Resumen
Greenhouse microclimate and crop response of winter cucumber cycles grown in unheated Mediterranean greenhouses with representative combinations of passive heating systems (fixed, plastic screen with and without black mulch; movable thermal screen with black mulch; and double-layer plastic covering with black mulch) were evaluated in Almería, SE Spain. In the first experiment, the black mulch in combination with a movable or fixed screen increased the marketable cucumber yield by 14%, which appears to be mostly attributable to higher substrate temperatures induced by the black mulch in the cold period. Moreover, the black mulch in combination with a fixed screen frequently led to screen water condensation. The use of a movable screen, rather than a fixed one, in combination with a black mulch increased the first-class cucumber yield in the second experiment and reduced the non-marketable one in the first experiment. This might be mainly attributable to the higher incoming shortwave radiation in the cold period. Moreover, the movable screen reduced the risk of water condensation on the screen and the crop. The cucumber in the greenhouse with the double-layer covering and black mulch, compared to that with movable screen and black mulch, received lower daily incoming shortwave radiation, particularly, during the second half of the cycle (mainly due to the formation of water condensation droplets on the lower surface of the external plastic film), which reduced crop yield. Further field research is needed to better quantify the most limiting factor for growth (substrate/soil or air temperature, radiation, or water condensation) in Mediterranean greenhouse crops.