Effects of cover optical properties on screenhouse radiative environment and sweet pepper productivity

The effects of cover optical properties on screenhouse radiative environment and sweet pepper productivity were investigated in a Mediterranean climate (Eastern Greece) under three covering materials (i) a pearl insect-proof screen (IP-78), (ii) a white insect proof screen (IP-59) and (iii) a green shade-screen (GS-62) with values of the transmittance to photosynthetically active radiation (τPAR) of 78%, 59% and 62%, respectively. All screens induced impoverishment in the blue wavelength band (B, 400-500 nm) and enrichment in near-infrared broadband (NIR, 700-1100 nm) with respect to photosynthetically active radiation (PAR) (400-700 nm), and modified photomorphogenetic parameters such as the ratio R:FR (Red:Far-Red). From a 2-year agronomic survey, it was found that the lowest performance in terms of crop productivity was observed in the open field, and the highest under the IP-78 screenhouse. At final harvest, aboveground dry biomass and total yield of screenhouse crops were linearly and positively correlated with τPAR, while plant height was negatively correlated with the ratio R:FR. The results suggested that (i) the more diffuse radiation regime prevailing under screenhouse is likely to be at the root of the higher crop performances observed under the screenhouses and (ii) τPAR and R:FR are likely to act synergistically in light capture. We conclude that both changes in transmittance and light quality must be accounted for when analysing productivity and yield regulation of screenhouse-grown crops. From a practical point of view, a shading factor no higher than 20% is recommended for sweet pepper cropping under Mediterranean conditions similar to those of this study.