Strict legislation on the release of water and nutrients into the subsoil exists in the Netherlands. Therefore, on-line monitoring and control systems are being developed to tune the supply of water and nutrients to the plants' demand for optimal control of production and reduction of system losses. In this context sensors and control systems are important tools. For Chrysanthemum, however, the effect of reduced irrigation on crop production is not well understood and more data from adequate sensors are required to establish critical soil moisture levels for an optimal crop water status and growth. We studied the effect of different soil water levels and soil desiccation in a climate chamber and under changing light intensities in a greenhouse to assess the critical soil moisture status for optimal growth of Chrysanthemum plants. Moreover, we studied the efficacy of infrared (IR) thermometry as a useful tool for on-line monitoring and control under the conditions of Dutch greenhouse horticulture. It is shown in this study that under moderate climate conditions plants start to suffer from water shortage when soil moisture potential pF reaches values below –32 kPa. Water status of the plant can very well be monitored on-line in a greenhouse as changes in leaf temperature due to water shortage can be detected by IR thermometry, especially at summer radiation levels. In the climate chamber however, leaf temperature did not respond to changes in soil water status, suggesting that the environmental settings of the climate chamber are unsuitable for these kind of experiments. In two of the three experiments at different levels of global radiation a change in leaf temperature is explained by a change in water regime. We conclude that regression analysis of high frequency on-line IR monitoring may be a useful tool for expost analyses of irrigation regimes at high light intensities, and may lead to interesting insights in crop responses.
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