Articles
TRANSPIRATION AND TEMPERATURE OF GREENHOUSE CROPS, IN RELATION TO INTERNAL AND EXTERNAL RESISTANCES
The partition of available radiative energy is determined, at least partially, by some resistance to transfer of a related flux.
Transfer of sensible heat is restricted by the resistance of the boundary layer (external resistance). On the other hand, since vapour is produced somewhere beneath the external surface of a leaf, an additional (internal) resistance has to be accounted for, when transfer of latent heat is considered.
Most climate control actions do affect many more variables than the one(s) they are meant to influence, and are thus likely to result in the alteration of one or both resistances; such variation may enhance or limit the outcome of the control action.
Accordingly, both the (undesired) effects on the canopy of any control operation and the effectiveness of any such procedure, when aimed at control of canopy temperature or transpiration, have to be carefully assessed.
Some first results are shown, of an experiment allowing for a concurrent appraisal of the internal and external resistances of a greenhouse tomato crop.
The magnitude and variations of both resistances are shown for a typical 24 hours period.
Although no conclusions can be drawn yet, it may be pointed out that the day/night cycle of the internal resistance is of overwhelming importance.
Smaller variations of the external resistance can be observed, as resulting from a change in temperature of the heating system and a sudden opening of the windows.