Recent studies have shown that, for greenhouse crops, high levels of humidity can lead to yield losses.
If optimal climate control strategies are to be developed in order to avoid these adverse effects and to optimize yield, it is necessary to quantify and predict the relationship between humidity or transpiration and yield.
This work shows how yield relates to both inside air humidity and to transpiration for greenhouse tomato crops.
Yield data were obtained during three multifactorial experiments at HRI Efford in 1987, 1988 and 1991. Tomato plants were grown under different day and night humidity set points (Air vapour pressure deficit between 0.1 and 0.8 kPa). Transpiration was calculated for the different compartments using Stanghellini's model, on the basis of measured climatic data.
Humidity treatments had a significant effect on total tomato yield.
Several relationship linking yield either to vapour deficit or to transpiration were taken into account.
As day and night vapour pressure deficit had a similar influence on yield but a different effect on transpiration, differences in yield were not directly related to the total absolute transpiration during the treatment period.
In fact, yield response to transpiration was higher at low levels of transpiration and the increase in yield proved proportional to the relative increase in transpiration, 89% of the yield variability being accounted for by such a model.
For prediction purposes, a simplified relationship can be used, linking the increase in yield to the increase in vapour pressure deficit, weighted by the inside solar radiation.
This relationship was validated by comparison with long term yields.
A relationship is also suggested for short time periods.