the recommended rate. Yield increased by 17 percent as the leaf number increased from 20 to 36. In contrast, both agronomic traits and chemical characteristics decreased accordingly. The ratio of nitrogen to nicotine that was unacceptable at leaf numbers above 28. Yield, value, and quality decreased as topping was delayed by seven to 21 days. Yield was reduced by 30 kg per hectare per day when topping was delayed by 21 days after the conventional cultivar reached the bud stage. Leaf lengths and widths of mammoth cultivars were larger than the conventional cultivar. Mammoth cultivars were higher yielding and had better quality at 112 kg ha⁻¹ N when compared to the conventional cultivar, although these differences were not significant. / Master of Science