Environmental Control of a Glasshouse Suite for Crop Physiology

Five commercial glasshouses, each with a floor area of 10 mx 5 m, are equipped for control of temperature, humidity, carbondioxide concentration and ventilation rate using a PDP 11 computer.Heating is provided by a hot-air gas-fired system with a fastresponse time and roof ventilators are mechanically operated.Humidity is increased by the injection of fine water droplets.The internal concentration of CO₂ is held close to the externalconcentration by the injection of pure CO₂ during the day. Air temperature is allowed to cycle in phase with the externaltemperature and an algorithm applies retrospective correctionswhich hold long-term mean temperatures within about 0.1 °C of set values. Control of saturation deficit is limited bythe vapour pressure of the external air and by the temperatureof the glasshouse walls. The dependence of crop photosynthesisrate on irradiance has been established from diurnal changesin the rate of CO₂ injection. Each glasshouse is large enough to allow regular harvestingof plants and the installation of tube solarimeters measuringlight interception, access tubes for neutron probes, tensiometersand auxanometers. The cost of the system was about £500per m², an order of magnitude less than figures quoted for conventionalgrowth cabinets and phytotrons. Key words: Environment, Computer control, Glasshouses