Use of augmented natural ventilation and ‘double roof’ with photoconversion films to improve crop photosynthesis inside greenhouses

The aim of this work was to analyze the effect on greenhouses microclimate and plant activity of a larger side ventilation surface and a double roof with spectrum photoconverter film (installed in the West sector of a multispan greenhouse), as alternative to the cover whitewashing (used in the East sector). To avoid excessive temperatures in summer, Mediterranean greenhouses are painted with calcium carbonate to reduce cover transmissivity. The drawback of the whitewashing is the severe reduction of transmissivity to the photosynthetically active radiation (PAR). As consequence, plants photosynthetic capacity and crop productivity can be significatively reduced. In commercial greenhouses of Almeria (Spain), transmissivity of whitewashed covers is reduced under 0.2 and their deficient ventilation capacity decreased the CO₂ concentration in the central area, reaching 370 ppm, well below the values of 420 ppm measured nearby the side vents (comparable to the outside concentration). A tomato crop and a pepper crop were developed inside the experimental greenhouse. The use of the photoconversion film installed as ‘double roof’, without whitewashing of the greenhouse cover, allowed to increase inlet net radiation (26-54%) and PAR at plant leaves (2-39%). In addition, the increase of 10% of the total ventilation surface (from 16.6 to 26.0% of ground surface) using larger side opening allowed to increase by 3% the concentration of CO₂ in the area occupied by the crop. As a consequence an increase of 10% in the photosynthesis of pepper plant was observed.