Psychrometric and ventilation constraints for vapor pressure deficit control

Constraints on air temperature, dewpoint temperature and ventilation rate i n plant growth and propagation chambers under vapor pressure deficit (VPD) control are presented. These constraints apply to any controlled environmen t space subjected to significant radiation in which both air temperature an d humidity can be simultaneously manipulated. Defining relations and logic necessary to implement two methods of VPD control (VPDair vs. VPDcrop-air) are described. Theoretical analysis and experimental results demonstrate th at constraints for VPD control are inevitable. and are exacerbated by low a ir flow rates and high levels of incident radiation. Interactions between V PDair and VPDcrop-air for rooted poinsettia cuttings showed that by maintai ning VPDair constant at 1500, 2000 and 2500 Pa, the corresponding VPDcrop-a ir increased directly with incident radiation from 50 to 300 W m(-2). Inter actions between crop, air and dew point temperature for VPDair control at 1 500, 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 roote d poinsettia cuttings. This work forms a basis for defining operational con straints necessary to develop a real-time VPD controller. (C) 2000 Elsevier Science B.V. All rights reserved.