Research on antitranspirants may have application in increasing the yield of water from watersheds by reducing transpirational losses from vegetation under certain environmental conditions. There are three broad groups of antitranspirants: (1) reflecting materials that decrease the heat load on the leaf. (2) film‐forming materials that hinder the escape of water vapor from the leaf, and (3) stomata‐closing materials that increase stomatal resistance. Antitranspirants are most effective in decreasing transpiration when other resistances in the passage of water to the roots, through the plant, and to the atmosphere are not great, when there is good coverage of the stomatal bearing leaf surfaces (except for reflecting materials), when new foliar growth following treatment is minimal, and when optimum concentrations and application rates are used.

An antitranspirant (AT) can retard excessive groundwater consumption by phreatophytes without eradicating them. To determine whether aerial spraying (the only realistic application method for most sites) of a 6% (vol/vol) wax-based AT emulsion would provide adequate spray coverage and reduce transpiration, multiple passes were made with (1) a fixed wing plane on salt cedar, cottonwood, and willow and (2) a helicopter on salt cedar. Spray coverage on the ground was 30-100% in the open (depending on the number of passes and wind drift) and 10-90% under the canopies (depending on vegetation density). Average coverage on tags ranged from 13% in the lower canopy to 75% in the upper (depending on species and application rate) after spraying by the fixed wing plane and 47 to 98% after spraying by the helicopter. Coverage in the center of dense bushes was 0-20%, but transpiration is only minimal there. Scanning electron microscope photomicrographs showed considerable AT on foliage in the upper canopy and lesser amounts in the lower; the film was detected even 24 days after spraying. Aerially applied AT increased resistance to leaf water vapor diffusion by 150% during the first few days and by 80% thereafter. Transpiration of outer foliage of salt cedar was reduced 50% initially and 20% after two weeks without phytotoxicity.

A wax-based antitranspirant (AT) was sprayed on leaves of three phreatophytes, salt cedar (Tamarix sp.), cottonwood (Populus sp.), and willow (Salix sp.), as an alternative to eradicating them to conserve water. Scanning electron micrographs gave information on the nature of the foliar surfaces and coverage by the AT wax. A 10% solution of AT (vol/vol) reduced the transpiration rates of container-grown phreatophytes by 35-75% 1 day after spraying and by 17-56% after 4 days. In gas exchange studies with salt cedar in the field, transpiration was reduced 40% by a 'light' application of 6% AT and 70% by a 'heavy' application. The AT also increased resistance to water vapor diffusion and the water potential of the sprayed leaves. On salt cedar, phytotoxicity occurred at high temperature and solar radiation only if spray applications were very heavy. The AT conserved water much more effectively when it was applied to the outer part of the canopy than when it was applied to the inner shaded foliage, where transpiration was already minimal. These data from sprays applied from the ground provided a basis for trials of AT application by aircraft.