Microclimate and agronomical effects of internal impermeable screens in an unheated Mediterranean greenhouse

In unheated greenhouses in mild-winter areas, low-cost, fixed, water-impermeable plastic screens are frequently installed in winter cycles of the vegetable crops more sensitive to fungal diseases. They are used to prevent rain and condensation falling on the crop and to improve the greenhouse air temperature. Two experiments were carried out to quantify how fixed and movable impermeable screens affect microclimate and crop behaviour in an unheated greenhouse in a mild-winter area. The fixed screen improved the night-time temperature and humidity of the air below the screen, and reduced the water condensation on its inner plastic surface or the proliferation of fungal diseases, but did not completely prevent it. Compared to the greenhouse without screen, the movable screen, usually unfolded at night, increased the night-time temperature of the air and the crop, reduced the night-time relative humidity of the air below the screen, prevented the water condensation on the screen or the crop, accelerated melon crop development, and significantly increased early marketable yield of melon fruits and their quality, but it did not substantially affect the substrate temperature, or the total marketable yield of melon fruits. In the comparison of the greenhouse with fixed versus movable screen, no substantial differences were found for a winter cucumber cycle in night-time temperature and relative humidity of the air below the screens, in shoot biomass or in fresh weight of total and marketable cucumber fruits. This can be mainly attributed to the small differences between both treatments in the shortwave radiation reaching the crop.