Influence of Temperature Gradients on Leaf Water Potential

Abstract

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 cyde, 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. Temperature gradients and fluctuations are a fact of life for plants. Temperature influences psychrometric water potential determinations, and temperature correction factors have been published (2, 10); We are not aware of studies in which water potentials were measured in parts of the same plant intentionally subjected to wide temperature differences. Here we report such studies on maize, the leaves of which are long enough to permit different temperature regimes on an individual leaf. Our experiments show that temperature gradients have little influence on the water potential of the affected or of other plant parts. MATERIALS AND METHODS Maize plants (Zea mays L.) were grown in soil, three plants/ 18-liter can, and were used when they were 9 to 12 weeks old and beginning to tassle. During the summer, they were grown in a glasshouse and during the winter in a growth cabinet with about 290 ,ueinsteins m-2 sec-1 illumination (quantum sensor LI 190, Lambda Instruments, Lincoln, Neb.) from both fluorescent and incandescent lights, 16-hr photoperiod, and a 30 C:25 C day-night temperature regime. The experiments were done in a growth room with 140 ,einsteins illumination; 12-hr photope-riod, 26 C:20 C day-night temperature regime. The treatments involved subjecting four regions of an intact, attached leaf to different temperatures as follows: base: ambient temperature; midbase: 18 cm, ambient, heated or cooled; mid-tip: 18 cm, ambient, heated or cooled; tip: ambient temperature. The midbase and midtip portions were enclosed in a Styrofoam picnic chest with an added vertical partition and a transparent plastic top. The chest was cut horizontally and the leaf with attached hygrometers was inserted between the two halves. ' Utah Agricultural Experiment Station Journal Paper 2061, in cooperation with Western Regional Project W-67. Leaf water potential was monitored with silver foil hygrome-ters (4) in earlier experiments and aluminum block hygrometers (3, 6; L51, Wescor Inc., Logan, Utah) in later experiments. Both types of hygrometers were calibrated in both the psychro-metric and dew point readout (HR33 microvoltmeter, Wescor Inc.) over standard solutions at temperatures ranging from 5 to 45 C. The cooling coefficient for the dew point readout was also corrected for temperature. Different schedules can be used to read the hydrometers. For earlier experiments, we read the psychrometric value immediately after 10-sec cooling, followed by 2-min dew point equili-bration and reading, followed without additional cooling by the psychrometric reading. Both schedules gave similar readings on calibrating solutions, but the delayed reading time schedule consistently gave higher water potentials on dryer leaves when stomates would be closed. Our interpretation is that condensing water on the junction temporarily lowers humidity within the chamber and that with prolonged time in the dew point mode, during which water neither condenses on nor evaporates from the junction, the humidity in the chamber more nearly approaches that of the mesophyll. With a leaf resistance of 80 cm sec-' and a hygrometer chamber depth of 2 mm, a 2-min delay is sufficient to reduce the difference in humidity between chamber and mesophyll to 0.1 % of the difference occurring immediately after the 10-sec cooling period. The data reported in Figures 1 and 2 are averaged dew point and psychrometric determinations by the delayed schedule. The dew point mode can also continuously monitor the water potential for periods extending to many hours (Fig. 3). The mounting procedure differed in some details from that described elsewhere. The leaf was gently washed with distilled H20 and a sponge 1 hr before the hygrometers were attached. Prior to attachment, a rubber washer (cut from 0.2 mm sheet rubber-dental dam) was cemented inside the leaf slit of the aluminum block to provide a base, or back stop, to press the leaf firmly but gently against the hygrometer unit. The aluminum block hygrometer housings, mounted on wooden dowels on a Styrofoam block base, were then assembled along both sides of the leaf with care to avoid leaf twisting or injury. Then the hygrometer cylinders themselves, each with a Parafilm gasket lightly coated on both surfaces with petrolatum, were inserted in the aluminum blocks, seated firmly against the leaf (with the rubber washer on the other side of the leaf), and secured with the setscrew. Reading could be made within 1 hr but often required 5 hr to equilibrate. With this procedure, we were able to mount and obtain satisfactory readings from as many as eight out of 10 hygrometers. The rubber back stop can be criticized because it insulates the leaf from the aluminum block, possibly resulting in temperature gradients. Such temperature gradients can be detected by non-zero voltages prior to cooling the junction; these were less than 0.3 ,uv in our system, equivalent to less than 1 bar. An alternate method of mounting the leaf directly against the aluminum block resulted in two problems: only a fraction of the units sealed, and the leaf often slipped within the unit. Both problems were aggravated by the need to mount a number of units on a single leaf.