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

Abstract

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 (Ac), potentially leading to crop yield losses. This study investigated to what extent Ac is reduced inside a modern greenhouse and under a shade net in comparison with that outside. A simple Ac model (i.e., canopy-scale photosynthesis-light curves) was parameterized based on the measurements of Ac 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 (Iout) with I inside arch-roofed, single-span greenhouses [enveloped with a diffuse ethylene tetrafluoroethylene (ETFE) film; Iin] and I under shade nets (composed of aluminum and polyester strips; Ish). An Ac simulation using these models indicated that on a typical sunny summer day in Japan, Ac inside the greenhouses and under the shade nets (Ac,in and Ac,sh, respectively) corresponded to 91% and 52% of Ac outside (Ac,out) for the paprika canopy (for the tomato canopy, Ac,in and Ac,sh corresponded to 90% and 48% of Ac,out, respectively). The simulated Ac loss was more serious on a cloudy day because of the linear Ac-I response under low I conditions (Ac,in/Ac,out and Ac,sh/Ac,out were 69% and 13%, respectively, for the paprika canopy). The loss of Ac,sh may be alleviated by limiting the shading period to only midday hours.