C. Stanghellini
The application in greenhouses of energy saving devices (as double cover, thermal screens or reduced air exchanges), results in a lower rate of vapour removal i.e. a higher ambient humidity. Most climate control systems are programmed not to allow the humidity to exceed some threshold. However, lessening of the ambient humidity by whatever means, is an expensive exercise and may dispose of the saving expected from applying better insulation. The manipulation of the crop transpiration through a "wise" control of the microclimate could indeed deliver a better humidity management in greenhouses. Such a wise control is possible only as long as the influence the microclimate exerts on the transpiration rate of crops is understood.

In this work some examples are provided of the relationship between the microclimate and the transpiration of a greenhouse crop, as resulting from a model whereby the transpiration is regarded as a physical process affected by the greenhouses climate. Some results of the model with respect to the expected influence on the microclimate (and thus on the transpiration rate) of a better insulation follow here:

  • Manipulation of the relative humidity in a greenhouse is no substitute for the manipulation of the transpiration rate.
  • The application of a screen does not necessarily reduce the transpiration rate of the crop.
  • A variation of the air movement within a greenhouse seldom has a detectable effect on the transpiration rate.
  • Longwave radiation from the elements of a conventional pipe heating system does not contribute significantly to the transpiration.

It is further suggested that the explicit use of the transpiration rate as a control criterion (a "transpiration set-point" instead of separate temperature and relative humidity set-points) would deliver quantitative rules about the application of some climate manipulations, now dictated by rules derived from substantial practical experience.

Stanghellini, C. (1988). MICROCLIMATE AND TRANSPIRATION OF GREENHOUSE CROPS. Acta Hortic. 229, 405-414
DOI: 10.17660/ActaHortic.1988.229.46

Acta Horticulturae