MICROCLIMATE AND TRANSPIRATION OF GREENHOUSE ROSE CROPS

Measurements of microclimate and transpiration of two greenhouse crops of roses (grafted and ungrafted) grown in rockwool were carried out during late spring and summer periods of 1989 to 1992 in the South of France. Results show that the crops have relatively low rates of transpiration compared with the latent heat equivalent to the global radiation incident on the crop. Analysis of the results indicates that this can be attributed to several causes: (1) for both crops, (a) stomatal closure as a result of high levels of the saturation deficit when the misting system was not operating (a value of 1.5 kPa was found to be the critical value above which stomatal conductance of the crop falls significantly) and (b) high substrate temperature during the afternoon and poor aeration of the root system may also reduce water uptake by the roots; (2) for the ungrafted crop, the low leaf area index (L approximate to 1) limits potential transpiration rate and also prevents cooling and humidification of the greenhouse air through a microclimatic 'feedback effect'. The use of the Penman-Monteith formula, incorporating computed values of the surface resistance to water vapour transfer vs. global radiation and saturation deficit, gives a good prediction of the hourly transpiration rate. The results confirm the importance of adequate control of greenhouse saturation deficit aod substrate temperature during summer conditions, especially when radiation load is high and canopy transpiration is not sufficient to cool and humidify the greenhouse environment.