Impacts on photosynthesis

Crop models mainly differ in the approach to simulate biomass accumulation. Two broad categories can be identified.

The first is formed by models which computes daily net photosynthesis as a function of daily radiation use efficiency (RUEd, g MJ-1), eventually corrected by limitation factors such as temperature, senescence, atmospheric CO2 concentration (e.g., CERES, Jones et al., 1984; CropSyst, Stockle et al., 2003; STICS, Brisson et al., 2003; WARM, Confalonieri et al., 2009). These models do not simulate maintenance and growth respiration and compute the rate of increase of aboveground biomass as linearly related to cumulative light interception (Monteith, 1977), with RUEd as the slope of the linear relationship. The impact of a foliar disease on this category of models is simulated according to the equation below:

Where RUEd-1 is the radiation use efficiency computed by the crop model at the previous time step and β is the ratio between the virtual  and the visible lesions. The β value is pathogen specific and have been estimated for different pathosystems (Bastiaans, 1991; de Jesus et al., 2001; Lopes and Berger, 2001).

The other main category of models compute daily gross assimilation by upscaling the leaf instantaneous CO2 assimilation to the canopy level. Maintenance and growth respiration of the different plant organs are computed as a function of the dry weight and of organ-specific coefficients (e.g., SUCROS, Spitters et al., 1989; WOFOST, Van Keulen and Wolf, 1982; ORYZA, Kropff et al., 1994). These models implement a photosynthesis-light response curve of individual leaves which is upper limited by the daily maximum CO2 assimilation rate (Amax,d, kg CO2 m-2 d-1), modulated by the crop phenological stage. The following equation is used to simulate the impact of a foliar disease in reducing the photosynthetic activity:

The impact of using different values for β in considering the impact of the epidemic on radiation use efficiency is shown in the Figure below:

Reduction of radiation use efficiency at increasing values of disease severity using different

beta values.

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