A model was constructed to predict shoot-tip temperature of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) according to an energy-balance equation by using five greenhouse environmental factors: dry-bulb, wet-bulb, and sky (glazing or shade screen) temperature; transmitted shortwave radiation; and air velocity. An experiment was conducted to collect the five environmental variables that were used as model inputs, and shoot-tip temperature data were used to validate the predicted shoot-tip temperature in a commercial greenhouse. The standard deviation of the difference between predicted and measured shoot-tip temperature was 0.798 and was calculated by using 8547 data points, and >84% of the actual and predicted data points were within 1 °C. A sensitivity analysis performed with the model indicated that, among the three temperatures measured, plant shoot-tip temperature was primarily influenced by the dry-bulb temperature. For example, shoot-tip temperature increased an average of 0.74 °C for every 1 °C increase in dry-bulb temperature when dry-bulb temperature varied from 28 to 42 °C, wet-bulb temperature was 27.8 °C, sky temperature was 39.8 °C, shortwave radiation (285 to 2800 nm) was 760 W·m-2, and air velocity was 0.44 m·s-1. Under these conditions and a dry-bulb temperature of 32.6 °C, an increase in shortwave radiation of 500 W·m-2 increased the shoot-tip temperature by an average of 3.3 °C. This developed model may be a useful tool to predict shoot-tip temperature and evaluate the effect of greenhouse environmental factors on shoot-tip temperature.
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