Light use efficiency approach

Total plant growth, C_Atm→a, is proportional to the global radiation absorbed by canopy, R_s,pl, (see Soil evaporation, snow and radiation processes) but limited by unfavourable temperature f(T_l), nitrogen f(CN_l) and water f(E_ta/E_tp) conditions represented by functions ranging between zero and unity as:

                          (5.9)

where ε_L is the radiation use efficiency and η is a conversion factor from biomass to carbon.

Optionally, this equation can be slightly modified to account for radiation saturation at high levels of radiation (see switch PhoSaturation) using a non-rectangular hyperbolic function:

         (5.10)

where p_max is the maximum level of photosynthesis given as a parameter.

The leaf temperature response, f(T_l), includes limitations because of too low or too high temperatures:

           (5.11)

where p_mn, p_o1, p_o2 and p_mx are parameters. See viewing function Assimilation – air temperature response.

The leaf nitrogen response, f(CN_l), is made linear as:

        

                                                                             (5.12)

where p_CN,Opt and p_CN,Th are parameters and CN_leaf is the carbon nitrogen ratio in the leaf. See viewing function Assimilation – nitrogen content in leaf response.

The response function for water f(E_ta/E_tp) is simply the ratio itself.

If the plant is developing grain or if the grain is maturing, eq. (5.9)  will be slightly modified, because during this period the plants radiation use efficiency is dependent on the development stage. Instead of using the photo radiation use efficiency, ε_L, directly, this parameter is therefore exchanged to a photo radiation response function, f(ε_L):

                                          (5.13)
where ε_Lred is the percentage reduction of radiation use efficiency due to grain development and G_fill is the degree of reduction due to development stage. G_fill is low when the plant starts to develop grain, which results in a low reduction of the radiation use efficiency, and it increases gradually towards 1 when the plant is in the grain maturing phase and the radiation use efficiency is then reduced by the whole ε_Lred. See viewing function Radiation use efficiency response function at grain filling.