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ISHS Acta Horticulturae 654: International Workshop on Models for Plant Growth and Control of Product Quality in Horticultural Production

DECISION SUPPORT FOR GREENHOUSE CLIMATE CONTROL USING A COMPUTERISED MOLLIER DIAGRAM

Author:   U. Schmidt
Keywords:   light use efficiency, phase diagram, photosynthesis, stomata conductivity, tomato, transpiration, vapour concentration difference, vapour pressure difference
Abstract:
At present, there are only a few aids available for estimating the momentary quality of the greenhouse climate, which concerns the set-up of optimal air temperature and humidity conditions. The combined effects of temperature and air humidity can only be illustrated inaccurately from measuring diagrams and, for production processes in commercial greenhouses, no information about the mass and energy fluxes of the plants is available. By using a digitised mollier-h,x-diagram calculations and representations can be carried out to aid air-conditioning technology. With the help of a thermodynamic model and a computer program, the diagram was digitised. The use of the equations for the thermal properties of wet air, make it possible to use the flexible plot of the diagram in the areas of temperature, absolute air humidity and air pressure. A special routine of the program allows the calculation and representation of the air condition point. An additional value can be imported into the database, which can be plotted by a colour scale. In a commercial production greenhouse for tomatoes, measurements were obtained with a phytomonitoring system that selected microclimate and plant data. With the help of leaf chambers and a gas exchange system, transpiration mass flow and net photosynthesis were measured at eight places every five minutes during the whole culture (nine months). Leaf temperature and vapour concentration difference (VCD) were calculated. From net photosynthesis and measured global radiation, light use efficiency (LUE) was calculated. The data were represented monthly in the Mollier diagram. As a result, the sectors of the highest LUE could be shown. In areas with high VCD, low LUE values were obtained, possibly due to closing stomata or photoinhibition. As an indication for stomatal closure low stomatal conductivity was measured at higher VCD.
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