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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,