Total plant growth, CAtm→a, is proportional to the global radiation absorbed by canopy, Rs,pl, (see Soil evaporation, snow and radiation processes) but limited by unfavourable temperature f(Tl), nitrogen f(CNl) and water f(Eta/Etp) conditions represented by functions ranging between zero and unity as:
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:
The leaf temperature response, f(Tl), includes limitations because of too low or too high temperatures:
The leaf nitrogen response, f(CNl), is made linear as:
The response function for water f(Eta/Etp) 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, εL, directly, this parameter is therefore exchanged to a photo radiation response function, f(εL):
where εLred is the percentage reduction of radiation use efficiency due to grain development and Gfill is the degree of reduction due to development stage. Gfill 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 εLred. See viewing function Radiation use efficiency response function at grain filling.