Effect of soil moisture stress on the correlation between heat pulse velocity and transpiration

(1) Heat pulse velocity in the stem of pine and sour orange seedling, measured by HUBER's heat pulse method, was correlated with transpiration as measured by determining the humidity of air passed over the plant. This correlation was used to calculate transpiration under natural conditions, when heat pulse velocity only was measured. (2) It was shown that heat pulse velocity at a given transpiration rate was related to water stress. When water potential in the soil decreased heat pulse velocity increased in pine seedlings and decreased in sour orange. This effect of water stress did not appear to be completely reversible. Under conditions of very low water potential in the soil, no useful correlation between transpiration and heat pulse velocity could be obtained either in pine or sour orange. (3) From the data obtained it is concluded that transpiration can be reliably calculated from heat pulse velocity measurements only if the calibration curve relating transpiration to heat pulse velocity is determined shortly beforehand under similar conditions of soil moisture and potential evapotranspiration. Even so, only total daily transpiration can be estimated by this method. Hourly figures of transpiration are unreliable, especially under conditions conducive to low water potential in the plant.