Studies of root systems and nitrogen uptake in diverse sorghum genotypes

The physiological basis for variation in dry matter (DM) partitioning, nitrogen (N) utilization, and nitrate (NO$\sb3\sp-)$ uptake kinetics among sorghum (Sorghum bicolor (L.) Moench) genotypes is not well understood. Greenhouse studies were conducted to evaluate differences in root branching and NO$\sb3\sp-$ uptake kinetics among sorghum genotypes of U.S. and Africa origin. The genotypes consisted of three U.S. derived sorghum genotypes (TX631, KS57, and PI8358), five African derived sorghum genotypes (Nagawhite, M90378R, CE-151-262, IR204, and WSV387), and two hybrids derived from one common U.S. parent and two African parents (TX631 x WSV387 and TX631 x M90378R). Root branching of ten sorghum genotypes grown in root boxes were evaluated by fractal analysis. Significant differences in fractal dimension (D) among genotypes were observed (1.44 $\leq$ D $\leq$ 1.89). The African genotypes were not only more branched but also had a greater proportion of their root system (30-35%) at the deepest profile layer than the U.S. genotypes which had 15-25%. The second greenhouse study evaluated four sorghum genotypes (TX631, M90378R, IR204, and WSV387) for differences in NO$\sb3\sp-$ uptake kinetics. They were grown hydroponically in six NO$\sb3\sp-$ concentrations ranging from 0.2 to 1.2 mM. Estimates of NO$\sb3\sp-$ uptake differed among the genotypes. The U.S. genotypes responded differently to increased NO$\sb3\sp-$ absorption at greater NO$\sb3\sp-$ solution concentrations in contrast to the African genotypes. The increased NO$\sb3\sp-$ absorption of the U.S. genotype appeared to be associated with a greater root absorbing power $(\alpha).$ Genotype TX631 had the greatest maximal rate of NO$\sb3\sp-$ uptake $\rm\lbrack I\sb{max};$ 26.5 $\mu$mol g$\sp{-1}$ root fresh weight (RFwt) h$\sp{-1}\rbrack$ while for the African genotypes, ranged from 2.78 to 4.22 $\mu$mol g$\sp{-1}$ RFWT h$\sp{-1}.$ Smaller values of root affinity $\rm(K\sb{m})$ and concentration at which net uptake rate is zero $\rm(C\sb{min})$ for the African genotypes were compensatory mechanism for a lower $\rm I\sb{max}.$ The sorghum genotypes used for the rooting branching study were evaluated in the field for differences in DM distribution and N utilization at different growth stages when grown in soils previously depleted of available plant N. Soils for the field studies were a fine, montmorillonitic mesic typic Argiudoll. The African genotypes were generally superior in partitioning DM to grain under depleted soil N conditions. Nitrogen accumulation and utilization was greater in the African genotypes much more than the U.S. genotypes