Sustained growth and yield in elevated greenhouse air temperatures through control of VPD

The availability of freshwater is a limiting factor to food production, including in the arid central valley of California, a major food production center for the United States. Limited supply is compounded by surface water containing agricultural runoff with high levels of fertilizer salts and naturally occurring selenium and boron. A novel method of utilizing the abundant solar radiation in arid lands to treat agricultural drainage water will integrate a humidification-dehumidification (HDH) system into a controlled environment greenhouse producing food crops. Due to the water holding capacity of warm air, the HDH system efficiency increases at air temperatures that are considered at, or above, upper limits of air temperatures for growing food crops in greenhouses. Two experiments were performed in which tomato crops were grown in optimal conditions of 24°C daytime air temperature, and then at upper threshold air temperatures of 30°C. In one experiment, greenhouse humidity and VPD were allowed to fluctuate naturally. In a subsequent experiment, VPD was kept at or below 2.0 kPa by adding moisture to the air with a low-pressure misting system. All plant growth and development measurements exhibited undesired characteristics when exceeding 2.0 kPa at 30°C without VPD control, but they maintained desired characteristics with VPD control to below 2.0 kPa. The goal of providing greenhouse conditions favoring effective operation of the HDH desalination system that also allowed desired plant growth and development was successfully demonstrated. When VPD was controlled, solar desalinization and crop production could be achieved within the same greenhouse production facility.