Effects of Vertical Gradient of Leaf Nitrogen Content on Canopy Photosynthesis in Tall and Dwarf Cultivars of Sorghum

The vertical profile of leaf nitrogen (N) content per unit leaf area (N-LA) is important for increasing crop productivity via optimizing N use for canopy photosynthesis. To investigate the effects of plant height on the optimality, we analyzed the N-LA profiles with respect to light gradient twice during vegetative growth in canopies of tall and dwarf cultivars of sorghum. The gradients of the N-LA profiles relative to the light gradients were similar in the two cultivars although the vertical light gradient was steeper in the dwarf cultivar with doubled leaf area density (LAD). This suggests that light attenuation is more influential on the N-LA profile than is plant height or LAD. The advantage of the observed N-LA profile for the canopy CO2 uptake as compared to the uniform N-LA profile was similar in the cultivars except when the N allocation rate to the canopy decreased relative to the leaf area expansion in the dwarf cultivar. These results suggested that the optimality of the advantage of the N-LA gradient may not be directly influenced by the plant height, but by the balance between N allocation and leaf area expansion. The balance may be altered by the difference in the biomass allocation in the shoot between the tall and dwarf sorghum. These factors are to be taken into consideration in breeding programs that target stature in order to potentially increase production in sorghum.