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TEXAS AGRICULTURAL EXPERIMENT STATION

R. D. LEWIS. Director, College Station. Texas

 

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Responses ofCotton
to 2,44)  

DAVID R. ERGLE and A. A. DUNLAP

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GIBB GILCHRIST. Chancellor

 

[Blank Page in Original Bulletin]

Pre face

Z/l-D is an organic chemical commonly referred to as one of the
hormone-type weed killers. At certain relatively heavy dosages, it is
capable of burning the foliage and killing cotton plants. Light dos-
ages cause the cotton plant to produce, a week or two later, abnormal
new growth which is badly distorted and useless in cotton production.

This bulletin gives the results of some experimental tests with
extremely light applications of ZA-D to cotton under partially con-
trolled greenhouse conditions. Certain applications of minute amounts
of Z/l-D caused some foliage distortion without noticeably affecting
the set of bolls or the yield of seed cotton. Slightly heavier applica-
tions caused much disturbance in normal growth of the plant, vari-
ations in certain chemical plant constituents and reduction in final
yield. Calculations based upon this work indicate that one ounce of
ZA-D could possibly cause serious damage to every plant in S5 acres
of cotton, if uniformly distributed.

CONTENTS

a Page
Preface . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Fundamental Data Taken on the Plants . . . . . . . . . . . . . . . . . . . . . . .. 6

Harvesting and Ginning of Cotton . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6

Fiber Analysis . . . . . . . . . . . . . . ._ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Chemical Analyses and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Elfect on Subsequent Vegetative Development . . . . . . . . . . . . . . . . . .. 8

Elfect on Fruiting and Yield of Cotton . . . . . . . . . . . . . . . . . . . . . . . . .. 10

Effect on the Lint Characters of Cotton . . . . . . . . . . . . . . . . . . . . . . .. 12
Elfect on the Chemical Composition of the Main-stalk Leaves . . . . . 13
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18

 

BULLETIN 713 SEPTEMBER 1949

Responses oi Cotton to 2,4-|I
DAVID R. ERGLE and A. A. DUNLAP*

IDESPREAD use of the relatively new hormone weed-killer,

2,4-D (the abbreviation for all common derivatives of 2,4-
dichlorophenoxyacetic acid) has resulted in many cases of un-
foreseen injury to susceptible crops. In the case of cotton, for
example, unfavorable effects from drifting of the.chemical have
been reported from many states in the Cotton Belt. Ignorance
of the fact that unbelievably small quantities of 2,4-D are capa-
ble of materially altering the growth and fruiting of sensitive
plants has been a major factor in the many unfortunate occur-
rences of this sort.

Experimental and observational e-vidence (1, 2, 18) show that
cotto-n is very sensitive to the 2,4-D compounds commonly used
in weed-control work. In addition, there is published informa-
tion (1, 2) on specific morphological responses of cotton to
2,4-D which has aided in making apparent to the layman the
characteristic symptoms of 2,4-D injury- However, since the
commercial use of this selective herbicide began in 1945 (8),
it is natural that much remains to be known of its effects on
cotton under various conditions of growth. Among the factors
most likely to influence the reaction of 2,4-D on cotton, such as
age and size of plant, form of 2,4-D (acid, salt or ester) and
amount of the chemical, the latter is probably the most critical.

A greenhouse experiment was conducted at College Station
during the winter 1948-49 to ascertain the exact effects of small
quantities of 2,4-D on cotton. It was the purpose of this experi-
ment to study the growth and chemical response of the cotton
plant to small amounts of the chemical. This information should
be useful in predicting probable effects of accidental applica-
tions. It should be of further use in distinguishing between mild
effects and those severe enough to produce plant injury which
would affect the final yield.

MATERIALS AND METHODS

Plant culture and application of the 2,4-D: Over 100 cotton
plants, Stoneville 2B variety, were grown singly in manured
Houston Black Clay in two-gallon glazed jars with a drain hole
in the bottom. The seed were planted September 15. On October
14, when the plants were about one foot high with 5 to 6 true

*David R. Ergle is chemist, Division of Cotton and Other Fiber Crops
and D1seases,\Bureau of Plant Industry, Soils and Agricultural Engineer-
mg, U. S. Department of Agriculture.

A. A. Dunlap is head, Department of Plant Physiology and Pathology,
Texas Agricultural Experiment Station.

6 BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION

leaves, they were divided into 5 groups of 23 plants each. One
group served as the control and received no 2,4-D while each
plant in the remaining four groups was given an amount of
sodium salt of 2,4-D (monohydrate) equivalent to 0.002, 0.01,
0.02 and 0.04 milligram (mg.) of 2,4-D acid, respectively. These
amounts were prepared in 4 aqueous solutions of the sodium
salt of 2,4-D in concentrations equivalent to 1, 5, 10 and 20
parts per million (p.p.m.) of the acid, respectively. In addition,
each solution contained two milliliters of Grasselli spreader per
liter of 2,4-D solution. At this time, one large leaf on each of
the plants in a group to be treated was dipped in a known vol-
ume of one of the 2,4-D solutions; the excess solution which

dripped from the leaf was caught in the container. After treat- i

ing all the plants in a given group, the volume of the remaining
solution was accurately determined to compute the amount of
solution applied to the plants. When application of the 2,4-D
was completed, the treated and untreated plants were random-
ized along the greenhouse benches in 23 blocks of 5 plants each;
each block contained an untreated plant and one from each of
the four 2,4-D treatments.

Fundamental Data Taken on the Plants

Flowering: Starting November 4, when the first flower ap-
peared on the experimental plants, and continuing for 35 suc-
cessive days, newly formed flowers on all the plants were tagged
and counted daily. During this 35-day period, the rate of flow-
ering reached a maximum and then declined to a low level.

Boll set:
viously tagged flowers were counted on each plant.

Fruiting and vegetative structures: The following counts,
measurements and weighings were made, except as noted, on
December 17:

(a) Height of main-stalk from the soil to the terminal bud-

(b) Number of main-stalk nodes, exclusive of the cotyledon
scars and terminal bud.

(c) Number of vegetative branches 30 centimeters or more
in length, per 23 plants.

(d) Length of longest fruiting branch per plant.

(e) Number of nodes on the longest fruiting branch.

(f)

Mean weight of main-stalk leaves collected November 29
and December 17 at the fourth node from the terminal
bud.

Harvesting and Ginning of Cotton

Cotton from the experiment was first picked January 6, 1949,
at which time approximately 50 percent of the bolls on the un-
treated plants were open. The second and final picking was made
January 27. The harvested cotton was ginned in a small roller-
type gin preparatory to obtaining lint samples for fiber analysis.

On December 29, the bolls that were set from pre- i

 

RESPONSES OF COTTON TO 2,4-D 7

Fiber Analysis

The usual fiber determinations were made on the lint samples,
grouped according to treatment and to date of picking. These
were made by the Cotton Testing Laboratory, Production and
Marketing Administration, College Station.

Chemical Analyses and Methods

The main-stalk leaves, previously noted in connection with
the plant data, were arranged in groups according to date of
collection and treatment. They were analyzed for sugar, starch,
hemicellulose, cellulose, soluble and total nitrogen, and organic
acids.

The analytical methods used for the labile carbohydrates de-
terminations were given in detail by Eaton and Rigler (3).
Briefly, the sugars. were extracted from the oven-dried leaves
with boiling 80 percent ethanol and determined by the semi-
micro method of Wildman and Hansen (20). Starch, contained

~in the residue from the sugar extraction, Was determined by

combined diastatic and acid hydrolysis- Hemicellulose was de-
termined in the starch-free residue by HCl hydrolysis. The
washed and dried residue from the hemicellulose determination
was treated with 72 percent H2804 (5) and followed with a 3
percent H280, hydrolysis. The reducing power of the hydroly-
sate, calculated as anhydrous dextrose, was used as an index of
the cellulose content of the leaf.

Total nitrogen was determined by the well-known Kjeldahl

method and the soluble fraction was based upon the amount of
leaf nitrogen extractable with water at 80°C. Residual nitrogen
was calculated as the difierence between total and soluble
nitrogen.

Total organic acids were ether-extracted from the leaf tissue
previously acidified to pH 1.0 and were determined by titrating
electrometrically between the limits of pH 7.8 and pH 2.6 (12).
The individual organic acids, (citric, malic and oxalic), were
determined by the methods of Pucher, et al., (13). The. quantity
of the unknown or unidentified acid fraction was obtained by
substracting the sum of citric, malic, and oxalic acids from the
determined value for total organic acids.

RESULTS

For simplicity of presentation, the equivalent amounts of
2,4-D acid applied to the cotton leaves, 0.002, 0.01, 0.02 and 0.04
milligram, will for the most part be referred to hereafter as
treatments 1, 5, 10 and 20, respectively. These numbers are of
further significance in that they also represent the relative
amounts of 2,4-D applied and the actual concentrations, parts
per million, of the 2,4-D solutions employed in treating one leaf
on each plant. -

8 'BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION

Elfect on Subsequent Vegetative Development

As previously stated, the plants at the time of the 2,4-D ap-
plications (October 14) were about one foot high and had five
t0 six true leaves. Seven days later, the terminal leaves of plants
in treatments 5, 10 and 2O were showing characteristic symp-
toms of 2,4-D injury; namely, crinkly margins and elongated
lo-bes on the young le-aves. A monthlater, the extent of injury
had increased as shown by the number of affectedleaves in Fig-
ures 1 and 2. Excepting treatment 1, which manifested no injury

Figure 1. Early manifestations of 2,4-D injury to cotton leaves; 1, 5,
10 and 20 received 0.002, 0.01, 0.02 and 0.04 milligram of 2,4-D, respec-
tively. C, not treated.

Figure 2. Two of the plants in Figure 1 showing mild (5) and severe
(20) leaf reaction to 2,4-D. Note the crinkly leaves without much dwarfing
in 5, whereas the affected leaves in 20 are greatly reduced in size, curved
and attenuated.

RESPONSES OF COTTON TO 2,4-D 9

at any time during the experiment, the degree of leaf injury
Was found to increase progressively with the amount of 2,4-D
applied to the plant. The symptoms were mild in treatment 5
and were severe in treatment 20. Injured leaves from the latter
group of plants were attenuated to such an extent that the
larger veins were close together and practically parallel. The
reduction in weight of individual main-stalk leaves from near

the top of the treated plants varied 27 percent in treatment 5

to 77 percent in treatment 20, as compared with leaves similarly
located on the untreated plants. However, leaf development on
many of the fruiting and vegetative branches Was not severely
affected. It was not until the plants were approaching maturity
(time of first lint harvest) that the main-stalk leaves developed

normally.

Plant height (Figure 3) and the number of main-stalk nodes
Were progressively increased above those of the untreated plants
by increases in the amounts of 2,4-D applied. With the exception
of the apparent increase in number of main-stalk nodes in
treatment 1, all were highly significant (Table 1)- The maximum
increases in plant height and in the number of nodes occurred
in treatment 20. The increases amounted to 38 percent and 21
percent, respectively, over the control.

A stimulation in the formation of longvegetative branches
Was also noted among the plants in treatments 1O and 20, Fig-
ures 3 and 4. Based on 23 plants in each treatment, the former
had a total of 14. such branches, 30 centimeters or more in

Figure 3. Elfect of 2,4-D on plant he-ight and the development of vege-
tative branches by the time o-f the first harvest of cotto-n. 1, 5, 10 and 20
received 0.002, 0.01, 0.02 and 0.04 milligram of 2,4-D, respectively. C,
untreated.

10

BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION

Table 1. Elfect of 2,4-D on the vegetative development .of cotton
Number of Number of Fresh
Length of fruiting vegetative weight off
Height of Number of longest nodes on branches, 23_ main-f
2.4-D per plant, mg. plant, cm. main-stalk fruiting longest 30 cm. Ystalk
nodes branch, fruiting or longer, leaves, .
cm. branch on 23 plants* gm.*| ,
None (control) 60.8 15.0 18.2 2.8 2. 82.
0.002 (treatment 1) 65 0 15.8 18.0 2.7 2. 83.
0.01 (treatment 5) 68.1 16.2 18.3 2.8 3. 60.
0.02, (treatment 10) 79.7 17.5 18.9 2.7 14. 41.
0.04 (treatment 20) 84.0 18.2 13.7 2.3 29. l9.
Percentage change as
a result of:
Treatment 1 6.9** 5.3 —1.l -—3.6 . . . . . . . . . . .. 1.2
Treatment 5 l2.0** 8.0** 0.5 0.0 . . . . . . . . . . . . —26.8 ‘
Treatment 10 31.l** l6.7** 3.8 —3.6 . . . . . . . . . . .. —50.0
Treatment 20 38.2** 2l.3** ——24.7** ———l7.9** . . . . . . . . . . . . ——76.8 i

*Data not treated statistically.
**Statistically significant by odds of 99 to 1.

length, while the latter treatment had a total of 29. These
branches, for the most part, produced fairly normal leaves and
bolls but none of the latter had reached maturity by the time
of the second harvest, January 27, 1949, of bolls from tagged

flowers.

The growth of the fruiting branches, as judged by their
length, appeared t0 be less affected by the 2,4-D applications
than did the vegetative branches. Only the heaviest application,
treatment 20, produced a significant effect, a 25 percent reduc-
tion in the length of the longest fruiting branch per plant and
an 18 percent reduction in the number of its nodes.

Elfect on Fruiting and Yield of Cotton

The average untreated plant produced a total of 18.3 flowers
over a 35-day period of flowering (Table 2). This value was not
significantly altered either by-the smallest application of 2,4-D
used, or by the next to the largest dose, treatment 10, which
caused considerable leaf injury. H0-wever, treatment 10 did
cause a reduction of 7 percent in number of flowers produced,
but the difference was too small for statistical significance. On
the other hand, the heaviest dose of 2,4-D, treatment 20, caused
a 33 percent reduction in flower production, which was highly

significant.

In keeping with the efiect of 2,4-D on flowering, only the
heaviest dosage reduced significantly the number of bolls set

A per plant, Table 2. Again, as in the case of flowering, the next

to the largest amount of applied 2,4-D, treatment 10, caused a
slight reduction (12 percent) in boll set which closely ap-
proached but did not attain the .05 level of significance. The

 

RESPONSES OF COTTON TO 2,4-D 11

Table 2. Effect of 2,4-D on flower production, boll set and yield of cotton

Yield of seed cotton, grams per 23 plants*

Number of Number of

l, 2,4-D per plant, mg. flowers bolls set Percentage of
per plant per plant First Second Total total obtained
harvest harvest yield at first harvest
e (control) 18.3 7. 6 391 353 744 53
I02 (treatment l) 18.3 7.6 412 380 792 52
ll (treatment 5) 18.5 7.8 336 392 728 46
V I2 (treatment l0) 17.1 6.7 162 431 593 27
.04 (treatment 20) 12.3 3.6 54 259 313 17
'~ Percentage change as
a result of: .
tlnent l 0.0 0.0 . . . . . . . . . . . . . . . . . . .. 6.5 . . . . . . . . . . . . . . . . ..
tment5 1.1 2.6 . . . . . . . . . . . . . . . . . . .. —-2.2 . . . . . . . . . . . . . . . . ..
tment 10 -a.s -11.s ' . . . . . . . . . . . . . . . . . . .. —20.3 . . . . . . . . . . . . . . . . ..
tment 2o --32.s** -52.0=i=* . . . . . . . . . . . . . . . . . . . . -57.9 . . . . . . . . . . . . . . . . ..

‘Data not treated statistically.
"Statistically significant by odds of 99 to 1.

Table 3. Effect of 2,4-D on lint characters of cotton

s Fiber Length Fiber
Lint, weight Percent Tensile
2.4-0 per plant, mg. percentage Upper per inch, mature strength
half mean Mean Uniformity micro- fibers 1000 lb.
inches inches ratio grams per sq. in.

Cotton from first picking

one (control) 33.0 1 . 14 .88 77 2.5 59 80
b.6002 (treatment 1) 32.5 1.10 .86 7s 2.1 e4 s2
L01 (treatment s) 32.7 1.13 .ss 7s 2.3 s2 s0
5.0: (treatment 10) 33.3 1.10 .87 79 2.4 s0 7s
Lu (treatment 20) 33.3 1.0a .84 7s 2.0 s4 73

Cotton from second picking
e (control) 34.8 1.00 .82 77 2.2 e4 72
L002 (treatment 1) 34.5 1.06 .84 79 3.0 63 72
L01 (treatment 5) 34.0 1.09 .80 70 2.6 67 74
p.02 (treatment 10) 34.3 1.07 .s3 7s 2.4 02 73
,0: (treatment 20) 35.0 1.09 .86 79 2.1 s2 70

 

, heaviest dosage, on the other hand, reduced the boll set by a
 highly significant amount, 53 percent.

The total yield of seed cotton from the 23 untreated plants
was 744 grams (Table 2). Treatment 1, which had no significant
effect on flower production and boll set, increased this amount
by 6.5 percent and the next dosage of 2,4-D (treatment 5) de-
creased the yield of seed cotton by 2.2 percent. The two largest

 

12 BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION

doses of 2,4-D reduced the yield 20 percent (treatment 10) and
58 percent (treatment 20) below that of the untreated plants.
In addition, these same treatments yielded at the first picking
only 27 percent and 17 percent, respectively, of their total yield
of cotton. In this connection, the results from the two lighter
treatments, though perhaps not significant, followed a similar
pattern. It may be seen, therefore, that as the dosage of 2,4-D
was progressively increased, the percentage yield of cotton ob-
tained at the first harvest decreased.

EEect on the Lint Characters of Cotton

Table 3 shows that all of ‘the treatments reduced the length
(upper half mean) of the lint in the first picking but that these
results, with the exception of treatment 1, were reversed at the
second picking. Also, a majority of the treatments at the time
of the first picking reduced the fiber weight per inch. In addi-
tion, treatments 1O and 20 caused a reduction in tensile strength
of the fiber. However, in the second picking, these trends were
absent,except in the case of treatment 20. In fact, the latter
was the only one of the four treatments that produced similar
effects on the cotton harvested at each of the two dates-

The 2,4-D had no effect on the percentage lint, and all of the
values obtained, untreated and treated alike, appeared to be
representative of the Stoneville 2B variety.

The percentage of mature fibers in cotton from both the
treated and the untreated plants was below the conventional

Figure 4. Three plants (C, 10, and 20 in Figure 3) showing normal leaf
development on the lon.g vegetative branches and near the apical bud of
the treated plants. Photograph taken approximately 3 months after 2,4-D

was applied.

RESPONSES OF COTTON TO Z,4-D 13

standard for “average,” 68 to 76 percent, and the cotton would,
therefore, be classed as immature. Any efiect of the 2,4-D on
this fiber property seemed to be limited to the largest dose
which reduced still more the percentage of mature fibers. l

Effect on the Chemical Composition of the Main-stalk Leaves

Table 4 shows that increased amounts of 2,4-D, up to and
including the maximum amount applied, tended to reduce pro-
gressively the concentration of reducing sugars in the main-
stalk leaves that were formed after the 2,4-D applications. At
the same time, the percentage of sucrose varied conversely and
tended to compensate for the loss in reducing sugars. Hence,
the total sugars of the leaf were not appreciably affected, and
they bore no relation to the degree of leaf injury. Likewise,
there appeared to be no close correlation between the starch
content and the degree of leaf injury as only the most severely
injured leaves were affected in this respect. In this instance,
2,4-D reduced the starch content 42 percent below that of leaves
from the untreated plants.

Among the less labile or cell-wall carbohydrates, hemicellu-

r lose and cellulose, there were small but consistent increases

associated with increased 2,4-D injury up to and including treat-
ment 1_0._But the next higher dosage of 2,4-D, which caused
severe injury, 1ncreased the concentration of hemicellulose and

Table 4. Elfect* of 2,4-D onuchemical composition of main-stalk leaves

Carbohydrates and nitrogen as‘ percent of dry weight.

. Nitrogen
D per plant, mg. Reduc- Total Hemi- Cell- Total
ing Sucrose sugars Starch cs-ll- ulose carbo- Sol- Res-

sugars _ ulose hydrates uble idual Total
 e (control) 1.90 0.47 2.37 7.44 3.23 3.42 16.46 0.65 2.48 3.13
 (treatment l) 1.84 0.53 2.37 7 93 3.48 3.53 17.31 .55 2.58 3.13
11 (treatment 5) 1.33 1.10 2.43 7.94 3.43 3.99 17.79 .52 2.52 3.04
 (treatment 10) 1.45 1.26 2.71 7.40 4.50 4.01 18.62 .58 2.55 3.13
(treatment 20) 1.58 1.04 2.62 4.35 7.01 7.14 21.12 .87 2.40 3.27

. “V"-Q2:E_:'~.Q'{'Q.x;- -..

Organic acids, m.e. per 100 g. of dry weight.

» Unident-

Citric Malic Oxalic ified Total

None (control) 97.7 81.6 9.1 34.4 222.8
0.002 (treatment 1) 97 2 79.9 10.0 2.4 189.5

0.01 (treatment 5) 79.8 88.0 10.5 1.8 180.0

I 0.02 (treatment 10) 64.7 96.6 12.2 1.0 174.5
 0.04 (treatment 20) 25.8 75.8 19.1 3.1 123.8

*Mean of two sets of samples.

14 BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION

of cellulose 117 percent and 109 percent, respectively, above
that in the leaves from untreated plants.

The sum of the carbohydrate fractions in the leaves increased,
without exception, with each increase in the amount of applied
2,4-D while the percent moisture (not tabulated) in the leaves
tended to vary inversely.

The organic acids of the main-stalk leaves were consistently
and markedly affected by the 2,4-D treatments. In contrast with
the behavior of the total carbohydrates of the leaf, the concen-
tration of total organic acids was found to vary inversely to the
2,4-D dosage (Table 4). In milliequivalents (m.e.) per 100
grams of dry leaves, the organic acid contents ranged downward
from 222.8 in the control to 123.8 in the most severely injured
leaves, a maximum reduction of approximately 50 percent.

The behavior of malic acid was. erratic, citric decreased pro-
gressively from 97.7 (control) to 25.8 m.e. (treatment 20), and
oxalic increased from 9.1 to 19.1 m.e. In the case of the uniden-
tified acid fraction, the response to the smallest amount of 2,4-D
was relatively great. It caused a reduction fro-m 34.4 (control)
to 2.4 m.e., and further increases of applied 2,4-D were without
additional efiect in this respect.

Of the leaf constituents that were determined, nitrogen seemed
to be the least affected by the 2,4-D (Table 4). The leaves that
were injured most severely contained a little more soluble ni-
trogen and total nitrogen than leaves from untreated plants.

DISCUSSION

Results of this study show that relatively small amounts of
2,4-D, in the form of the sodium salt, may stimulate the growth
and formation of certain vegetative structures of the cotton
plant. All of the 2,4-D applications, for example, increased
plant height, the number of main-stalk nodes, and the two
largest amounts caused extensive development of vegetative
branches. On the other hand, the reaction of the fruiting activ-
ities of co-tton to the application of 2,4-D varied from none, in
the instance of the two smaller amounts, to a 58 percent reduc-
tion in the yield of plants receiving the heaviest dose. This re-
duction in yield appeared to be the summation of several dif-
ferent but closely interrelated responses to the 2,4-D. Correlated
with the suppression in yield of the treated plants, significant
reductions were found in the (a) length of fruiting branch and
number of nodes, (b) flower production and (c) bo-ll setting.

In view of the reported (9, 19) favorable hormonal action of
2,4-D in reducing (a) the preharvest drop of certain fruits and
(b) the shedding of cotton bolls, as reported by Rakitin, et aL,
(14), the results from the use of the two smaller amounts of
2.4-D appear to be noteworthy. As shown in Table 1 these
amounts, While causing no serious injury to the leaves, had no

5i

 

RESPONSES OF COTTON TO 2.4-D 15

significant effect on the flowering 0r boll-setting of the Stone-
ville 2B cotton- It is believed that favorable conditions for a
fruiting response is shown by the following facts: (a) there
was a strong manifestation of 2,4-D activity during, and even
beyond, the 35-day flowering period, and (b) the plants. were
grown under naturally reduced light intensity and length of
day, September 15 to January 27 (11).

The above results, though negative and at variance with those
reported by Rakitin, et al., are in line with those from the use
of two other hormones. Thus Eaton (4), found that dusting
cotton plants during periods of active shedding with 100 p.p.m.
sodium p-chlorophenoxyacetate and 1000 p.p.m. alpha-naphtha-
leneacetic acid, or spraying weekly with 20 p.p.m. sodium p-
chlorophenoxyacetate did not increase boll retention. On the
contrary, the latter treatment reduced the number of bolls per
plant.

Because of the diagnostic value that is often placed upon the
morphological response of the cotton leaf to 2,4-D, it seems
pertinent to examine briefly the leaf injury as observed in this
study- This should be done in relation to the yield of cotton
which, after all, is the most practical basis for evaluating 2,4-D
damage. Using the reduction in weight of main-stalk leaves as
an index of injury, it was found that the 2,4-D caused as much
as a 27 percent reduction in Weight of certain leaves (at the
4th node below the terminal bud) without appreciably altering
the total yield of seed cotton. The next higher dosage of 2,4-D
(0.02 mg.) reduced the weight of leaves 50 percent and the
corresponding yield of cotton 20 percent. Thus, the cotton plant
may sustain some leaf injury (typical 2,4-D formative reactions
and reduction in weight) without an accompanying reduction
in its yield. However, it should be emphasized that the amount
of 2,4-D required to exceed the tolerance level of cotton, under
the conditions of this experiment, is of a very small magnitude,
being only the difference between 0.01 and .02 mg. per plant.

To understand why the tolerance of cotton for 2,4-D is fre-
quently exceeded by drifting dusts o-r sprays from nearby weed-
killing operations, one has only to convert the rates of this
experiment to an ounce-per-acre basis and compare them with
those commonly used in weed control. Thus, assuming 20,000
cotton plants to an acre, the rates for the treatments 1, 5, 10
and 20 become 1 ounce to 700 acres, 1 ounce to 140 acres, 1 ounce
to 70 acres and 1 ounce to 35 acres, respectively. The amount
of 2,4-D acid-equivalent applied in weed control varies from 20
to 80 ounces (12; to 5 pounds) per acre. Therefore, it is readily
apparent that the minimum rate for weed control is 700 times
the maximum rate used in this work.

When sensitive plants are treated with 2,4-D, reductions in
carbohydrates and accumulation of nitrogen (7, 10, 15, 16, 17)
are usually noted as typical chemical responses The data from

16 BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION
the present study deserve some discussion for they indicate
that the cotton plant was not affected similarly by the 2,4-D
applied. As previously stated, none of the applications through
the next to the largest dose appreciably altered either the con-
centration of carbohydrates (total sugars and starch) or of
nitrogen. On the other hand, the largest dose resulted in a
marked decrease in leaf starch, but it was not accompanied by
a substantial increase in nitrogen. However, in this single in-
stance of carbohydrate reduction it seems doubtful that accel-
erated carbohydrate utilization is indicated for it must be re-
membered that in the most severely deformed leaves, Figure 1,
there was a drastic reduction in the chlorenchyma with abnor-
mal development of vascular tissue. Consequently, it would seem
more logical to attribute the low starch level to a reduction in
starch production rather than to accelerated utilization. Fur-
thermore, the observed increases in sucrose, hemicellulo-se and
cellulose, with increased doses of 2,4-D, are believed to be a
reflection and an indirect measure of the differential effect of
the synthetic hormone on the anatomy of the cotton leaf.

One of the most interesting of the chemical responses of the
cotton leaf to 2,4-D was the behavior of the organic acids, which
were found to decrease in concentration (total) with an increase
in the amount of 2,4-D applied. Data presented in another paper
(6) show that the organic acids of cotton together with the
inorganic anions, like those of other plants, appear to function
primarily as neutralizers of the inorganic bases that accumulate
during plant growth. Also, it was noted in that work that the
total organic acid content of the leaf was little affected by
utilization or accumulation of carbohydrates that did not involve
a change in the ratio of inorganic cations to inorganic anions.
Logically, therefore, the observed decrease in total organic acids
in this study would appear to be a reflection of changes in the
mineral nutrition of the leaf. In other words, the 2,4-D applied
to the cotton plant probably inhibited the normal accumulation
of inorganic bases in the affected leaves and reduced simultane-
ously the synthesis of organic acids. But, as in the case of the
leaf carbohydrates, this efiect of 2,4-D was indirect and sub-
ordinate to the effect of changes in leaf anatomy. As further
evidence of this, the magnitude of the values for citric, malic,
oxalic and total organic acids in the severely injured leaves is
strikingly similar to that found in the cotton petiole (6), which
has comparable tissues.

 

RESPONSES OF COTTON TO 2,4-D 17

SUMMARY

Four groups of cotton plants in the prebloom stage were
treated with so-dium salt of 2,4-D equivalent to 0.002, 0.01, 0.02
and 0.04 milligram of 2,4-D acid, respectively, and the subse-
quent growth and chemical respo-nses were studied. A close re-
lationship was found between the quantity of 2,4-D used and
the degree and type of response manifested by the treated
plants. The principal results may be summarized as follows:

Growth: Plant height and the number of main-stalk nodes
increased in proportion to the amo-unt of 2,4-D applied; in case
of the heaviest dose, the increase Was 38 percent and 21 percent,
respectively. Also, the formation and growth of vegetative
branches was stimulated by the two largest applications. The
largest amount of 2,4-D, on the other hand, repressed develop-

ment of fruiting branches; and injury to the main-stalk leaves, a

as reflected by reductio-n in leaf weight, was. increased progres-
sively from 27 percent (0.01 mg.) to 77 percent (0.04 mg.).
The smallest amount of 2,4-D (0.002 mg.) produced no visible
signs of leaf modification or injury.

Fruiting and fiber: Compared with untreated plants, the
0.02 and 0.04 mg. applications suppressed flower production 7
percent and 33 percent, respectively, (highly significant), and
reduced the number of bolls set per plant 12 percent and 53
percent, respectively, (highly significant). Likewise, the yields
of seed cotton were reduced 20 percent and 58 percent, respec-
tively, by these amounts. Only the results from the 0.04 mg.
application were indicative of a trend which was toward a re-
duction in (a) fiber weight per inch, (b) percentage of mature
fiber and (c) tensile strength.

Leaf constituents: In the main-stalk leaves, the concentra-
tions of sucrose, hemicellulose and cellulose tended to increase
with increased amounts of 2,4-D applied. Varying with equal
consistency, but inversely to the amount applied, were reducing
sugars and total organic acids. Leaf starch appeared to be
affected by only the heaviest dosage of 2,4-D.

18

18.

19.

20.

. Mitchell, J. W. and

BULLETIN 713, TEXAS AGRICULTURAL EXPERIMENT STATION

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