MODELING SHOOT-TIP TEMPERATURE IN THE GREENHOUSE ENVIRONMENT

An energy-balance model is described that predicts vinca (Catharanthus roseus L.) shoot-tip temperature using four environmental measurement s: solar radiation and dry bulb, wet bulb, and glazing material temper ature. The time and magnitude of the differences between shoot-tip and air temperature were determined in greenhouses maintained at air temp eratures of 15, 20, 25, 30, or 35 degrees C. At night, shoot-tip tempe rature was always below air temperature, Shoot-tip temperature decreas ed from 0.5 to 5 degrees C below air temperature as greenhouse glass t emperature decreased from 2 to 15 degrees C below air temperature. Dur ing the photoperiod under low vapor-pressure deficit (VPD) and low air temperature, shoot-tip temperature increased approximate to 4 degrees C as solar radiation increased from 0 to 600 W . m(-2). Under high VP D and high air temperature, shoot-tip temperature initially decreased 1 to 2 degrees C at sunrise, then increased later in the morning as so lar radiation increased. The model predicted shoot-tip temperatures wi thin +/-1 degrees C of 81% of the observed 1-hour average shoot-tip te mperatures. The model was used to simulate shoot-tip temperatures unde r different VPD, solar radiation, and air temperatures. Since the rate of leaf and flower development are influenced by the temperature of t he meristematic tissues, a model of shoot-tip temperature will be a va luable tool to predict plant development in greenhouses and to control the greenhouse environment based on a plant temperature setpoint.