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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.

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