Using early-season leaf traits to predict nitrogen sufficiency of burley tobacco

Abstract

Burley tobacco (Nicotiana tabacum L.) requires large amounts of fertilizer N to produce high yields of good-quality air-cured leaf. On well-drained soils, fertilizer N typically is applied as much as 4 wk before transplanting. This results in a delay of up to 9 wk before the onset of rapid growth and N accumulation at about 5 wk after transplanting. Abundant rainfall often occurs during this period and creates the potential for losses of soil and fertilizer N. An early-season N-sufficiency tissue test could guide the tobacco grower in adjusting fertilizer needs of the crop before the last cultivation, which normally occurs about 5 wk after transplanting. The objective of this study was to determine relationships among selected N-nutrition-related leaf traits of tobacco measured 3, 4, and 5 wk after transplanting and to determine if these traits are related to yields of air-cured leaf. Burley tobacco was grown in 1993 at two locations, with 0 to 336 kg N ha(-t) applied before transplanting. One site had a well-drained soil; the other site, moderately well-drained. Total N, extracted chlorophyll, and chlorophyll meter readings were measured on leaf lamina disks taken from lower leaves. Leaf yields at both sites were equivalent at 336 kg N ha-1, with increases in yield due to added N being less for the well-drained soil. The response patterns of the leaf traits to fertilizer N were different for the two soil types and corresponded to the respective yield response patterns for each site. Neither extracted chlorophyll (335-760 mg m-2) nor chlorophyll meter readings (23.0-42.6 SPAD units) plateaued with increasing total N of leaf disks (3.50-6.26 g m-2; 41.2-66.6 mg g-1 dry wt.). Models for linear relationships among leaf traits were significant (R2 = 0.60-0.66; P < 0.001). A 20% increase in relative yield corresponded to about a 25% increase in concentrations of total N and extracted chlorophyll, but only a 5% increase in chlorophyll meter readings. After more testing and calibration with comparisons to a high-N-fertility reference strip, these early-season leaf tissue traits may prove useful in predicting sidedress fertilizer N requirements.