While most crop models consider the impact of light on photosynthesis and crop growth, not many consider the impact of within-canopy light and nitrogen distribution. Uncertainties remain about how nitrogen distribution affects plant growth and photosynthetic properties within different canopy layers. Therefore, in this study, seasonal variations in leaf photosynthetic, morphological and biochemical properties within canopy profile were investigated in two potential biofuel species, switchgrass (Panicum virgatum) and miscanthus (Miscanthus x giganteus), under two nitrogen treatments (control and +200 kg/ha).

Results/Conclusions

Photosynthetic properties, such as photosynthetic capacity at light saturation (A_max), maximum rate of carboxylation (V_cmax) and maximum rate of photosynthetic electron transport (J_max) all decreased significantly from top to bottom canopy layers, but were not affected by nitrogen fertilization. In contrast, nitrogen fertilization increased leaf area index (LAI) for Panicum virgatum and Miscanthus × giganteus.  Stomatal conductance (g_s) was affected by both fertilization and canopy position. Distribution of leaf nitrogen concentration followed irradiance gradients and was increased by nitrogen fertilization. Chlorophyll content was increased by nitrogen fertilization, but did not differ within canopy layers. Specific leaf area (SLA) increased with decreasing irradiance level but was not affected by fertilization. Our results therefore suggest that the photosynthetic capacity of switchgrass and miscanthus depends on canopy position, but not nitrogen fertilization. These two species adapted to the light environment by adjusting their leaf morphological and biochemical properties and to the nitrogen treatment mainly by increasing LAI. These results should aid in developing models that can accurately estimate carbon dioxide flux in earth systems and biomass production in crop ecosystems.