A small portable instrument for measuring the freezingpoint depression of plant tissue has been developed for field use. The instrument is easy to operate and can be constructed from materials costing less than $100.Moisture stress measurements made with the Freezingpoint meter on a variety of plants were compared with vapor pressure psychrometer measurments. Variation between duplicates m the freezing point averaged 1.2 bars, but differences between stress measurements made with the psychrometer and freezing-point instrument averaged 2.6 bars.Additional key words: Freezing-point depression, Diffusion pressure deficit, Plant water relations.
The vapor pressure psychrometer was used as a tool to study the physical state of water in plant xylem vessels. The experimental procedure involved measuring the change in diffusion pressure deficit (DPD) of corn and tomato plants when the stem was cut. When the DPD was greater than 4 bars in tomatoes and 28 bars in corn, the water in xylem vessels no longer appeared to flow in response to hydrostatic pressure gradients. The limiting value of DPD increased as the xylem radius decreased. A mechanism is suggested which describes the physical state and the movement of water through xylem tissue under high DPD. The proposal is based on the pressure difference across a curved air-water interface and on the concept of an electrostatic double layer with its associated osmotic pressure.
A portable freezing point meter was used in the field to measure the water potential gradients in sunflower (Helianthus annuus), beans (Phaseolus vulgaris), corn (Zea mays), wheat (Tritium aestivum), pumpkin (Cucurbita pepo), potato (Solanam tuberosum), alfalfa (Medicago sativa), and sugarbeets (Beta vulgaris). The measurements were made between daybreak and sunrise, and again during the middle of the afternoon on days when the potential evapotranspiration varied between 6.5 and 8.0 mm of water.The gradients varied from a maximum of 0.2 bar per cm in a wheat, down to an undetectable value for pumpkin. Although most of the soil in the root zone was kept at potentials above -1 bar, the bulk of the root tissue had water potentials of -5 to -10 bars. Differences in water potential between shaded and unshaded leaves, and between leaf tissue and guttation fluid suggested a similar drop of several bars between xylem elements and the surrounding leaf tissue in some plant species. The implications of such drops are discussed with respect to plant water transport equations and pressure cell potential measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.