Ratio of cut surface area to leaf sample volume for water potential measurements by thermocouple psychrometers.

Evaporative losses from the cut edge of leaf samples are of considerable importance in measurements of leaf water potential using thermocouple psychrometers. The ratio of cut surface area to leaf sample volume (area to volume ratio) has been used to give an estimate of possible effects of evaporative loss in relation to sample size. A wide range of sample sizes with different area to volume ratios has been used. Our results using Glycine max L. Merr. cv Bragg indicate that leaf samples with area to volume values less than 0.2 square millimeter per cubic millimeter give psychrometric leaf water potential measurements that compare favorably with pressure chamber measurements.

In situ field measurement of leaf water potential using thermocouple psychrometers.

Thermocouple psychrometers are the only instruments which can measure the in situ water potential of intact leaves, and which can possibly be used to monitor leaf water potential. Unfortunately, their usefulness is limited by a number of difficulties, among them fluctuating temperatures and temperature gradients within the psychrometer, sealing of the psychrometer chamber to the leaf, shading of the leaf by the psychrometer, and resistance to water vapor diffusion by the cuticle when the stomates are closed. Using Citrus jambhiri, we have tested several psychrometer design and operational modifications and showed that in situ psychrometric measurements compared favorably with simultaneous Scholander pressure chamber measurements on neighboring leaves when the latter were corrected for the osmotic potential.

Measuring water potential (activity) from free water to oven dryness.

Water activities (potentials) in plant materials were measured over the range from free water to oven dryness with a Spanner thermocouple psychrometer. In a two-step procedure, water was first condensed on the thermocouple junction for several minutes. The sample was then inserted under the wet thermocouple and the maximum psychrometric cooling was measured in about 10 seconds. Calibration was with saturated salt slurries of known water activities. Psychrometric cooling was a nearly linear function of the water activity and of the negative log of the water potential. The psychrometric cooling to water activity relationship agreed with wetbulb temperature depression to relative humidity relationships given in tables. Water activities of wheat grains and leaves decreased sharply in a curvilinear fashion as their water contents decreased. Some problems of the procedure are discussed.

A Rapid Psychrometric Procedure for Water Activity Measurement of Foods in the Intermediate Moisture Range 1.

Commercial vapor pressure thermocouple psychrometers (hygrometers) are now generally accepted for measuring water activity, aw (water potential) in plants and soils, and commercial instruments are available. We have adapted them for aw measurements in the 0.99 to 0.60 range using a two-step procedure. Water is first condensed on the thermocouple; then the sample is inserted in the thermocouple chamber and the psychrometric cooling of the wet thermocouple measured. The procedure is calibrated with a series of saturated salt slurries of known aw values. Typical aw values (with standard deviations) for a variety of foods were: Cheddar cheese, 0.95 ± 0.03; Parmesan cheese, 0.76 ± 0.03; milk powder, 0.75 ± 0.02; milk chocolate, 0.60 ± 0.04; luncheon meat, 0.96 ± 0.03; bread, 0.95 ± 0.03; dried raisins, 0.82 ± 0.02; corn syrup, 0.60 ± 0.02; and orange juice concentrate, 0.80 ± 0.03. The coefficients of variation ranged from 1.9 to 5.8%. When compared with published values obtained by other methods, these figures were within the standard errors of measurement. The thermocouple detector did not foul since it had only vapor contact with the sample. An economical sample chamber and instrument is described. Analysis time is 4 to 8 min. The procedure is accurate, convenient and rapid.

Influence of temperature gradients on leaf water potential.

Water potential was monitored at nine locations along single maize (Zea mays L.) leaf blades with aluminum block in situ thermocouple hygrometers. Water potential showed a continuous decrease toward the tip, with a 2- to 4-bar difference between leaf base and tip under both moist and dry soil conditions. The water potential difference between the soil and the leaf base was about 4 bars. Water potentials decreased during the day and during a drying cycle, and increased at night and after irrigation. Heating a band of a leaf to 40 C or cooling it to 7 C had no influence on the water potential of the affected portion when this was corrected for hygrometer output over standard calibrating solutions at the respective temperatures. Heating or cooling a portion of a leaf had neither short nor long term effects on water potential of more distal leaf portions continuously monitored by hygrometers in dew point readout. Water potential fluctuated with an amplitude of about 1.5 bars and an irregular period of 10 to 30 minutes. Measurements with silver foil in situ psychrometers gave similar results.

Influence of moisture, heat, and light stress on hydrogen fluoride fumigation injury to soybeans.

Soybean (Glycine max (L) Merr.) plants were exposed to a single fumigation with hydrogen fluoride at concentrations sufficient to cause visible injury within 2 days. They were subjected to soil moisture or osmotic stress prior to, during, or after fumigation. Moisture stress before or during fumigation reduced injury because of stomatal closure and reduced fluoride uptake. Moisture stress after fumigation markedly accentuated the injury resulting from a single fumigation compared to plants kept continually under optimum soil moisture conditions. Full sunlight following the fumigation accentuated injury, while shade reduced it. Higher temperatures following fumigation also increased severity of symptoms.

Influence of hydrogen fluoride fumigation on the water economy of soybean plants.

The influence of hydrogen fluoride fumigation on water economy was studied using soybean (Glycine max [L] Merr.). Fumigation caused partial stomatal closure in 1 hour and practically complete closure within 4 hours. The transpiration rate was greatly reduced by fumigation, while the leaf temperature was increased. Water potential increased after 1 day but fell drastically when necrosis occurred. Effects of interrupted fumigation during the day were somewhat less severe in all respects than those of continuous fumigation; nighttime fumigation caused only minor effects. Fluoride uptake was also much less from nighttime than from daytime fumigations.

Matric potential of several plant tissues and biocolloids.

The pressure membrane apparatus was used to study the matric potential (imbibition pressure or moisture tension) of plant tissues and of several organic colloidal preparations.The moisture release curves of aqueous 2% agar, 12% gelatin, and filter paper were smooth parabolic curves between matric potentials of -0.1 and -15 bars. When logarithms of the matric potentials were plotted against logarithms of the moisture content, the data yielded straight lines for agar and filter paper.Slices of fresh tissue lost little water after 2 days in the apparatus at maximum pressure of 15 bars. Osmotic forces in conjunction with cell membranes are able to retain moisture against pressure of this magnitude. After the cells were disrupted by freezing and thawing, up to 90% of the original moisture was removed by a 15 bar pressure, with lesser amounts removed at lower pressures. The results gave a parabolic relationship, and straight lines could be fitted to log-log plots of data from potato tuber and young asparagus stem slices. Sections from the tips of asparagus stems held less moisture at all matric potentials than more basal sections.The method permits the study of the matric potential of tissues independently of the osmotic potential. As measured, however, the matric potential is a composite of matric potentials of colloidal substances in the protoplasm and cell walls after disruption of cells by freezing and mixing of the contents. The value is therefore only an approximation of the matric potentials occurring in the living tissues.