Field characterization of maize photosynthesis response to light and leaf area index under different nitrogen levels: a modeling approach
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A better characterization of nitrogen (N) demand is essential for an improved estimation of N requirements by maize crops. Crop growth models can provide a tool for greater understanding of the variable responses of yield to different N levels observed experimentally. Photosynthesis-based models require a detail characterization of the effect of N on the photosynthetic machinery of the leaves and leaf area index (LAI) for capturing N variability and demand. Photosynthetic light response curve at different N levels (0, 90, and 225 kg ha-1) were measured in a field experiment conducted at the Agronomy Research Farm, Iowa State University during 2011. Additionally, light interception, LAI, and leaf nitrogen concentration were determined during the growing season. Main parameters of the light curves (Amax: the maximum assimilation rate under saturated light intensities, φ: quantum efficiency, and Rd: dark respiration) were optimized using a C4 photosynthesis model based on Collatz et al. (1992). The model MaizeGro (BioCro R-package) with a LAI model (Lizaso et al., 2003) was parameterized and calibrated with field data. The objectives were (1) characterize the photosynthetic response to light and LAI under different N levels throughout the growing season (V4 to R5 stages); (2) incorporate in a crop growth model a dynamic relationship between photosynthetic parameters and leaf N concentration taking in consideration the effects of developmental stage and LAI calibrated for different N levels. Amax was linearly dependent on leaf N concentration and its response to N concentration was higher for the leaf 14 (V14-R5) than vegetative stages (V4-V10). There was a significant linear relationship between Rd and leaf N concentration. Apparent quantum efficiency (&rho) did not show a relationship with leaf N concentration or developmental stage. The model provided a framework that allowed the incorporation of a dynamic relationship between photosynthetic and LAI parameters with leaf N concentration. A better prediction of the N-limited LAI and canopy photosynthesis by crop models would eventually lead to a more accurate assessment of N supply and demand in the cropping systems that might provide a powerful opportunity for mitigating nitrous oxide emissions from agricultural soils and leached N loss as well.