Estimation of Photosynthesis Loss Due to Greenhouse Superstructures and Shade Nets: A Case Study with Paprika and Tomato Canopies

In horticultural greenhouses, the photosynthetic photon flux density ( I ) is inevitably lower than that outside because of interference from greenhouse superstructures (e.g., reflection and absorption of radiation by greenhouse coverings and superstructures). In addition, during hot seasons in many regions, I can be lowered by shade nets installed to reduce excessive radiation. These reductions in I can cause a decrease in the canopy photosynthetic rate ( A c ), potentially leading to crop yield losses. This study investigated to what extent A c is reduced inside a modern greenhouse and under a shade net in comparison with that outside. A simple A c model (i.e., canopy-scale photosynthesis-light curves) was parameterized based on the measurements of A c and I for paprika and tomato canopies using the open-chamber method. In addition, based on the measurements of I , linear regression models were derived that related outside I ( I out ) with I inside arch-roofed, single-span greenhouses [enveloped with a diffuse ethylene tetrafluoroethylene (ETFE) film; I in ] and I under shade nets (composed of aluminum and polyester strips; I sh ). An A c simulation using these models indicated that on a typical sunny summer day in Japan, A c inside the greenhouses and under the shade nets ( A c,in and A c,sh , respectively) corresponded to 91% and 52% of A c outside ( A c,out ) for the paprika canopy (for the tomato canopy, A c,in and A c,sh corresponded to 90% and 48% of A c,out , respectively). The simulated A c loss was more serious on a cloudy day because of the linear A c - I response under low I conditions ( A c,in / A c,out and A c,sh / A c,out were 69% and 13%, respectively, for the paprika canopy). The loss of A c,sh may be alleviated by limiting the shading period to only midday hours.