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The measurement of transpiration of water by trees in the field with tritiated water as a tracer depends upon a new application of established theory of radionuclide dynamics in steady-state systems. The techniques required are non-destructive to the trees and probably have negligible disturbing effects on transpiration. Average transpiration rates ranging from 1.75 to 372 liters per day per tree were measured by the proposed method on tropical forest trees which ranged from the understory to the canopy in size. Statistical errors range from 12.1% on the largest tree to 6.2% on the smallest for one standard deviation. Non-random sources of error in the method may include ( 1) loss of tritium from leaves due to rainfall; and (2) possible enrichment of tritiated water in leaves due to differences in vapor pressure and molecular diffusion coefficients between tritiated water and ordinary water. These require further experimental evaluation. The method may be generally applicable to field measurements of transpiration in trees.

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Rates of uptake and loss of tritiated water (HTO) were measured in grass vegetation in the field after the plants had been exposed to an atmosphere containing HTO. Uptake of H T O from the atmosphere by leaves required 66 min to achieve 50% equilibrium activity while stems required 1970 min. Loss of HTO from leaves and stems during daylight however took place at rapid and comparable rates. The half time for loss in stems was 25 min while in leaves it was 35 min. Delabeling of contaminated leaves and sterns in darkness took place at much reduced rates. The half time for delabeling of leaves in the dark was 400 min while for stems it was 2180 min. Rapid labeling and delabeling of plant tissue after exposure from the atmosphere takes place through leaf stomata and by direct transfer through the epidermis. Loss of HTO from plants is primarily dependent on transpiration. In the event of accidental HTO release to the atmosphere near the ground rapid contamination of vegetation could be expected. When the atmospheric source is dissipated, rapid decontamination could also be expected during periods of normal plant transpiration but not at night or other times when transpiration flow is not taking place.