Characterization of the Spectrum of Solar Irradiance under Different Crop Protection Coverings in Mediterranean Conditions and Effect on the Interception of Photosynthetically Active Radiation

Abstract

Plants use visible light and part of adjacent ultraviolet and near infrared regions for photosynthesis. Crop protection coverings enable plant cultivation in areas or seasons not suitable open field. However, the use of covering materials is a detriment to solar irradiance, which may decrease the photosynthetic rate. Here, the effect of two different covering materials, tempered glass and white polyethylene mesh, on solar irradiance was compared to open field (control) under real farming conditions. Relative irradiance (RI) and photosynthetic photon flux density (PPFD) were recorded along 380-780 nm wavelength spectrums in the two conditions at 10:00 h and 13:00 h. Also the efficiency of Capsicum peppers in capturing solar irradiance was evaluated in leaves as the reflectance of both RI and PPFD under the mentioned growing conditions. Low differences in RI among the three conditions were found, and the lowest values corresponded to glasshouse conditions. Differences were more obvious in PPFD and, compared to open field, both mesh greenhouse and glasshouse conditions provoked remarkable decreases in all the spectral bands, 50-55% and 75-80% respectively. Covering materials also differed on the ratio of reflected PPFD and incident PPFD. Glasshouse plants displayed the highest reflectance at both 10:00 h and 13:00 h (0.05-0.20), followed by mesh greenhouse (0.05-0.10), suggesting that glasshouse conditions might decrease the photosynthesis rate due to both PPFD decrease and reflectance, although the effect of polyethylene mesh should not be disregarded as it also decreases considerably PPFD. Our results have important implications for the physiology and the productivity of crops under different covering materials.