EFFECT OF STABLE BORON ISOTOPES ON GROWTH
OF TOBACCO IN SOLUTION CULTURES
J. E. MCMURTREY, JR. AND H. B. ENGLE
BUREAU OF PLANT INDUSTRY, BELTSVILLE, MARYLAND
Received December 20, 1951
It has been stated and is generally accepted that the isotopes whetlher
stable or radioactive behave relative to biological functions as do the commonly occurring forms. It appears to have been assumed that they will
function in all phases in the physiology of a green plant in the same way.
However, lethal effects (7) and other effects (1) on the plants have been
reported but appear to be associated with the level of radioactive material
to which the plant is exposed. There were some reductions in growth even
when the concentration was below what was considered the critical level.
These findings have been supported by recent work (2).
The extensive studies (3) now under way with radioactive isotopes of
the elements essential to plant growth including carbon, calcium, nitrogen
and phosphorus, would appear to be intimately related to the assumption
that they function biologically as do the commonly occurring forms.
The micronutrient elements have not apparently received any considerable attention. One paper by STOUT and M1EAGHER (8) deals with radioactive
molybdenum effects in plant growth. No publication dealing with stable or
radioactive isotopes of boron effects on plant growth has been noted.
The stable isotope of boron, B10, recently made available by the Atomic
Energy Commission appeared to offer an interesting approach to study any
differences in growth from stable boron isotopes. Comparisons using boric
acid commonly available as the reagent grade and boric acid prepared from
the complex supplied by the commission in which 96% of the boron occurred
as B'0 were carried out in solution cultures using tobacco as the test plant.
The cultures were located on three adjacent tables with the B" groups
on one table and the B'0 groups on another with the groups with no boron
on a third table placed between the two. The tables were 13.5 feet long
and one foot seven inches wide and were located one foot 10.5 inches apart.
The plants were thus grown in an area 8.5 feet by 13.5 feet so that if there
were a position effect on growth it should not be of a serious nature.
The procedures used in culturing the plants were essentially those described previously (5, 6). The solutions used in these studies were of the
same composition as those previously used (6). They contained the following voluine molecular concentrations: Ca (NO3) 2 4H20, 0.00617; KNO3,
0.00108; Mg(NO3)2 6H20, 0.00062; KH2PO4, 0.00211; and NH4Cl, 0.00140.
The parts per million of the various elements supplied were: Nitrogen, 225;
phosphorus, 66; potassium, 125; calcium, 247; magnesium, 30; sulphur, 20;
and chlorine, 50. The following elements were supplied in parts per million
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PLANT PHYSIOLOGY
in the amounts indicated as sulphates. Copper, 0.125; manganese, 1.0; and
zinc, 0.5. Iron as the sulphate was added to supply 1.0 p.p.m. of this element on April 8, the starting date of the experiment, and thereafter on April
11, 12, 13, 14, 15 and 22.
The Connecticut Broadleaf variety of tobacco was used in these studies.
The seeds were sown in a flat of steam-sterilized greenhouse potting soil
mixture, and after approximately one month they were transplanted to twoinch pots containing the same soil sterilized in the same manner. After the
plants were allowed to develop in these pots for about two weeks the roots
were washed free of soil, and the plants were transferred to the starter solution without covers as described (4) for about two weeks. When the largest
leaves were about three and one half by seven inches the plants were placed
in the experimental solutions.
The variable under investigation consisted of a comparison of boron isotopes as a source of the element. In one group the boron was derived from
reagent grade of boric acid, which is made up of the usual occurring proportions of the isotopes of boron, 18.4% B10 and 81.6% B"1, which is referred
to in the following discussions as B11. Another lot was made up from that
containing 96%o B10 and 4%o B"1 and referred to in the following discussion
as B10. All calculations were made on the basis of above analyses to supply
a given amount of boron atoms, but are referred to below as the equivalent
concentrations that would have been supplied by natural boron.
There were five groups of treatments. One group of 12 plants received
no boron. One group of six plants received 0.1 p.p.m. B11. One group of
six plants received 0.5 p.p.m. B'1. One group of six plants received 0.1
p.p.m. B'0. One group of six plants received 0.5 p.p.m. B'0.
The plants were placed in the experimental solutions April 8, 1949, and
on April 14 the first sign of boron deficiency was evident on one plant of the
12 growing in the solution to which no boron was added. The base of the
younger leaves making up the terminal bud showed a faint light green color.
The next leaf below was brittle and broke when handled. At this date there
was no apparent evidence of abnormality in any other plants.
On the tenth day after the start of the experiment all twelve plants that
were growing in the solution to which no boron was added showed characteristic boron deficiency symptoms culminating in breakdown of the terminal
bud. Also, one plant receiving 0.1 p.p.m. B" showed the first indication of
boron deficiency 10 days after transfer to the experimental treatment. After
18 days, a second plant receiving 0.1 p.p.m. B" was showing deficiency
symptoms, while none of those receiving 0.1 p.p.m. B'0 showed any symptoms, and appeared to be growing more uniformly than those receiving B".
There was no abnormality in any plants receiving 0.5 p.p.m. boron in either
form. After 21 days, five plants out of six with 0.1 p.p.m. B" were showing
boron deficiency, while two out of six with 0.1 p.p.m. B'0 were showing deficiency. After 25 days, all plants with 0.1 p.p.m. B" showed deficiency,
and four out of six with 0.1 p.p.m. B'0 showed deficiency. After 28 days,
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BRIEF PAPERS
the remaining two plants in the 0.1 p.p.m. B'0 solution showed the characteristic breakdown symptoms associated with boron deficiency. All plants receiving 0.5 p.p.m. boron in either form showed no abnormality and were
starting to flower.
All cultures received 100 cc. of stock nutrient solution three weeks after
the start of the experiment. This was added to the solutions in which the
plants were growing and no aditional micro-elements were added. On the
twenty-eighth day (MAay 6) all plants were harvested and hung in the greenhouse to air-dry.
The results from this test are shown in table I. The air-dry weight of
leaf produced from the two rates and sources of boron are in close agreement. There was a striking increase in yield of leaf when boron was used
either as B'0 or B" over that obtained when no boron was used. The most
striking effect is manifested by the stalk weights when the no-boron treatment is contrasted with the two rates and sources. There was also a decided
TABLE I
AVERAGE AIR-DRY WEIGHT PER PLANT FOR TOBACCO GROWN IN
NUTRIENT SOLUTIONS WITH TWO FORMS
AND RATES OF BORON.
Boron
Form
Rate
Leaves
None
B81
B10
B'1
B'0
p.p.m.
None
0.1
0.1
0.5
0.5
gm.
6.18
13.66
14.77
14.33
14.50
Air-dry weight
Stalk
Roots
gm.
1.07
4.18
6.68
9.84
10.13
gm.
1.49
3.69
3.88
4.43
4.49
Total
gm.
8.74
21.53
25.33
28.60
29.12
Standard
error
of total
±
±
±
±
±
2.69
2.77
1.38
2.36
1.48
increase in the air-dry weight of stalks with the higher compared to the
lower rate of boron. There appears to be a slight advantage for the B'0
treatment of a given concentration in yield of stalk although the statistical
significance of this difference is questionable. The yield of roots was greatly
increased by both forms of boron at the two rates as contrasted with the
no-boron treatment. The higher rate of boron has produced a higher airdry weight of roots. The total air-dry weight shows much the same trends
as that of the stalks. There is a decided increase in total yield for both
forms and rates of boron. The higher rate of boron has resulted in a significantly higher yield than the lower rate. As previously pointed out, this
difference is largely the result of a higher yield of stalk.
In every instance, the total yields are higher from the B'0 than from the
B" but the difference, based on weights of individual plants, was not found
to be statistically significant at the higher concentration. The difference
appears to be significant at the lower concentration. During the growth of
the plants there was no observable difference in growth effects from the two
isotopes of boron with the exception of that previously noted, namely, the
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130
PLANT PHYSIOLOGY
earlier occurrence of deficiency symptoms where the low rate (0.1 p.p.m.)
B" was used. This difference was not due to extra molecules of boron from
the B'0 source since the two forms were added to supply an equal number
of molecules of each form. Actually there was less weight added in the B'0
form than in the B" form.
It would appear from this test that B'0 produces essentially the same
growth responses as B" in tobacco plants when used to supply the element
boron in solution cultures.
The stable isotope of boron was secured from the Isotopes Division,
Atomic Energy Commission, as the BF3CaF2 complex containing 6.9% of
elemental B of which 96%o was B'0. Mr. Frank 0. Lundstrom of the
Bureau's Division of Fertilizer and Agricultural Lime kindly converted this
compound to boric acid according to the miiethod of Dr. Walter B. Keighton.
1.
2.
3.
4.
5.
6.
7.
8.
LITERATURE CITED
ARNASON, T. J., CUMMING, ELAINE, and SPINKS, J. W. T. Chromosome
breakage induced by absorbed radioactive phosphorus p32. Canadian Jour. Res. 26C: 109-114. 1948.
BLUME, J. M., HAGEN, C. E., and MACKIE, RUTH M. Radiation injury
to plants grown in nutrient solutions containing p32. Soil Science
70: 415-426. 1950.
BURRIS, R. H. Isotopes as tracers in plants. Bot. Review 16: 150-180.
1950.
ENGLE, H. B. and MCMURTREY, J. E., JR. Effect of algae in relation to
aeration, light and sources of phosphorus on growth of tobacco in
solution cultures. Jour. Agr. Res. 60: 487-502. 1940.
MCMURTREY, J. E., JR. The effect of boron deficiency on the growth of
tobacco plants in aerated and unaerated solutions. Jour. Agr. Res.
38: 371-380. 1929.
MCMURTREY, J. E., JR. Distinctive effects of the deficiency of certain
essential elements on the growth of tobacco plants in solution cultures. U. S. Dept. Agr. Tech. Bull. 340. 1933.
SPINKS, J. W. T., CUMMING, ELAINE, IRWIN, L. B., and ARNASON, T. J.
Lethal effect of absorbed radio-isotopes on plants. Canadian Jour.
Res. 26C: 249-262. 1948.
STOUT, P. R. and MEAGHER, W. R. Studies of molybdenum nutrition of
plants with radioactive molybdenum. Science 108: 471-473. 1948.
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