‘eee sewer __UND J. J. KORANDA 1453 movement of tritiated water in vascular plants, .ae will be retained as interstitial water. ~qtiated water (THO)is absorbed by the vas~lar plant along with stable water (HHO), yihough isotopic discrimination must occur at sarious Jevels in the plant metabolic processes have been concerned mainly with herbaceous species for practical experimental reasons. Woodyplants present more complex problems in their secondarytissues and pathways of water pants. RANEY) reviewed the literature con- of tritium in 500 ml of water into sandy soils at eat regulate the mass movement of water in eemed with plant~waterrelationships, and also those relationships concerned with the movement sfdeuterated (DHO)andtritiated water (THO) ito and within plants. He grew tobacco plants in tritiated Hoagland’s solution for 80 days, and the following levels of equilibration with the gutrient solution were observed in the various planttissues: Tissue Per cent equilibration with the root solution Stem 97 Leaf veins 92 Mesophyll of leaf 62 The stem of this plant was 48 cm long. He also demonstrated similar results in an HHO-grown wnflower plant; in this case, the tritiated water was shown to be 92 per cent equilibrated with the rootsolutionin 8 h. Therefore, in herbaceous plants, at least, we can expectrelatively rapid absorption of tritiated water and movementinto the shoot tissues after its appearancein the root zone. This rapid near-equilibration was also demonstrated by Cxine'® who found that Phaseolus vulgaris shoot tissues contained 45-65 er cent of the tritium concentration in the nutrient solution within 12h. Tissue-bound writium in all shoot tissues was only about | per cent of the nutrient solution at 72 h. According to Raney," the lack of complete equilibration (20-30 per cent below root soluuon THO concentrations) is attributed to the inflow of unlabeled water from the atmosphere and the dilution of the THOarriving from the root system by metabolic water formed in respiration in the leaves. Tests carried out in a growth chamber having predried atmosphere gave THOlevels in leaf tissues which were near equilibrium (95 per cent) with the root solution concentration. Studies by Raney,'® Cire,Raney and Vaapia™ and Broputpx and Cory,'®) which have produced muchuseful information on the movement. Woops and O’NEAL)injected 5 Ci three depths and followed its uptake by various species of oak in North Carolina. They collected transpired water from leaves by enclosing them in plastic bags. The largest amount oftritium recovered from one branch (5-20 leaves) in 1 day was 4 wCi/ml of transpired water. The amountof dilution of the injected tritium by stable soil water was not measured. In the shallowest depth of tritium application, the transpired water of the trees reached approximately 400 times the background level on the day of application, and by the end of the second day, the high value of 4 ~Ci/ml wasattained in the transpired water vapor. These data point out that in woodyplants as well as in herbaceous species a rapid movement of tritiated soil-water takes place into the root and shoot system, and that changesin soil-water tritium levels would be reflected in the loosewater tritium content of leaves in a very short time, probably within a day. These results were also demonstrated by Lewts and Burcy!®) who injected microcurie amounts of tritium into wells at depths up to 83 ft in northern California. They detected the tritium activity in transpired water vaporfrom oaktrees located 55 ft from the point of injection in 1 day. In general, the results of these physiological studies indicate that tritiated water absorption and movement in plants approximates that of stable water and, for gross movements of water in plants, tritium may be regarded as an excellent tracer for loose or unbound water within the plant. Furthermore,the rapid rate of equilibration with the THO in the root zone indicates that the loose water of the plantis representative of the tritium concentration of the current ground water in the root zone. Changes in levels of tritium in thesoil water will show in the loose water of plant tissues within I-2 days. With respect to the tissue-bound water of plants, isotopic effects are certainly present in the many biochemical reactions that involve the synthesis of new organic matter. Furthermore,