Bench heating for potplant cultivation: analysis of effects of root- and airtemperature on growth, development and production.

This thesis deals with the application of bench heating systems for potplant cultivation, which were developed for application of low temperature heating water from flue gas condensers and external waste heat sources. Compared to the traditional way of heating, a 'reversed' temperature gradient is created on heated benches, with a high root-zone temperature compared to the air temperature. Knowledge of root-zone temperature requirements and - tolerances is necessary in order to fully exploit the potentials of bench heating systems.The influence of bench heating on microclimate was established for an aluminium bench heating system, which was mainly used in this study. It provided a uniform horizontal temperature distribution, and a rather constant vertical temperature gradient up to 45 cm crop height. Humidity did not detoriate with increasing root-zone temperatures.Growth and flowering of Saintpaulia were enhanced by increased root-zone temperature (23-26°C). Flowering of Begonia showed narrow tolerances for the air temperature, whereas the root-zone temperature was of minor importance in the range 18- 26°C. Elevated root-zone (19-30°C) and/or air temperatures (19-24°C) affected growth rate of Ficusbenjamina and Schefflera positively during the first three weeks of growth, but in later stages of growth no effect of root temperature could be detected. Flowering and growth of Spathiphyllum were largely influenced by the air temperature, whereas root-zone temperature was of less importance for flowering in the range 20-26°C. Increasing root-zone (19-26°C) and air temperatures (19-22°C) promoted flowering of Guzmania with out negative effects on size of inflorescences. Plant quality and keepability were not unfavourably affected in the crops under investigation. For Saintpaulia and Spathiphyllum, the number of lateral shoots was reduced at increased root temperatures.Response patterns of plant processes affected by root temperature were analysed with respect to optimal range and tolerances. There is no evidence that water and nutrient uptake were limiting growth of most potplants in the investigated root temperature range. Growth was influenced more by the air temperature than the root-zone temperature. The growth response of Saintpaulia was an exception, where a lower plant dry matter percentage was observed at increased root-zone temperature (resulting in increased plant fresh weight), which might have been due to a reduced resistance for water uptake at these temperatures. Effects of the root temperature were most pronounced on development processes (flowering, shoot formation). For crops with a shoot meristern situated close to the root-zone, effects of increased root-zone temperature on earliness of flowering were most likely due to the locally increased temperature of the shoot apex. Consequences for the use of bench heating systems in commercial practice are discussed with respect to bench construction, climate control, crop management and economical perspectives