Effect of nitrate/ammonium ratio on biomass production, nitrogen accumulation, and use efficiency in sorghums of different origin

Plant nitrogen (N) uptake, growth, and N use efficiency may be affected by N form NO3-, or NH4+) available to the root. The objectives of this study were to determine the effect of mixed N form on dry matter production and partitioning, N uptake, and biomass N use efficiency defined as total dry matter produced per unit plant N (NUE1) U.S. and tropical grain sorghums [Sorghum bicolor (L.) Moench]. The U.S. derived genotype CK 60 and three tropical genotypes, Malisor-7, M 35-1, and S 34, were evaluated in a greenhouse trial using three nutrient solutions differing in their NO3-/NH4+ ratio (100/0, 75/25, 50/50). Shoot and root biomass, N accumulation, and NUE1, were determined at 10-leaf and boot stages. Averaged over all genotypes, shoot and root biomass decreased when NH4+ concentration was increased in the solution. Shoot biomass was reduced by 11% for 75/25 and 26% for 50/50 ratios, as compared to 100/0 NO, MH,'. Similarly, root biomass reduction was about 34% and 45% for the same ratios, respectively. Increasing NH4+ concentration also altered biomass partitioning between shoot and root as indicated by decreasing root/shoot ratio. Total plant N content and NUE1, were also reduced by mixed N source. Marked genotypic variability was found for tolerance to higher rates of NH4+. The tropical line M 35-1 was well adapted to either NO3- as a sole source, or to an N source containing high amounts of NH4+. Such a characteristic may exist in some exotic lines and may be used to improve genotypes which do not do well in excessively wet soil conditions where N uptake can be reduced.