Nitrate reductase activity of diverse grain sorghum genotypes and its relationship to nitrogen use efficiency

High crop N use efficiency is desirable for reducing the cost and reliance on fertilizer N, and it may help reduce ground water pollution. The objectives of this study were to (i) characterize and compare the seasonal trends in nitrate reductase activity (NRA) of seven grain sorghum [Sorghum bicolor (L.) Moench] genotypes of tropical and U.S. origin, (ii) determine how NRA is related to total plant N, dry matter, grain yield, and N use efficiency, and (iii) examine how NRA and plant N accumulation affect N use efficiency for biomass (NUE(1)) and grain production (NUE(2)). The genotypes were evaluated in field trials in 1994 and 1996 at applied N levels of 0 and 100 N kg ha(-1) on a Sharpsburg silty clay loam soil (fine, smectitic, mesic Typic Argiudoll). Plant biomass, reduced N, NRA, NUE(1), and NUE(2) were determined at three growth stages (vegetative, anthesis, and physiological maturity). Genotypes did not differ for NRA at either N level; however, there was a definite tendency for the four tropical lines and two U.S. adapted lines to have greater NRA values at all growth stages than the one hybrid. Nitrate reductase activity decreased throughout the growing season, with the sharpest decline from anthesis to maturity. Nitrate reductase activity did not correlate with grain yield or shoot biomass, but did correlate with grain N concentration. Shoot and grain N concentration correlated with NUE(1), while grain and shoot N contents and shoot N concentration were correlated with NUE(2). Neither NUE(1) nor NUE(2) were significantly affected by the NRA level. Tropical lines had greater preanthesis N uptake accumulation (1.45 g plant(-1)) and greater NUE(1) than the hybrid (0.90 g plant(-1)) and U.S. adapted lines (0.73 g plant(-1)). In terms of grain N use efficiency, the hybrid had a 14% greater value than U.S. adapted lines and a 17% greater value than the tropical lines. It appears that NRA is not a factor that limited N accumulation and use efficiency in grain sorghum.