Simulating seasonal patterns of increased greenhouse crop production by conversion of direct radiation into diffuse radiation

The conversion of direct solar photosynthetically active radiation (PARDir) into diffuse radiation (PARDif) and its effects on greenhouse crop production were analyzed with simulation models. PARDir can be converted into PARDif by increasing the haze of greenhouse covers with a minimal loss of radiation transmission. At specific sun incidence angles, PARDif penetrates more deeply into the canopy than PARDir, thereby decreasing saturation effects in upper, and increasing photosynthesis rates at deeper layers. Increased profits can be realized if the crop is intercepting more incoming radiation at more efficient parts of the canopy. A dynamic crop growth model was used to quantify the yearly production of tomato, cucumber and sweet pepper, calculated on a daily basis. For different degrees of haze, six virtual leaf compartments were simulated in the canopy. These layers were analyzed for PAR availability, PAR absorption and photosynthesis rates. Model results showed that the conversion of direct to diffuse radiation shifted the vertical distribution of PAR to deeper canopy layers. Total photosynthesis in the top layer of the canopy was reduced by 0.5%, but photosynthesis per MJ absorbed PAR increased. Deeper canopy layers received and absorbed more PAR, thereby increasing total canopy photosynthesis. If all incoming PARDir was converted to PARDif, cucumber photosynthesis increased by 1.5% (winter), 4.0% (summer) and 3.4% (fall). For sweet pepper it was 1.5% (winter), 4.3% (summer) and 4.2% (fall) and for tomato it was 1.4% (winter), 3.5% (summer) and 2.6% (fall). Different reactions between crop types were related to differences in leaf area index evolution in the course of the growing season. About 55% of all benefits were obtained in summer months (May-June-July), when the amount of PARDir is the greatest and leaf area index (LAI) is sufficiently high