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This article in AJ

  1. Vol. 78 No. 2, p. 239-244
    Received: Nov 23, 1984

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Comparison of Water Potential Components Measured with a Thermocouple Psychrometer and a Pressure Chamber and the Effects of Starch Hydrolysis1

  1. J. M. Bennett,
  2. P. M. Cortes and
  3. G. F. Lorens2



Recent reports have shown that discrepancies exist between water potential components measured with a pressure chamber and those determined with a thermocouple psychrometer. It has been hypothesized that the differences might be attributable to a number of reasons including starch hydrolysis within the tissue during the time required for equilibration when using thermocouple psychrometers. We conducted a series of experiments to (i) compare xylem water potential measured with a pressure chamber (ΨX) with leaf water potential (ΨL) and leaf osmotic potential (ΨS) measured with a thermocouple psychrometer, and (ii) examine changes in starch concentration during the time period required to measure ΨL and ΨS with thermocouple psychrometers for field-grown alfalfa (Medicago saliva L.), soybean [Glycine max (L.) Merr.], and corn (Zea mays L.) leaves. Good agreement was found between ΨX and ΨL except for severely stressed corn leaves where ΨX was lower than ΨL When measuring ΨS and ΨL with thermocouple psychrometers on flaccid leaves assumed to be at zero turgor potential, ΨS determined after freezing and thawing of the tissue agreed well with ΨL measured before freezing, suggesting that dilution of cell sap by apoplastic water may be either very small or fully compensated for by production of solutes during equilibration of the tissue. There was a tendency for some starch hydrolysis to occur during the 4-h period between harvest of leaf material and determination of ΨL with a thermocouple psychrometer, especially with tissue sampled at midday and late afternoon when leaf starch concentrations were high. However, significant starch hydrolysis between freezing and thawing of the leaf tissue and measurement of ΨS was not detected. Starch hydrolysis was dependent on time of day and species; however, except for soybean leaves having high concentrations of starch, starch hydrolysis during equilibration could potentially account for only small changes (generally <0.1 MPa) in measured ΨL and ΨS

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