Psychometric and ventilation constraints for vapor pressure deficit control

Constraints on air temperatures, dewpoint temperature and ventilation rate in plant growth and propagation chambers under vapor pressure deficit (VPD) control are presented. These constraints apply to any controlled environment space subjected to significant radiation in which both air temperature and humidity can be simultaneously manipulated. Defining relations and logic necessary to implement two methods of VPD control (VPD(air) vs. VPD(crop-air)) are described. Theoretical analysis and experimental results demonstrate that constraints for VPD control are inevitable, and are exacerbated by low air flow rates and high levels of incident radiation. Interactions between VPD(air) and VPD(crop-air) for rooted poinsettia cuttings showed that by maintaining VPD(air) constant at 1500, 2000 and 2500 Pa, the corresponding VPD(crop-air) increased directly with incident radiation from 50 to 300 W m^-2. Interactions between crop, air and dew point temperature for VPD(air) control at 1500, 2000 and 2500 Pa under different levels of incident radiation are also presented. Under the constraints identified, VPD control from manipulating dew point temperature, air flow rate and cooling is demonstrated for rooted poinsettia cuttings. This work forms a basis for defining operational constraints necessary to develop a real-time VPD controller. (C) 2000 Elsevier Science B.V.