INFLUENCE OF HUMIDIFICATION AND DEHUMIDIFICATION ON GREENHOUSE CLIMATE AS WELL AS WATER RELATIONS AND PRODUCTIVITY OF CUCUMBER .1. INFLUENCE ON GREENHOUSE CLIMATE

In two year experiments with cucumbers planted in May, the influence o f humidification and/or dehumidification measures on air humidity were investigated. Air humidity was controlled by setpoints of water vapou r pressure deficit (vpd). Humidification was performed by on top spray ing impulses, dehumidification by ventilation, in one treatment suppor ted by heating pipes inside the crop. The vapour pressure deficits of the 24 degrees C- and 28 degrees C-ventilation control treatments were similar. Vpd daytime averages over the entire growing seasons in the 28 OC ventilation control were 0.78 kPa (1988) and 1.17 kPa (1989). Th e nighttime vpd came to 0.21 kPa (1988) and 0.41 kPa (1989). Depending on year and intensity of climatisation measure, humidification decrea sed the vpd by 0.23-0.44 kPa during day and 0.06-0.23 kPa at night. De humidification increased the vpd by 0.04-0.10 kPa during day and 0.02- 0.20 kPa at night. The combination of humidification and dehumidificat ion decreased the vpd by 0.08-0.35 kPa during day and increased the vp d about 0.02-0.12 kPa at night. With the application of simple climati sation measures extreme dry conditions during day and extreme humid co nditions at night could be prevented.