Articles
A PHOTOSYNTHESIS MODEL FOR GREENHOUSE ROW CROPS
It is based on three main steps: i) light penetration into the greenhouse including modifications of the diffuse to global ratio due to the greenhouse optical properties, ii) light penetration into the crop taking into account sunlit and shaded foliage, iii) leaf photosynthetic response to incident PAR altered by the age of the leaf.
The irradiance incident onto the crop is determined assuming that the inside direct irradiance comes from the outside one and that the inside diffuse irradiance comes from both outside diffuse and direct irradiances (the direct radiation is partly scattered by the greenhouse glazings and structures). The crop is divided into cells and the distribution of leaf area with respect to incident PAR radiation in each cell is determined cell using a probabilistic approach, taking into account multiple scattering.
In each cell the leaf photosynthetic response function is combined with the leaf area distribution to compute the cell photosynthetic activity.
The crop carbon gain is computed as the sum of the cell photosynthesis.
Measurements of the greenhouse characteristics, crop geometry and leaf photosynthesis were used; the model has then been validated against in situ measurements of the CO2 exchange of whole plants.
The predictive quality of the model reveals to be good but for an under-estimation of the crop photosynthesis under high irradiances.
The model has been used to compare the crop photosynthesis under fixed outside irradiances but with varying Diffuse to Global ratio.
There appears an interaction with the row orientation and the sun height.
When the sun is high, the direct radiation is poorly usefull; scattering due to the greenhouse improves the light interception efficiency of the crop.