Influence of NIR-Reflecting Shading Paint on Greenhouse Environment, Plant Temperature, and Growth and Flowering of Bedding Plants

Abstract

During greenhouse production, growers often utilize shading strategies to reflect shortwave radiation (SWR; 300 to 2,700 nm) and reduce the thermal load inside a greenhouse. Greenhouse glazing or shading materials that photoselectively filter more near-infrared radiation (NIR; 770 to 2,700 nm) than photosynthetically active radiation (PAR; 400 to 700 nm) can be used to reduce thermal energy transmission. We quantified environmental conditions and bedding plant growth and flowering inside a glass-glazed greenhouse (lat. 42.7° N) that received an application of either a commercially available NIR-reflecting (NIR-R) or neutral (N) shading paint. During two summer seasons, shading paints were applied to the glazing exterior of different greenhouses so that the mean daily transmission of PAR was similar. Poinsettia (Euphorbia pulcherrima Willd. ex Klotz) or bedding plants were grown inside each greenhouse. The NIR-R paint transmitted 67%, 8%, 24%, 30%, and 29% less ultraviolet-A (315 to 380 nm), red (R; 600 to 700 nm), far-red (FR; 700 to 800 nm), NIR, and SWR, respectively, than the N paint. Transmission of blue (400 to 500 nm) and green (500 to 600 nm) light was 4.7% and 4.5% greater, respectively, under the NIR-R versus N paint. The ratio of transmitted PAR per unit of SWR under the N and NIR-R paints was 1.8 and 2.6 µmol W-1 s-1, respectively. During the day (1100 to 1800 h), mean greenhouse air, shoot-tip, and leaf temperatures were 0.4°C to 1.5°C, 0.4°C to 1.2°C, and 0.7°C to 1.5°C higher, respectively, under the N paint compared with the NIR-R paint. From 0900 to 1700 h, the ratio of R to FR light under the NIR-R paint ranged from 1.44 to 1.79, whereas the N paint and outside the greenhouse had an R:FR ratio of 1.11 to 1.22. Blue salvia (Salvia farinacea Benth.), pansy (Viola × wittrockiana Hook.), and petunia (Petunia ×hybrida Vilm.-Andr.) flowered a mean of 1 to 3 d earlier under the N versus NIR-R paint, but plant heights were similar.