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

Abstract

AbstractRecent 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 ΨLand Ψswith thermocouple psychrometers for field‐grown alfalfa (Medicago salivaL.), soybean [Glycine max(L.) Merr.], and corn (Zea maysL.) leaves. Good agreement was found between Ψxand ΨLexcept for severely stressed corn leaves where Ψxwas lower than ΨLWhen measuring Ψsand ΨLwith thermocouple psychrometers on flaccid leaves assumed to be at zero turgor potential, Ψsdetermined after freezing and thawing of the tissue agreed well with ΨLmeasured 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 ΨLwith 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 Ψswas 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 ΨLand Ψs