Errors in the estimation of pre-excision plant water potential

Summary

Over the same water potential range, Pinus ponderosa (Laws.) seedling shoots lost less water when dried by sap expression than when air dried either before or after shoot excision (Fig. 1). It is hypothesized that this phenomenon was caused by air in the xylem elements of air-dried tissue and entrapment of the air during subsequent pressure chamber measurements. When shoots were dried by sap expression and pressure was released, the shoot water potential estimate became less negative unless pressure was reapplied immediately (Fig. 2). The pressure chamber reading of shoots dried intact, however, did not change after one hour of equilibration at atmospheric pressure (Fig. 2). It was concluded that there was air in the xylem of intact-dried shoots before excision but little or no air entry into xylem elements after excision. For the seedling shoots used in this study, therefore, it would be appropriate to calibrate the pressure chamber with thermocouple psychrometer measurements (Fig. 3).