IIIIIIIII

EFFECT OF SEVERAL SEED AND FOUAR
THATMENTS ON FLOWERING TIME
OF ”RED AND HYBRID MAKE
' INDUCTION 0" MALE STERMTY IN
1mm W W W!“ ”RAYS

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Michigan State
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EFFECT OF SEVERAL SEED AND FOLIAR TREATMENTS

ON FLOWERING TIME OF INBRED AND HYBRID MAIZE

INDUCTION OF MALE STERILITY IN INBRED MAIZE
WITH GIBBERELLIN SPRAYS

by

Paul Milton Nelson

AN ABSTRACT

Submitted to the School of Graduate Studies of
Michigan State University of Agriculture and
Applied Science in partial fulfillment
of the requirements for the degree of

MASTER OF SCIENCE

Department of Farm Crops

1958

— ,,»;j"7
Approved /€,6’:(T Oily-4W

 

2

PAUL MILTON NELSON ABSTRACT
PART I

It is sometimes desirable in the production of corn
hybrids to cross inbreds or single crosses which do not
shed pollen and silk at the same time. To synchronize
floral development, the earlier parent is planted an ap-
propriate number of days after emergence of the later parent.
This study investigated the possibilities of using various
seed and foliar treatments to delay the early flowering line
or hasten that of the later flowering line so both parents
could be planted at the same time.

Paraffin, silicone, enamel, and gelatin capsules were
tried as seed coatings to delay germination. Gibberellin,
in solutions, was used for soaking seed and slurry treat~
ments alone and in combination with the insecticide-fun-
gicide, Delsan A-D, to hasten germination and seedling
emergence. The influence of gibberellin in foliar sprays
on flowering was also studied. The length of time gib—
erellin, in combination with Delsan A-D, would retain its
activity on seed corn in storage was investigated. R53, a
relatively early flowering inbred, and Oh51, a relatively
later flowering inbred, and a double-cross hybrid Exp.
54-56 were used to measure the response of the various
treatments. A

The trials with enamel seed coatings were the most ef-
fective delaying treatment. Three coatings, in field trials,

delayed flowering nine days for R53 and delayed flowering

3
PAUL MILTON NELSON ABSTRACT

five days for OhBl with only slight reductions in
stand.

Enclosing seed in gelatin capsules delayed emergence
four to five days and appears to merit further trials.

Seedling emergence from seed soaked in gibberellin
was similar to untreated controls in most treatments.
Gibberellin seed treatments were largely ineffective in
hastening seedling emergence.

Gibberellin-Delsan A-D slurry seed treatments were
not effective in delaying or hastening flowering. Seedling
growth was increased for the first 18 days after emergence.
Stalks were weak and spindly, leaves were narrow, and the
plants lodged. These differences soon disappeared and
there was no difference in height of mature plants.

Seed treated with gibberellin-Delsan A-D and stored
six months still showed stimulation of early seedling growth.

Gibberellin in foliar sprays induced sterile and par-
tially sterile tassels and caused delays of four to five
days in pollen shedding on OhSl. The resultant male
sterility (Part II) would discourage this treatment to

delay or hasten flowering.
PWTII

Since the large—scale application of cytoplasmic male

sterility in the production of corn hybrid, the possibilities

A
PAUL MILTON NELSON ABSTRACT
of chemical induction of male sterility have received less
attention. Chemical induction, if reliable, would still be
very helpful in seed production until cytoplasmic male
sterility could be induced into the appropriate inbreds.
The unexpected appearance of male sterility on plants
sprayed with gibberellin in the greenhouse led to field
experiments designed to study chemical induction of sterility
with gibberellin.

Foliar spray applications of gibberellin induced male
sterility in two inbred lines, R53 and Oh5l. Complete male
sterility occurred in Oh5l planted July 8.

Consistent induction of male sterility depended upon
the concentration of gibberellin, directing the spray into
the leaf whorl, and the stage of plant development. The
estimated effective range of gibberellin was 5 to 35 milli-
grams per plant. The critical stage occurred when plants
had developed approximately one inch of immature tassel.
There were no "carry-over" effects on pollen production in

the succeeding generation.

EFFECT OF SEVERAL SEED AND FOLIAR TREATMENTS

ON FLOWERING TIME OF INBRED AND HYBRID MAIZE

INDUCTION OF MALE STERILITY IN INBRED MAIZE

WITH GIBBERELLIN SPRAYS

by

Paul Milton Nelson

A THESIS

Submitted to the School of Graduate Studies of
Michigan State University of Agriculture and
Applied Science in partial fulfillment
of the requirements for the degree of

MASTER OF SCIENCE

Department of Farm Crops

1958

5’1/l—‘

ACKNOWLEDGMENTS

The author wishes to acknowledge the guidance of
Dr. E. C. Rossman. The assistance of Professor H. M.
Brown with statistical analysis and the suggestions of
Dr. S. H. Wittwer are acknowledged.

The author also expresses sincere gratitude to his

wife, Sharon, for her unselfish support.

EFFECT OF SEVERAL SEED AND FOLIAR TREATMENTS

ON FLOWERING TIME OF INBRED AND HYBRID MAIZE

INDUCTION OF MALE STERILITY IN INBRED MAIZE

WITH GIBBERELLIN SPRAYS

TABLE OF CONTENTS

PART I Page
Introduction. . . . . . . . . . . . . . 1
Review of Literature . . . . . . . . . . . 3
Methods and Materials. . . . . . . . . . . 5
Results . . . . . . . . . . . . . . . 10
Discussion . . . . . . . . . . . . . . 29
Summary . . . . . . . . . . . . . . . 32

PART II
Introduction. . . . . . . . . . . . . . 3A
Review of Literature . . . . . . . . . . . 35
Methods and Materials. . . . . . . . . . . 36
Results . . . . . . . . . . . . . . . 38
Discussion . . . . . . . . . . . . . . 46
Summary . . . . . . . . . . . . . . . 49

LITERATURE CITED . . . . . . . . . . . . . SO

PART I

EFFECT OF SEVERAL SEED AND FOLIAR TREATMENTS

ON FLOWERING TIME OF INBRED AND HYBRID MAIZE

INTRODUCTION

Some outstanding corn hybrids involve crosses of
inbreds or single crosses that differ in maturity so that
pollen shedding of one parent does not match silking of
the other parent. In these cases, the earlier parent,
which is usually the pollen parent, is planted an appro-
priate number of days after emergence of the latter parent.

In Michigan and other northern areas early maturity
of corn is a very important characteristic of corn hybrids.
Hybrids using two relatively late maturing inbreds and two
relatively early maturing inbreds are frequently more pro-
ductive and equal in maturity to hybrids composed of four
relatively early inbreds. The two late lines must possess
outstanding combining ability and the two early lines must
carry a high degree of genetic dominance for early maturity.

The double—cross hybrids may be produced by crossing
(Ll x L2) X (B1 x E2) or (L1 x El) x (L2 x E2) where L1 and
L2 represent the two late inbreds and El and E2 represent
the two early lines. In the former situation, the producer
of the double-cross seed would need to delay planting the
single-cross pollen parent. A much larger acreage of
delayed plantings would be necessary than in the latter
case. Here the producer of single-cross seed would need

to delay planting of both pollen parent inbred lines in the

production of seed for the two single crosses. A much
smaller acreage of delayed planting would be involved. The
double-cross seed producer could plant both parents at the
same time.

Theoretically, one might expect more uniformity in
maturity of double-cross plants from crosses of (L1 x L2) x
(El x E2) than from crosses of (Ll x El) x 0.2 x E2).
However, some hybrids of the latter type are equal in uni-
formity of maturity when early maturity is highly dominant.

There are several difficulties associated with delayed
plantings. Variations in environment make it difficult to
estimate the exact length of delay to assure matching.

Rain may interfere with timely planting of the delayed
parent. Crusting and hardening of the seedbed frequently
result in poorer stands in the delayed rows. Weeds may
get a head start in these rows and clean cultivation be-
comes difficult or impossible.

The purpose of this study was to investigate the
possibilities of using various seed treatments and
gibberellin to delay flowering of the early parent or

hasten that of the later parent.

LITERATURE REVIEW

'Green (A) delayed flowering in inbred corn by flaming
treatments with a four-row flame cultivator. The longest
delay (approximately 3 days) occurred when plants two
inches tall were flamed and followed by another flaming
when the plants had again reached a two inch height. Re-
ductions in yield were significant but not great.

Dungan and Gausman (3) retarded pollen shedding in
inbreds and single-cross hybrids with timely pruning of
seedlings. Delays of one week resulted when plants approxi-
mately EA inches tall were clipped three inches above the
growing point. A 50 per cent reduction in grain yield
accompanied this treatment. Plants clipped slightly above
growing point did not recover. Plants of single crosses up
to 16 inches tall and inbreds up to 12 inches tall gave
significant delays to clippings made level with the soil.

Wittwer and Burovac (8) reported that slurry treat-
ments of gibberellin in combination with Delsan A-D on
sweet corn did not induce earlier seedling emergence except
with concentrations exceeding 1,000 ppm. Then, the seedlings
were weak and survival was low. In greenhouse trials pre-
liminary to the present investigation, Bell (1) soaked

inbred corn seed in solutions containing 10, A0, 60, 80,

and 160 ppm gibberellin for various lengths of time up to
48 hours. There was no significant effect on time of
emergence.

Gibberellin sprays hastened flowering of stocks
petunia, larkspur, English daisy, China aster and gerbera(5),
tomatoes (2), and induced long-day plants (lettuce, endive,
radish, mustard, spinach, and dill) to flower under short—

day conditions (9).

MATERIALS AND METHODS

Preliminary work was done in the greenhouse during
the winter and spring of 1957. Field trials were conducted

on University Farms during the summer of the same year.

Greenhouse Trials

 

Seed Treatment. Paraffin, silicone, enamel, and

 

gelatin capsules were tried as seed coatings to delay
germination. Gibberellin was tried in solutions for
soaking seed and in slurry treatments with Delsan A-D to
hasten germination and seedling emergence. -

Silicone is a water soluble concentrate chemically
related to glass or quartz. When applied to glass and
plastic surfaces, it gives a coating that is physiologically
inert and under normal conditions is moisture repellent and
unaffected by heat and most common chemicals. Gelatin
capsules were the kind used in pharmacy. The enamel was
a water-proof type material used to water-proof paper, wood
surfaces, etc. Clear and flat black colors were tried.
Potassium salt of gibberellic acid was used in all
gibberellin seed treatments.

Seed of a relatively early flowering inbred line,
R53, a relatively later flowering inbred line, Oh5l, and

a double-cross hybrid, 54-56 (wrg x sss2873) (onu3 x Mseiu),

6
were used to measure the response of the various treatments.
Seed, except those treated with paraffin and silicone, was
treated with the combination insecticide-fungicide, Delsan
A-D, before the various coating materials were applied.

Paraffin coated seed was germinated in 6-inch pots
containing sand. Enamel coated and gibberellin treated
seed were tested in flats containing soil and also in the
germinator. Silicone coated seed were tested in the germ-
inator. Daily emergence was observed and recorded.

Paraffin seed coatings were applied by quickly dip-
ping the seed with a table fork into and out of melted
paraffin (iuo-iuso F).

Silicone seed coatings were applied according to the
directions given for applying this moisture repellent com-
pound to a plastic surface. Seed was soaked in various
solutions (5, IO, 15, 25, 50, and 100 per cent) of the
silicone concentrate for various lengths of time (5, 15,
30, and 60 minutes). Excess silicone was gently rinsed off
the seed with water. The remaining silicone coating was
allowed to harden for 2A hours.

The water-proof enamel was sprayed on seed with the
hand dispensing ("bomb") container. One to six coatings
were applied. Seed was sprayed on a wide mesh (3/16” x
3/16") screen to facilitate quick and uniform drying. This
method was satisfactory for small amounts of seed, but

would not be practical for treating large quantities of

seed. The seed could be stored without sticking together
when the enamel hardened before the seed was mixed.

Solutions containing 1,000, 2,000, 5,000, 10,000,
15,000, and 20,000 ppm of gibberellin were used to soak
seed for 6 and 2A hours.

Gibberellin-Delsan A-D slurry seed treatments were
prepared by adding one gram of Delsan A-D to five milli-
liters of gibberellin solution. Gibberellin solutions
ranged from 1,000 to 5,000 ppm. The combination slurry
was mixed with seed at the rate of one tablespoon per

pound of seed.

Foliar Spray Treatments. The influence of gibberellin

 

foliar sprays on flowering dates of R53 and Oh5l inbred
lines were studied. Plants were grown in soil in 12-inch
pots. Nutrient solution was added when needed. Plants
were sprayed at various stages of plant development starting
when the immature male inflorescence (immature tassel) was
approximately one-fourth of an inchin length and continuing
up to the time the tassel emerged from the leaf whorl.
Concentrations of 100, 500, and 1,000 ppm gibberellic acid
and Tween 20 (a wetting agent) were used. The spraying
apparatus was an ordinary insect spray gun. Stage of

plant maturity was determined by defoliating test plants

to measure the length of the immature male inflorscence.

Storage of Gibberellin-Treated Seed. Gibberellin in

 

solution loses its potency after about two weeks. An exper-
iment was conducted to determine how long gibberellin, in
combination with Delsan A—D, would retain its effect on
stored seed corn. Double-cross seed of the hybrid experi-
mental 54-56 treated with the combination slurry was stored
at the Farm Crops Department seed storage with average tem-
perature of 65oF. A similar treated lot was stored at 35-400
F. At the end of six months (July-December) the seed was
tested in the greenhouse in flats containing soil. Rate of
emergence and average height, 15 and 19 days after emergence,

were recorded.

Field Trials

 

Experiment 1. Four replications of a split plot ex-

 

periment using R53 and Oh5l inbred lines as main plots and
12 foliar and seed treatments as subplots were planted on
June 1. The seven foliar treatments were: 100, 500, and
1,000 ppm gibberellin, each sprayed A8 and 56 days after
planting and a control. The seed treatments were: one,three,
and six seed coatings of clear enamel, seed soaked 12 hours
in 10,000 ppm gibberellin, and in tap water as a soaking

control.

Experiment 2. Double-cross hybrid seed of Experi-

 

mental 54-56 with 12 different seed treatments were planted
June 1 in a randomized block designed with four repli-
cations. Concentrations of 1,000, 5,000, 10,000, 15,000,

and 20,000 ppm gibberellin were each applied alone

and in combination with Delsan A—D as slurry seed treat-

ments. Controls were Delsan A-D alone and no seed treatment.

Experiment 3. R53 was planted on July 1 in a random-

 

ized block design with four replications of five treatments--
control, gibberellin foliar sprays of 1,000 ppm applied 39
and A6 days after planting, a foliar spray of 2,500 ppm
gibberellin applied 39 days after planting, three coats
of clear enamel on seed, and two coats of black enamel on
seed.

The potassium salt of gibberellic acid was used
throughout. Tween 20 at 0.1 per cent was used as a wetting
agent. Spraying apparatusvmmsa”Funnel-Top" 6-quart sprayer.

Emergence, flowering dates, and plant height was recorded.

Gelatin Capsules. Two seeds (R53) were inserted into

 

each of 10 No. 00 gelatin capsules and planted in the field

in early August. Rate of emergence was recorded.

RESULTS

Greenhouse Trials

 

Paraffin and silicone were unsuccessful as seed
coating materials to delay germination. Paraffin seed
coatings seriously reduced germination and only slight
delays in germination occurred with silicone coatings. No

data are presented.

Enamel Seed Coatings. Water—proof enamel seed

 

coatings showed promise as a method for delaying germin-
ation and dispersing seedling emergence over a longer
period. Table I shows the rate of seedling emergence for
R53 inbred as influenced by various numbers of enamel seed
coats. Three coats delayed emergence one to five days

and dispersed emergence over a six day period while the
control emerged in two days. Four and six coats dispersed
emergence through a six day period with a slight decrease
in total emergence. Six coats delayed emergence three to

seven days.

Gibberellin Seed Soaking Treatment. Figures 1, 2, 3,

 

and A, show seedlings of Oh5l inbred and Experimental
hybrid 54-56 from seed soaked 6 and 24 hours in various
concentrations (1,000 to 20,000 ppm) of gibberellin. Rate

of emergence is given in Table 2.

ll

TABLE 1. Daily and total per cent emergence in
greenhouse trials of R53 seed with
various numbers of water-proof enamel

seed coats.

 

 

 

 

No. of Days from Planting to Emergence

Seed Total
Coats 7 8 9 10 ll 12 13 la 15 Emergence
Control 9O 10 100
1/2* 40 50 90
2-1/2** 50 no 10 100
1 10 90 100
2 2O 6O 2O 1 OO
3 10 #0 2O IO 10 10 100
u I 10 0 30 IO 20 20 90
6 40 O 10 30 O 10 90

 

 

* Seed was coated on one side only.

** Seed had two coats on one side.

12

 

 

Figure 1.

 

Figure 2.

Seedlings from Oh5l inbred seed soaked 6 hours
in gibberellin. Left, 8 days after planting;
right, 13 days after planting. Treatments were,
left to right, control (no soaking), 15,000 ppm,
10,000 ppm, water-soaked control, 20,000 ppm,
1,000 ppm, 2,000 ppm, and 5,000 ppm.

Seedlings from 0h51 inbred seed soaked 24 hours
in gibberellin. Left, 7 days after planting;
right, 12 days after planting. Treatments were,
left to right, 5,000 ppm, 2,000 ppm, 1,000 ppm,
control (no soaking), 20,000 ppm, 10,000 ppm,
water-soaked control, and 15,000 ppm.

13

     

. ‘ L/

11‘1“» 1' I
"I! “IN

 

Figure 3. Seedlings from Exp. 5A-56 hybrid seed soaked 6
hours in gibberellin. Left, 8 days after
planting; right, 13 days after planting. Treat-
ments were, left to right, control (no soaking),
15,000 ppm, 20,000 ppm, water-soaked control,
2,000 ppm, 1,000 ppm, 10,000 ppm, and 5,000 ppm.

 

Figure 4. Seedlings from Exp. 5A-56 hybrid seed soaked 24
hours in gibberellin, 7 days after planting.
Treatments were, left to right, 2,000 ppm, con-
trol (no soaking), 5,000 ppm, 10,000 ppm, water-
soaked control, 15,000 ppm, 20,000 ppm, and
1,000 ppm.

14

TABLE 2. Cumulative daily emergence in a greenhouse
trial of Oh5l inbred and Experimental 54-56
hybrid seed soaked 6 and 24 hours in several
concentrations of gibberellin.

 

 

Per Cent Emergence

 

 

‘40h51 Exp. 54456‘HybrIdI
Gibberellin Days after Planting Days after Planting
(ppm) 4 5 6 7 8 4 5 6 7 8

 

 

6 hours soaking

 

 

 

No soaking 0 0 6O 90 90 O O 70 100

Water (O) O O 60 100 0 20 100

1,000 O 0 6O 9O 90 O 40 100

2,000 O 0 6O 90 90 O 40 100

5,000 O 10 80 100 O 30 100

10,000 0 IO 60 8O 90 O 50 100

15,000 0 O 90 100 0 40 100

20,000 0 IO 70 100 O 50 90 90 90
24 hours soaking

No soaking 0 0 60 100 0 0 100

Water (0) o 60 100 10 100

1,000 10 80 100 70 100

2,000 O 90 100 20 90 100

5,000 0 60 100 BO 100

10,000 10 90 100 70 90 100

15,000 O 80 100 50 90 90 100

20,000 0 80 100 70 9O 90 100

 

 

15

Seedlings from soaked seed emerged one to two days,
on the average, before seedlings from unsoaked seed.
Addition of gibberellin to the soaking solution did not
increase appreciably the rate of emergence.

Average seedling heights 13 days (6 hours soaking)
and 12 days (24 hours soaking) after planting are presented
in Table 3. Addition of gibberellin to the soaking solution
markedly increased seedling growth. Seedlings from
gibberellin soaked seed were two to three times taller than
those from unsoaked seed or seed soaked in water. Increase
in seedling height appeared to reach a maximum with a con—
centration of 10,000 ppm. Soaking seed for 24 hours
resulted in more rapid seedling growth than soaking for
6 hours.

Seedlings from gibberellin-soaked seed had narrower
stems and leaves than control seedlings. With concen-
trations above 5,000 ppm, the seedling stalks were weak

and lodged readily.

Foliar Spray Treatments. Excessive variability in

 

flowering time of R53 and Oh5l inbred lines under green-
house conditions made it impossible to determine if
gibberellin foliar sprays influenced flowering dates. Male
sterile tassels were observed on plants sprayed with 500

and 1,000 ppm gibberellin. Investigation of this phenomenon
was conducted in more detail in the field and results are

reported in Part II.

 

16

TABLE 3. Average seedling height (centimeters) in a
greenhouse trial of Oh5l and Experimental
54~56 hybrid seed soaked 6 and 12 hours in
several concentrations of gibberellin.

 

Con- Concentrations of Gibberellin (ppm)
COP“ tr°1 0 1,099 2,339 5,099 13,300 15,000 20,000

 

 

6 hours soaking -— 13 days after planting

 

‘J

Oh5l 9.0 9.5 9.3 11.0 15.5 15.0 15.5 16.

Exp.

54-56 12.5 16.0 18.5 21.5 23.0 27.5 27.5 29.5

 

24 hours soaking —- 12 days after planting

 

U

Oh5l 6.0 9.3 15.5 15.0 17.5 19.3 18.0 19.

Exp.
54-56 9.5 17.3 24.3 24.5 29.0 33.5 28.0 29.

‘J

 

Storage of Gibberellin-Treated Seed. Figures 5 and 6

 

show seedlings from gibberellin-Delsan A-D treated seed oijx-
perimental 54-56 stored six months. Rate of emergence and
average seedling height 15 and 18 days after planting are
given in Table 4.

Gibberellin in combination with Delsan A-D appeared
to retain, at least, part of its potency on seed stored
six months in the Farm Crops seed building (average 650 F)
and in refrigerated storage (35-400 F). There was no way
to determine if gibberellin had lost any of its activity

because seed was not tested before storage. There was no

Figure 5.

17

 

  
  

 

  

\

 
   

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I,

‘1.".” ii" in,“ 1““
J

.__._.- wfi- _-- -

. D ~O' . ’ .‘“H”‘ "o
-_.- -n...oo my,

, w

Seedlings from Exp. 54-56 hybrid seed treated
with several concentrations of gibberellin in
combination with Delsan A-D and stored 6 months
at Farm Crops Department seed building. Top,

13 days after planting, bottom, 19 days after
planting. Left to right, control (Delsan only),
10,000 ppm, 15,030 ppm, control (Delsan only),
5,000 ppm, and 1,000 ppm.

Figure 6.

l8

 

 

   
 

_ 4me -. c.-. -

Seedlings from Exp. 54-56 hybrid seed treated
with several concentrations of gibberellin in
combination with Delsan A—D and stored 6 months
in refrigerated storage (35-400 F). Top, 13 days
after planting, bottom, 19 days after planting.
Left to right, control (Delsan A-D only), 10, 000
ppm, 15, 000 ppm, control (Delsan A- D only),

5, 000 ppm and l, 000 ppm.

l9

 

 

 

 

 

 

 

TABLE 4. Cumulative daily emergence and average
seedling height 15 and 18 days after
planting from Experiment 54-56 hybrid seed
treated with several concentrations of gib-
berellin in combination with Delsan A—D
and then stored six months.

Per Cent Emergence Average Height(cm.)

Gibberellin Days after Planting Days after Planting

(ppm)
11 12 13 14 15 18
Normal Storage (65oF)
Deslan only 0 2O 70 100 4.0 12.0
Deslan only 0 40 100 6.5 17.5
1,000 40 70 100 7.5 20.0
5,000 20 90 100 9.0 21.0
10,000 30 93 93 130 13.5 25.0
15,333 43 93 93 93 13.5 24.5
Refrigerator Storage (35-4OOF)

Delsan only 0 40 90 100 6.5 14.0
Delsan only 0 70 100 6.5 17.0
1,000 70 130 9.0 20.0
5,033 90 90 103 13.5 22.5
13,030 30 100 10.5 23.5
15,000 70 90 100 11.5 25.5

 

 

20
appreciable difference in response between the two storage

locations.

Field Trials

 

Experiment 1. Data are presented in Table 5.

 

Analyses of variance for dates of pollen shedding and
silking are given in Table 6. Since birds destroyed a
number of plants during emergence, rate of emergence could
not be determined accurately and no data are presented.

Gibberellin in foliar sprays did not significantly
influence flowering dates of R53 inbred except when 1,000
ppm gibberellin was applied 56 days after planting. Silking
was delayed three days with this treatment.

Gibberellin sprays of 500 and 1,000 ppm,applied 48
days after planting,on Oh5l significantly delayed pollen
shedding four to five days. Silking was significantly de-
layed three days when 500 and 1,000 ppm gibberellin were
applied 56 days after planting.

Gibberellin in foliar sprays induced male sterility.
Data on male sterility from this experiment are reported
in Part II.

Seed coatings with water-proof enamel were the most
effective in delaying flowering time. Three coats on the
seed resulted in nine days delay in pollen shedding and
silking of R53 and five days delay for Oh5l. One coat

produced a significant delay of three days in pollen

21

 

 

 

 

 

 

 

 

 

 

TABLE 5. Flowering time of R53 and Oh5l inbred lines
as influenced by various seed and foliar
treatments. Experiment 1.

Seed and No. of Days to
Foliar Enamel Gibber- Days Flowering
Treat- Seed ellin to
ments Coatings (ppm) Spraying Pollen Silk

R53 Inbred

Control 0 0 0 61.1 62.5

Spraying - 100 48 61.3 62.8

Spraying - 100 56 62.1 63.4

Spraying - 500 48 62.9 64.6

Spraying - 500 56 62.4 64.2

Spraying - 1,330 48 61.1 63.0

Spraying - 1,030 56 62.3 65.7*

Seed coating 1 - - 64.3** 65 6*

Seed coating 3 - - 70.3** 71.9**

Seed coating 6 — - 67.5** 69.3**

12 hr.seed - 3 - 62.5 64.1

soaking

12 hr.seed - 10,033 - 62.1 63.5

soaking
Oh5l Inbred

Control 0 3 O 72.1 73.3

Spraying - 103 483/ 74.7* 73.8

Spraying - 133 561 73.0 74.4

Spraying - 533 48a/ 76.3** 73.8

Spraying - 533 561 74.2 76.1*

Spraying - 1,033 48~/ 77.1** 73.7

Spraying — 1,330 56 72.7 76.4*

Seed coating 1 - - 72.9 73.6

Seed coating 3 - - 76.9** 78.3**

Seed coating 6 - - 76.8** 74.4

12 hr. seed - 3 - 73.7 74.4

soaking

12 hr. seed - 13,000 - 72.2 71.9

soaking
* Significant at 5% level. Silking LSD = .7
Pollen LSD = .4
** Significant at 1% level. Silking LSD = .6
Pollen LSD = .2
1/ Immature tassels were approximately one nch length.

22

TABLE 6. Analyses of variance for dates of pollen
shedding and silking. Experiment 1.

 

 

Source Degrees

 

 

of of Pollen Shedding Silking
Variation Freedom MS F MS F
Total 891/
Main Plots
Lines 1 2942.6 2144.2
Replications 3 42.3 25.4
Error 1 3 30.6 81.4
Sub Plots
Treatments 11 34.6 6.4** 33.7 5.0**
T x L 111 11.2 2 1* 8.8
Error 2 60—/ 5.4 6.7
Treatments
Control vs
Treatments 1 43.4 8.0** 29.9 4.5*
Spraying vs
Other 1 56.2 10.4** 41.7 6.2*
Within Spray-
ings 5 6.3 1.2 9.2 1.4
Within Others
Seed coating
vs Soaking 1 139.4 25.8** 130.8 19.5**
Within Seed
Coatings 2 53.0 9.8** 62.1 9.3**
Within
Soakings l 3.5 9.6 1.4

 

* Significant at 5% level.
** Significant at 1% level.

1/ Degrees of freedom minus 6 missing plots.

23
shedding and silking of R53, but did not influence flowering
of Oh5l. Six coats significantly delayed pollen shedding
six days and delayed silking seven days for R53; and signi—
ficantly delayed pollen shedding four days for Oh5l.

Soaking seed for 12 hours in 10,000 ppm gibberellin

did not significantly influence flowering time.

Experiment 2. Data from gibberellin and gibberellin-

 

Delsan A-D slurry treatments on hybrid seed of Experimental
54-56 are presented in Table 7. Analyses of variance for
plant height 18 days after planting, number of suckers, and
suckers with tassels are given in Table 8. Bird damage
altered seedling population so that rate of emergence is
not reported. Observation indicated that high concentra-
tions of gibberellin (5,000 to 20,000 ppm) hastened
emergence approximately 12 to 24 hours.

Plants from hybrid seed treated with 5,000 ppm or
more of gibberellin were significantly taller for 18 days
following planting than those from seed receiving no
gibberellin (Table 7, Figures 7 and 8). There were no
significant differences 25 days after planting. Seedlings
from treated seed were taller, more slender, and weak
stalked. Three weeks after planting, seedlings from
treated seed began to straighten and new leaves were normal
in length and width.

Gibberellin in seed slurry treatments produced no

significant effects on flowering dates or yield of

24

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TABLE 8. Analyses of variance for plant height 18

 

days after planting, number of suckers,
and suckers with tassels for Exp. 54-56
hybrid in seed treatment. Experiment 2.

Average Number
Plant Height Average Number of Suckers

 

 

soggce 18 Days of Suckers Per Plant
Vari- after Planting Per Plant with Tassels
ation DF MS F MS F MS F
Total 47

Rep1. 3 59.5 21.3** 0.67 16.8** 0.70 38.9**
Treat. 11 61.2 21.9** 0.13 3.3** 0.21 11.7**

Error 33 2.8 0.04 0.018

 

 

** Significant at 1% level.

26

 

 

 

iliil'i'i "-7;- 4 {fl -4~W ' \ I /

Figure 7. Seedlings 14 days after planting Exp. 54-56
hybrid seed treated with 0, 5,000, 10,000,
and 20,000 ppm gibberellin alone.

 

.' v
'.‘.._.-_

4 — -‘ ————_- I

Figure 8. Seedlings 14 days after planting Exp. 54-56
hybrid seed treated with 0, 1,000, 5,000, 10,000,
and 20,000 ppm gibberellin and Delsan A-D.

 

27
Exp. 54-56 hybrid. It did, however, stimulate early growth
of seedlings but they were spindly and lodged. It was
equally effective used alone or in combination with Delsan

A—D.

Experiment 3. Enamel seed coatings delayed pollen

 

shedding about two days and silking about three days
(Table 9). The three day delay in silking was statistically
significant. The two day delay in pollen shedding was
statistically significant when the foliar spray treatments
were removed from the statistical analysis. Analyses of
variance for dates of pollen shedding and silking are given
in Table 10.

Gibberellin in foliar sprays did not significantly
influence flowering time, but did induce male sterility as

reported in Part II.

Gelatin Capsules. Enclosing seed in gelatin capsules

 

appeared promising as a technique to delay emergence (and
possibly flowering) in a very preliminary field trial.
Twenty seeds, two to a capsule, were planted in the field.
Emergence was delayed four to six days when compared with
the controls. All of the seeds germinated.

This was a very limited trial and the method should
be studied in more detail. A delay of one week in emergence
may be reduced to only one or two days at flowering. The
chore of inserting seeds into capsules is obviously a

tedious one unless done mechanically.

28

 

 

 

 

 

TABLE 9. Flowering time of R53 inbred as influenced
by enamel seed coatings and foliar appli-
cations of gibberellin, planted July 1.
Experiment 3.
No. of
Enamel
Coats Time of Gib. Days to
or Conc. Application Flowering
Treatments of Gib. (Days after ——
(ppm) Planting) Pollen Silk
Control none - 60.9 61.7
Gib. spray 1000 each 39 & 46 63.1 63.0
Seed coating 3 clear
enamel - 63.3 64.5*
Gib. spray 2500 39 60.1 62.3
Seed coating 2 black -
enamel - 62.9 64.5*

 

* Significant at 5 per cent level.

TABLE 10.

Analyses of variance for dates of pollen
shedding and silking of R53 inbred with
enamel seed coat treatments and foliar
sprays of gibberellin, planted July 1.
Experiment 3.

 

 

 

Source of

Dates of Pollen Shedding Dates of Silking

 

 

 

V riation
a DF17 MS F DF MS F
Total l2 l9
Replications 3 3
Treatments 2 6.4 14.9** 4 6.2 6.6**
Error

6 0.43 12 0.93

 

** Significant

at 1 per cent level.

1/ Foliar spray treatments removed from statistical analysis.

DISCUSSION

The two inbreds, R53 and Oh5l, in this study are used
in the seed parent single-cross Oh5l x R53 for the pro-
duction of double-cross seed of Michigan 250 (Oh5l x R53)
(W10 x M8206). In producing the single crosses Oh5l x R53
and W10 x M8206, the pollen parent inbreds R53 and M8206
flower earlier than the female parents, thus their planting
is delayed. Seedlings of Oh5l should average two inches
tall before R53 is planted and W10 should average four
inches tall before M8206 is planted.

A given number of seed coatings or a specific con-
centration of a chemical seed treatment that would con-
sistently delay seedling emergence a given length of time
under specific conditions of soil moisture and temperature
would permit simultaneous planting of both parents. Since
the environment following planting and emergence exerts
considerable effect on matching of pollen shedding of one
parent with silking of the other parent, a specific delay
that matches in one year may not match well another year
with a different environment. Thus, a mixture of different
numbers of coatings or several concentrations of chemical
treatment applied to seed of the delayed parent would
provide a spread in emergence and flowering that would more

nearly assure matching of the two parents.

30

Paraffin and silicone coating treatments as used in
this study were not effective delay treatments. Germin-
ation of paraffin treated seed was low. Temperature of
the melted paraffin may have been too high and injured
germination or the coating may have been too impervious
to air and moisture for germination. Only slight delays
in germination occurred with silicone treated seed. The
material was water soluble, and rinsing the seed after the
silicone treatment may have removed too much of the coating.

The preliminary trials with enamel seed coatings were
the most effective delaying treatments. In the field,
pollen shedding and silking of R53 were delayed about three,
nine, and six days when the seed was treated with one,three,
and six coats, respectively. Corresponding delays in
flowering of Oh5l were zero, five, and four days for one,
three, and six coats, respectively.

Gibberellin-Delsan A-D slurry seed treatment did not
effect flowering time. Seedlings from treated seed grew
two and three times faster than those from untreated seed
during the first 18 days after emergence. Stalks were
weak and spindly, leaves were narrow, and the plants lodged.
These differences soon disappeared and there was essentially
no difference in height of mature plants.

Foliar sprays of gibberellin applied 48 and 56 days
after planting were largely ineffective in delaying

flowering of R53. Delays of four to five days in pollen

31

shedding of Oh5l occurred with treatments of 500 and 1,000
ppm 48 days after planting. The development of male
sterile tassels (discussed in Part II) as a result of
foliar sprays would eliminate them as treatments to delay
or hasten flowering.

Enclosing seed in gelatin capsules delayed emergence
four to six days. Further studies should be made comparing
enamel seed coatings, gelatin capsules, and clipping treat-

ments for their effects on flowering time.

SUMMARY

Several seed and foliar spray treatments were inves-
tigated for their effects on emergence and flowering time

of inbred and hybrid corn.

Seed Treatments

 

Paraffin and silicone seed coatings as used in these
experiments were not effective treatments to delay emer-
gence and consequently flowering time. The paraffin coat
reduced germination.

One to six enamel seed coatings delayed and only
slightly decreased germination in greenhouse trials. Three
enamel coatings, in field trials, delayed flowering nine
days and five days for the two inbreds R53 and Oh5l,
respectively. There was a slight reduction in stand.

Gelatin capsules in preliminary trials showed promise
for delaying germination without decreasing germination.
Capsuling seed was more convenient than applying enamel
seed coatings with comparable delays in germination.

Seedling emergence from seed soaked in gibberellin
was similar to untreated controls in most cases. Gib-
berellin seed treatments were largely ineffective for

delaying or hastening seedling emergence.

33

Gibberellin in slurry treatments increased early
seedling growth of Exp. 54-56 for about 18 days. Plants
were spindly and lodged easily. There was no difference
in mature plant height. Gibberellin was equally effective
alone or in combination with Delsan A-D.

Hybrid seed of Exp. 54-56 treated with gibberellin
and Delsan A-D and stored six months still showed stim-

ulation of early seedling growth.

Foliar Spray Treatments

 

Gibberellin in foliar sprays effected some signifi-
cant delays in flowering of Oh5l. The resultant male
sterility (Part II) would discourage foliar sprays aimed

to delay or to hasten flowering.

PART II

INDUCTION 0F MALE STERILITY IN INBRED MAIZE

WITH GIBBERELLIN SPRAYS

INTRODUCTION

Prior to the practical use of cytoplasmic male
sterility in hybrid seed corn production, there was intense
interest in the possibilities for chemical induction of
male sterility to eliminate the tedious task of detasseling
seed fields. Cytoplasmic sterility must be introduced into
one of the inbreds of the single cross seed parent through
a backcrossing program of five or more generations. A few
inbreds are difficult or impossible to "sterilize" in this
manner. Thus, chemical induction of male sterility, if
reliable, would still be very helpful in seed production
until cytoplasmic male sterility could be introduced into
the appropriate inbreds. Observation of sterile and
partially sterile tassels on inbred corn sprayed with gib-
berellin in the greenhouse (Part I), led to field experi-
ments designed to investigate the possibility of using this

chemical to induce male sterility in seed fields.

LITERATURE REVIEW

Moore (6) reported that 600 ppm of maleic hydrazide
applied to corn at a critical stage of plant development
induced sterile tassels. The critical stage of plant
development was not indicated. Naylor (7) reported in-
duction of male sterility in corn with 250 ppm of maleic
hydrazide applied when plants were one meter tall with no
visible male inflorescence, and suggested the use of maleic
hydrazide as a means of inducing pollen sterility for

production of hybrid seed corn.

MATERIALS AND METHODS

The effects of gibberellin on flowering of inbred
corn were studied in the greenhouse during the winter of
1957. Gibberellin sprays were applied at various stages
of plant development starting when the immature male in-
florescence (immature tassel) was approximately one-fourth
of an inch in length and continuing up to the time the
tassel emerged from the leaf whorl. Plants sprayed with
gibberellin at 500 and 1,000 ppm, when the immature male
inflorescence was approximately one inch in length, devel-
oped sterile and partially sterile tassels (Part I).
Possibilities of chemical induction of male sterility in
corn were investigated with two inbred lines of corn in
the field in 1957.

A relatively early flowering inbred line, R53, was
planted June 1 and July 1 and a relatively later flowering
inbred line, Oh5l, was planted June 1 and July 8. The
potassium salt of gibberellic acid in concentrations of
100, 1,000, 2,000, and 2,500 ppm and a wetting agent (Tween
20 at 0.1 per cent) was used as a foliar spray. Gibberellin
sprays were applied at several stages of plant development
ranging from approximately one to three inches of immature

male inflorescence. Estimated amount of gibberellin per

plant ranged from 1.0 to 10 mg for the planting on June 1,

37
and 12 to 35 mg for the two later plantings. The increase
of gibberellin per plant in the later two plantings was
caused by directing a greater amount of spray into the leaf
whorl and a higher concentration of gibberellin. A "Funnel-
top" six quart sprayer was used for spraying.

Stage of plant development was determined by meas-
uring the length of the immature male inflorescence
(immature tassel). Plants were defoliated (with a razor
blade) to expose the immature tassel for measurement. Test
plants from each planting were used to estimate the stage
of plant maturity for the remaining plants.

The number of sterile and partially sterile tassels,

plant height at maturity, and flowering dates were recorded.

RESULTS

Male Sterility

 

Gibberellin-induced male sterility ranged from
tassels barren of all floral parts to tassels which ap-
proached normal pollen shedding. Most sterile tassels
developed all floral parts except stamens (pollen and
pollen sacs). On partially fertile tassels the upper
portion of the central spike or the terminal portions of
the lateral spikes, or both, developed staminate spikelets
and shed pollen. The number of anthers extruding from
staminate spikelets during flowering were recorded as
trace, light, moderate, and normal as a measure of relative
fertility (Table 11). Figure 9 shows fertile (normal),
partially fertile, and sterile tassels.

More sterile tassels were induced by gibberellin at
all concentrations applied 48, 39, and 36 days after
planting (first spraying for the three dates of planting).
At the time of treatment plants were about knee high and
had developed approximately one inch of immature male in-
florescence. It was therefore concluded that the critical
stage of plant development for effective induction of male
sterility occurred when the immature tassel was approxi-
mately one inch in length. Variability in maturity among

plants at the time of spraying was a factor influencing

9
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TABLE 12. Analyses of variance for dates of pollen
shedding and silking and average plant
height of R53 and Oh5l as influenced by
gibberellin in foliar spray applications.

Source DF SS MS F

Oh5l, June 1 Planting, Days to Pollen Shedding
Total 19 59.7
Repl. 3 7.4 2.5
Treat. 4 33.4 8.“ 5~3*
Error . 12 18.9 1.6

Oh5l, June 1 Planting, Days to Silking

Total 19 161.2
Repl. 3 59.0 19.7
Treat. A 70.9 19.2 9.1**
Error 12 25.3 2.1

Oh5l, June 1 Planting, Plant Height-at Maturity
Total 19 308.0
Repl. 3 11.0 3.7
Treat. 4 250.0 62.5 16.0**
Error 12 47.0 3.9

R53, July 1 Planting, Plant Height at Maturity
Total 11 661.7
Repl. 3 83.1 27.7
Treat. 2 532.5 266.2 3M.6**
Error 6 u6.1 7.7

Oh5l, July 8 Planting, Days to Silking

Total 15 22.9
Repl. 3 1.5 0-5
Treat. 3 13.0 4.3 4.66*
Error 9 8.4 0.93

 

 

* Significant at 5% level.
** Significant at 1% level.

 

.it“. ‘. ll‘ll‘l! .

 

‘\

Figure 9.

 

k ‘ \ XV
. . - ' \l ‘. ,
{ J"_fi_.l__~ , .#_____ i L1__1H1__ m

42

 

    

 

Top: left, normal (fertile) tassel on untreated
sterile tassel from treated plant.

plant; right,
Bottom: Partially fertile tassels on treated
plants. Left to right, "trace," "light," and

"moderate" fertility (see footnote of Table 11).

43
the amount and degree of sterility induced in the
tassel.

The early flowering line, R53, planted June 1 showed
no sterility in any of the treatments and no data are
presented. The immature tassels of this earlier flowering
line, being considerably more than one inch in length when
treated 48 and 56 days after planting, had apparently
passed the critical stage of development for chemical
induction of male sterility.

The later flowering line, Oh5l, planted June 1
showed 32 per cent and 46 per cent totally sterile tassels
when treated with 500 and 1,000 ppm, respectively, 48 days
after planting (Table 11). Both concentrations of gib-
berellin (500 and 1,000 ppm) applied 56 days after planting
induced fewer sterile tassels on the main stalk, but it
was noted that there were more sterile tassels on the
"suckers" (lateral shoots) than when the same treatment
were applied 48 days after planting. Concentrations of
100 ppm at both times of spraying induced no completely
sterile tassels and only a few partially sterile tassels
and data were not recorded.

Applying gibberellin closer to the suggested critical
stage of plant development and increasing the concentration
two and one-half times induced male sterility in the R53
inbred planted July 1. Both treatments (2,500 ppm applied

39 days after planting and 1,000 ppm applied 39 and 46

44
days after planting) resulted in 87 per cent sterile
tassels. Similar treatments on Oh5l planted July 8 induced
100 per cent sterility.

In the two later plantings, flowering occurred in
early September when temperature and day length were less
favorable for pollen development and shedding. These en-
vironmental conditions also may have favored chemical
induction of male sterility. It is impossible to separate
environmental effects present during the late plantings

from the effect of increased concentration.

Flowering Time

 

Date of pollen shedding for partially fertile tassels
of Oh5l planted June 1 was significantly delayed four and
five days for the two treatments applied 48 days after
planting. There was no significant effect on date of
pollen shedding for treatments applied 56 days after
planting. Silking was significantly delayed three days
for both concentrations (500 and 1,000 ppm) applied 56 days
after planting, but the same concentrations applied 48 days
after planting had no effect.

Flowering dates for R53 planted July 1 were not
significantly influenced by any of the treatments. Silking
dates of 0h5l planted July 8 were significantly hastened
with gibberellin at 2,000 ppm applied 36 days after

planting.

45
The effects of gibberellin on flowering time were
small and inconsistent. The differences, with the accom-
panying induction of partial male sterility, were not
encouraging for the use of gibberellin sprays to delay or
hasten flowering time in seed production of hybrids with

early and late flowering parents.

Plant Height at Maturity

 

Oh5l planted June 1 was significantly reduced in
height by gibberellin at 1,000 ppm applied 48 days after
planting, but significantly increased in height when tested
56 days after planting. Height of Oh5l planted July 8 was
not significantly influenced by gibberellin. Both treat-
ments (1,000 ppm at 39 and 46 days, and 2,500 ppm at 39
days) significantly increased plant height of R53 planted

July 1.

Genera1#0bservations

 

Silks on treated plants appeared functional. Yield
was not taken, but size and number of ears of treated and
untreated plants were comparable. Lodging of treated
plants was increased only in Oh5l planted July 8. No
increase in insect and disease susceptibility of treated

plants was noted.

DISCUSSION

The present methods of eliminating pollen from the
female parent in the production of hybrid seed corn are
not 100 per cent efficient. Detasseling by hand is slow
and requires going through fields at least once a day and
sometimes twice a day to pull out tassels which are shed-
ding or about to shed pollen. At times, the seed producer
is unable to do or neglects this frequent detasseling. If
the field inspector finds an excess of "shedders" the seed
field is rejected. Cytoplasmic male sterility eliminates
detasseling but 100 per cent male sterility is not assured.
When cytoplasmic male sterile parents are used, the seed
producer is advised to rogue the field. For unknown reasons
a relatively high per cent of "shedders" are sometimes
found. Likewise, chemical induction of male sterility may
be of practical value even though it may not be 100 per
cent effective. It should be assumed that its practical
use would require some roguing of seed fields.

Chemical induction of male sterility, if as reliable
as cytoplasmic male sterility, could be used when cyto-
plasmic male sterile inbreds were not available. If 90 per
cent as reliable as detasseling (e.g., left 10 per cent
partially fertile tassels), it would (theoretically)

reduce detasseling time, number of shedders, and pollen

ll 7

shed from ”shedders,‘ and still be of practical importance
to hybrid seed production.

Educating the grower as to how and when to apply the
chemical may be a factor in the practical application of
chemically induced male sterility. Uniformity of spraying,
drift of spray, and other inconsistencies associated with
spraying and spraying equipment may discourage the large-
scale application of this chemical "sterilization."

Gibberellin effectively induced male sterility in one
field trial. Experiments are needed, however, to determine
a definite range in milligrams of gibberellin per plant
required to assure this induction of male sterility. Sterile
tassels occurred with applications of gibberellin at both
low (500 ppm) and high (2,500 ppm) concentrations. Gib-
berellin at low concentrations (500 and 1,000 ppm) may be
sufficient to consistently induce sterility if the spray
is directed into the leaf whorl and applied at the critical
stage of plant development. A second spraying, a few days
after the first spraying, may be advisable to "sterilize"
both early and late maturing plants and early maturing
"suckers."

Greenhouse plantings from open-pollinated seed
harvested from male sterile 0h5l plants showed full pollen
production during the winter 1957-58. Thus, there would
be no need for pollen restoration genes or blending of

seed as is necessary in hybrid seed production with cyto-

plasmic male sterility.

48

Only two inbred lines, R53 and 0h5l, were used in
these experiments. With wide differences in response of
inbreds and hybrids for many characteristics, it is likely
that they may vary also in response to gibberellin for
chemical sterilization. Some may be more completely
sterilized than others. Dosage requirements may differ
for various varieties.

The largest acreage, by far, is devoted to double-
cross seed production in which seed is produced in vigorous
single-cross plants. Results were obtained from inbred
lines used in single-cross seed production. These experi-
ments should be repeated with single-cross hybrids.

Additional field trials are needed to determine the
reliability of chemical (gibberellin) induction of male
sterility and its influence on the plant in general. Yield,
lodging, disease and insect susceptibility, plant abnormal-
ities, and carry-over effects should be studied in greater
detail before large-scale application in the production of

hybrid seed corn.

SUMMARY

Foliar spray applications of gibberellin induced male
sterility in two inbred lines, R53 and Oh5l. Complete male
sterility occurred in Oh5l planted July 8.

Consistent induction of male sterility depended upon
the concentration of gibberellin, directing the spray into
the leaf whorl, and the stage of plant development. The
estimated effective range of gibberellin was five to 35
milligrams per plant. The critical stage occurred when
plants had developed approximately one inch of immature
tassel.

Gibberellin effects on flowering time were small and
inconsistent. Effects on mature plant height were also
inconsistent.

There were no 'carry-over" effects on pollen pro-

duction in the succeeding generation.

LITERATURE CITED

Bell, Duane. Effect of gibberellin seed soaking
treatment on emergence of inbred corn. Un-
published data, Mich. State Univ.

Bukovac, M. J., Wittwer, S. H., and Teubner, F. G.
Gibberellin and higher plants: VII Flower
formation in the tomato. Mich. Agr. Exp. Sta.
Quart. Bul. 40:207-214. 1957.

Dungan, George H., and Gausman, Harold W. Clipping
corn plants to delay their development. Agron.
Jour. 43:90—93. 1950.

Green, John M. Effect of flaming on growth of inbred
lines of corn. Agron. Jour. 41:144-146. 1949.

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