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Root Development of Field-Grown Flue-Cured Tobacco¹

A flue-cured cultivar of tobacco (Nicotiana tabucum L. ‘NC 2326’) was grown on a Norfolk fine sandy loam soil (Typic Paleudult) in 1977 and 1978 to characterize the development of roots in field culture and additionally in 1978 to determine the extent to which band placement of fertilizer and removal of the flowering head by topping at anthesis influence the pattern of development. Both band placement of fertilizer and topping are common cultural practices. Root development was observed at intervals during the growing season using a pinboard method. Pinboard monoliths, 100-cm wide, 60-cm deep, and 24-cm thick, were taken transverse to the row and the soil rinsed away with a fine spray of water to expose the roots. Because of rotational schedules, the same field could not be used in both years, but adjacent fields within a continuous soil mapping unit were used. The single distinction in soil properties between fields was the thickness of the A, horizon. In 1977 the A₂ horizon was 25 to 35-cm thick, and in 1977 the A₂ was 0 to 5-cm thick.

In both years root growth until anthesis was confined to the Ap horizon and then penetrated into subsurface horizons. In 1977 root growth after anthesis was contained within the A₂ and did not penetrate into the B₂. In 1978 roots penetrated through the thin A₂ horizon and into the B₂. The eventual depth of rooting in 1978 was greater than in 1977; however, the mass of roots recovered in 1977 was nearly three times greater than in 1978. Soil moisture data for both years and measurement of penetrometer resistances in 1978, along with published data, indicate that the variations in rooting depth and dry weight may have been associated with the effects of differences in soil moisture on resistance to penetration at the A to B interface and on shifts in direction of assimilate flow within the plant in response to changes in turgor potentials within the plant.

For the cultural variables imposed in 1978, the pattern of root development was not different between removal of the floral parts at anthesis, together with chemical inhibition of lateral meristem development, and permitting uninterrupted floral development. Fertilizer placement did alter the rate of root development for a period following anthesis, but not the final root mass, with a greater rate during this interval for banded than for broadcast placement.

Key Words: Soil moisture stress • Nicotiana tabacum L. • Fertilizer placement • Root penetration

² Former graduate research assistant and professor, Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27650.

Received for publication February 23, 1981.