Multi-compartmental modeling of nitrogen translocation in sorghums differing in nitrogen use efficiency

Some cultivars of sorghum [Sorghum bicolor (L.) Moench] are more efficient than others in using nitrogen (N) from the soil to produce grain. A study was conducted to test the hypothesis, that the processes involved in the translocation of N differed between two sorghum lines, China 17, a N-efficient genotype, and Tx623, a less efficient genotype, during their ontogenetic development. Both cultivars were grown in the field and periodically sampled to determine the amount of N in above-ground parts of the plants, i.e. stalk, leaves, and grain. Compartmental modeling, which is based on sets of differential equations that describe the flow of material through compartments, was used as the basis for analysis of the rates and rate constants of N which had moved among parts of the plant. Compartmental models are relatively simple, systematic mathematical representations that can be used to aid in understanding how different plant parts influence plant-N dynamics. China 17's rate constants between upper parts of the stalk and between the upper stalk and grain were larger in the earlier part of the season and became smaller toward the end than Tx623 meaning that China 17 was more capable of moving N to the upper parts of the plant earlier in the season than was Tx623. However, no differences in rate constants were detected for stalk to leaves and within leaves rate constants, meaning that the N-movement processes in the leaves were essentially the same between the two genotypes. The model also detected pseudo-compartments associated with the upper leaves in both genotypes which indicated that there are several processes or other rate-controlling factors associated with N flow in the leaves. A multi-compartmental model, based on rather simple assumptions and which only required N for each plant part over time, was used to describe N translocation between plant parts for two different sorghum genotypes. The model was able to detect fundamental differences between the genotypes in terms of N translocation processes, however more work is needed to understand the physiological processes underlying the results.