INTEGRATION OF WATERING, SPRINKLING AND CO2 INTO THE GREENHOUSE PROGRAMME
These three factors are all closely interdependent. Higher temperature caused by insolation increases transpiration. Water loss influences the opening of stomates and thereby the CO2 uptake. Increase of CO2 concentration will raise the photosynthetic rate to a higher level and due to the dark partial process of CO2 assimilation, a higher temperature can be tolerated and is often even very beneficial.
Many experiments with light and CO2 under controlled optimal conditions demonstrate yield increase from 50 to 200%, but in the majority of glasshouse crop experiments over an extended period only give a 10–30% effect. A study of the conditions for CO2 uptake in glasshouse crops makes it evident that the possible periods for applying CO2 are too short and fall in periods when light is low or when plants are not in optimal conditions for uptake. During the best part of the day the CO2 level cannot be kept high due to ventilation loss and in the same period the plants are often stressed and the stomates closed.
In our CO2 research we often found uptake curves for CO2 like shown in Fig. 1.
The temperature in a glasshouse is closely related to solar radiation and varies throughout the season.
Fig. 2 shows the average of 10 years solar radiation in Denmark.
The variations during winter time from 20–150 calories cm-2 per day give no heat problems, but variations from dull spring and summer days from 50 to 800 calories make it difficult to make an efficient CO2 application.
In Fig. 3 the true variation in radiation through the year is illustrated.
The CO2 level in a producing greenhouse crop, tomato, cucumber or carnation will be 250–290 dpm with open ventilators and indicate that the air change rate in insufficient within a greenhouse canopy.
Experiments with application of CO2 from the bottom of the canopy through perforated plastic tubes in fully ventilated greenhouses gave no or non-significant yield increase. Our explanation is that the stomates may be closed and measurements often show that they really are. As a result: The leaf temperature increases up to a maximum of 13° above air temperature.