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E. B. (Ernest Benton) Earley.

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I26b
no. 747
cop. 8



UNIVERSITY OF

ILLINOIS LIBR/.RY

AT URBANA-CHAMPAIGN

AGRICULTURE



CIRf.












Earshoot
Development
of Midwest
Dent Corn



Earshoot Development

of Midwest Dent Corn (Zea mays L.)

E. B. Barley, J. C. Lyons, E. Inselberg, R. H. Maier, and E. R. Leng



Bulletin 747

Agricultural Experiment Station

College of Agriculture

University of Illinois at Urbana-Champaign



The Illinois Agricultural Experiment Station provides equal opportunities in programs and employment.

URBANA, ILLINOIS JUNE, 1974

PUBLICATIONS IN THE BULLETIN SERIES REPORT THE RESULTS OF INVESTIGATIONS MADE OR SPONSORED BY THE EXPERIMENT STATION.



op.



TABLE OF CONTENTS



Vegetative bud development 5

Shank ears 5

Pattern of earshoot development 5

Effects of rate of planting 8

Genetic influence 9

Simultaneous pollination of all silking earshoots 10

Covered earshoots 11

Removal of earshoots before silking 11

Earshoot coverage versus earshoot removal 16

Effects of date of removal of the first earshoot 16

Shaded plants 22

Length of time between silking of earshoots 24

Length of time between pollination of earshoots 24

Conversion of nonfunctional earshoots into functional earshoots 26

Chemical composition of earshoots 26

Theory of earshoot development 31

Summary 33

Appendix: Influence of synthetic growth-regulating chemicals

on earshoot development 34

Literature cited . .... 42



E. B. Earley, Professor of Plant Physiology, and E. R. Leng, Professor of Plant
Breeding and Genetics, are members of the Department of Agronomy. The other
authors, former graduate students at the University of Illinois at Urbana-Cham-
paign, are as follows: Edgar Inselberg, Associate Professor of Biology at Western
Michigan University, Kalamazoo, Michigan; Robert H. Maier, Vice-Chancellor of
the University of Wisconsin at Green Bay; and J. C. Lyons, Director of Market
Development, Agrico Chemical Company, Tulsa, Oklahoma.



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EARSHOOT DEVELOPMENT OF CORN



YIELD OF CORN GRAIN PER ACRE is determined by the
number of plants and by the number and size of ears
(pistillate inflorescences with grain) per plant. One or
more of the six to eight earshoots pistillate inflores-
cences with no grain usually develop into ears, depend-
ing upon the hybrid, rate of planting, and cultural
conditions (soil fertility, water, light, temperature, and
so on) . Earshoots that are normally capable of producing
grain are called "functional"; those normally incapable
of producing grain are called "nonfunctional."

In searching for methods of further increasing grain
yields, the following factors that affect the number and
size of ears per plant were investigated : rate of planting,
earshoot coverage and removal, simultaneous and dif-
ferent times of pollination of earshoots, amount of sun-
light reaching the plants, and chemical treatment of sev-
eral parts of the plants.

VEGETATIVE BUD DEVELOPMENT

Morphological studies of the development of floral
structures of corn plants have been published by Martin
and Hershey (1934), Sharman (1942), Bonnett (1948,
1953), and Kiesselbach (1949). Generally, each of the
lower eight or so nodes of the corn plant has an axillary
vegetative bud that is initiated acropetally (from the base
of the plant upward) and transformed into an earshoot
basipetally (from the top bud downward) at intervals of
about one-half to four days. The earshoots develop into
one or more functional earshoots always basipetally (Sass
and LoefTel, 1959; Sass, 1960 and 1962; Siemer, 1964:
Siemer et al., 1969). The yield of grain per plant is not
limited by lack of potential ears but by failure of one or
more of the earshoots to develop into sizable ears.

It is not known why the axillary buds usually stop
forming at the eighth node from the base of the plant
rather than at the top node, but the explanation seems
to be closely associated with the initial elongation of the
apex of the main stalk and with tassel differentiation
(Bonnett, 1948; Leng, 1951). This observation is
strengthened by data in Table 1 and by the plants illus-
trated in Figure 1. Applying gibberellic acid and
Tween-20 (polyoxyethylene-sorbitan-monolaurate, a wet-
ting agent) to plants before normal elongation of the
stem apex occurred caused the plants to stop forming
axillary buds. 1 Presumably, gibberellic acid initiated
elongation of the stem apex sooner than normal, causing
vegetative buds to stop forming in the axils of leaves that
subsequently developed. As a result, ears developed on
treated plants from lower axillary buds than on untreated
plants. There was no sign of vegetative bud development
in the axils of the leaves above the bud that developed
into the first (top) ear. Further experiments with growth-
regulating substances are described in the appendix.

1 All experiments described in this bulletin were conducted in
fertile soil on the Agronomy South Farm of the University of
Illinois at Urbana-Champaign.



It should be pointed out that, although the literature
indicates the presence of an axillary vegetative bud at
each of the lower eight or so nodes of the corn plant,
extensive field work with L317 x R4 disclosed that many
plants did not produce a vegetative bud in the axil of
each of the lower leaves. At such points, consequently,
there was no earshoot development and no groove in the
corresponding internode (Fig. 2). Occasionally, more-
over, the first (top) axillary bud produced only the
prophyll, which appeared to be the beginning of a func-
tional earshoot; in these few cases the first ear developed
from the second axillary bud rather than from the first
axillary bud. Figure 3 shows corn plants with prophylls
at the first node above the top ear. Their origin and
structure have been reported by Kiesselbach (1949),
Anderson (1950), Bonnett (1953), Collins (1963), and
Sass and Loeffel (1959).

SHANK EARS

Each node of the shank (connecting tissue between
stalk and ear) has a vegetative bud identical to the buds
in the axils of lower leaves of the main plant. Some
hybrids, including WF9 x Hy2, have a greater tendency
than other hybrids to develop sizable earshoots from
axillary buds of the first and second shanks (Fig. 4).
Usually, in order for the plant to produce sizable ears
on the shank, the first earshoot has to be removed, cov-
ered, or partially damaged around silking time. Initiation
of axillary buds and the development of earshoots on
shanks have also been reported by Freeman (1940).

PATTERN OF EARSHOOT
DEVELOPMENT

The basipetal pattern of earshoot development (Fig. 2
and 5) and the production of grain by the upper non-
functional earshoots when functional earshoots are re-
moved (see pages 11-22) suggest that growth-promoting
substances are synthesized in the upper part of the plant
(probably in the leaves) and move down the stalk into
the earshoots. The first earshoot is seemingly in the most
favorable position to receive growth-promoting substances
if the plants possess no mechanism for distributing these
substances to the lower earshoots. The basipetal pattern
is emphasized by the decrease in grain yield per ear from
the top ear downward, as reflected in the data for check
plants reported in this publication and in results reported
by Krantz and Chandler (1954), Bauman (I960), and
Collins and Russell (1965).

The pattern of earshoot development of corn plants is
apparently similar to fruit set in the inflorescence of
tomato plants, about which Gustafson (1946) stated,
"Flower buds formed first in an inflorescence of the
tomato plant have more hormone and are more likely
to set fruit than those formed later. The second, third,
and fourth flower set have progressively less hormone
and less chance of setting fruit."



BULLETIN NO. 747



1/2 mg. GA PER PLANT r 796Q
H y 2 x 07




2 mg. J3APERIPLANT
WF9x 07H960




Figure 1. Effects of a combination of gibberellic acid and Tween-20 (polyoxyethylene-sorbitan-monolaurate) on the height
at which ears are produced on corn plants. Dates given are dates of application.




No. I



1956



Figure 2. Pattern of
earshoot development.
Upper arrow indicates
absence of axillary bud
at node on the plant at
right.




Figure 3. Three plants at right show prophylls at first node
above top ear. The plant at the left is the normal check.
The two plants at far right have had the two upper ear-
shoots removed.



EARSHOOT DEVELOPMENT OF CORN




Figure 4. The plant on the left shows fully developed ear-
shoots on the shank of the first ear. The other plants show
shank ears formed after the primary earshoots had been
removed or covered before silking.



The developmental patterns of earshoots of WF9 x
C103 (one ear per plant) and LSI 7 x R4 (two ears per
plant) are shown in Figure 5 (Inselberg, 1956). The
seeds were planted May 21, 1954, and thinned to one
plant per 40" by 40" on June 28. By July 19 (59 days
after planting) , all of the above-ground axillary vege-
tative buds had been transformed into visible earshoots.
From this time until the plants reached full size, the two
upper earshoots of both hybrids developed at a much
faster rate than the lower ones. Moreover, the two upper
earshoots of the two-ear hybrid developed more nearly
at the same rate than those of the single-ear hybrid.

At this low rate of planting, the lower nonfunctional
earshoots of both hybrids ranged from very small with no
silks showing to silks about the size of the functional ones.
The second earshoots of plants of WF9xC103 and the
third and occasionally the fourth earshoots of plants of
L317xR4 produced silks but seldom produced grain.
However, the lower nonfunctional earshoots of these and
other hybrids were maintained in a potentially functional
condition until about eight days after silking of the first
earshoot (see pages 16-20 and Figs. 11-13).

Quantitative studies of the rate of development of
upper earshoots of several single-cross corn hybrids dur-
ing the two-week period before silking of the top earshoot




No. 2 WF9XCI03 1954




1954



Figure 5. Developmental pattern of earshoots from plants of WF9 x C103 and L317 x R4; the top earshoot is at the right
end of each row. Asterisks designate approximate silking date of top earshoot.



8



BULLETIN NO. 747



have been reported by Collins (1963) and by Collins
and Russell (1965). Data on earshoot development of
corn have also been presented by Freeman (1940), Kies-
selbach (1949), Sass and Loeffel (1959), Sass (1960),
and Han way (1966). These findings are in general
agreement with the developmental pattern of earshoots of
one- and two-ear hybrids that is presented in Figure 5.

Under conditions of low fertility or at a high rate of
planting, some plants of most hybrids fail to produce
even one ear (Fig. 6) . Normally, earshoots that become
functional do so before silk emergence (Kiesselbach,
1922). Sass and Loeffel (1959) state that the failure of
plants to produce ears (barrenness) "is the result of fail-
ure of silk emergence during the pollen-shedding period,
rather than the failure of formation of floral organs."
However, first earshoots that silk as much as 10 days
later than the check plants, 3 or 4 days after the pollen-
shedding period, may be functional (Figure 17, page 23,
shows the results of hand-pollinating first earshoots that
silked late because of shading) .

Data presented by Siemer (1964) and by Siemer et al.
(1969) show that several inbreds and hybrids differ con-
siderably in the number of days between initiation of the
first and second earshoots and of the first and third ear-
shoots. Average time between initiation of first and sec-
ond earshoots of inbreds ranged from days for M14
to 4.7 days for C103; for hybrids the interval ranged
from 0.4 days for B14 x Oh43 to 2.5 for Hy2 x C103.

Unfortunately, Siemer (1964) did not record the num-
ber of ears per plant in 1962, and the data for 1963 were
insufficient to show whether the time between initiation
of the first and lower earshoots on each plant correlated
with the pattern of earshoot development and the num-



ber of ears per plant. It would be desirable, therefore, to
determine the interval of time between initiation of the
first and lower earshoots and the number of ears per
plant for inbreds and corresponding hybrids when grown
one plant per hill at 4,000, 8,000, 12,000, 16,000, and
20,000 plants per acre. This would help determine how
rate of planting, genetics, and cultural conditions (soil
fertility, water, light, and so forth) interact to affect the
interval of time between initiation of the first and the
lower earshoots. It would also help explain the relation-
ship between that interval of time, the pattern of earshoot
development, and the number of ears per plant.

EFFECTS OF RATE OF PLANTING

The effect of three rates of planting on the number
of ears per plant of hybrid L317 x R4 are reported in
Table 2. Figure 6 illustrates the effect of rate of planting
on the number and size of ears per plant for Hy2 x Oh7
and WF9xC103. Similar results, which also show the
large influence of rate of planting, have been reported by
Brown and Garrison (1923), Long (1953), Krantz and
Chandler (1954), Lang et al. (1956), Dungan et al.
( 1958) , Zuber et al. ( 1960) , Collins and Russell ( 1965) ,
Russell and Teich (1967), and Brown et al. (1970).

The data of Collins and Russell illustrate the inter-
action of rate of planting and genetics of one-ear x two-
ear hybrids and two-ear x two-ear hybrids on the number
of ears per plant. The percentages of plants with two
ears when one-ear x two-ear hybrids were grown at
8,000, 12,000, and 16,000 plants per acre were 17.5, 4.4,
and 0.6, respectively; the corresponding percentages for
two-ear x two-ear hybrids were 85.6, 39.7, and 2.8. Such



Figure 6. Effects of rate of
planting on number and size
of ears per plant. Upper num-
bers give plants per hill and
hill size; lower numbers are
plants per acre.




EARSHOOT DEVElOP/VIENr OF CORN



Table 2. Effecf of Rate of Planting on Number of Ears per
Plant of L317 x R4, 1955







Rate of planting




Number of


1 per hill


1 per hill


3 per hill


plants with


(80" x 80"


) (40" x 40")


(40" x 40")


One ear








44


Two ears


13


103


8


Three ears


66


1





Four ears












results underline the necessity of studying inbreds and
hybrids over a range of planting rates and cultural con-
ditions; only so can the interaction of these factors on
the interval of time between initiation of earshoots and
the number of ears per plant be clarified.

Decreasing the land area per plant by increasing the
rate of planting decreases the number and size of ears
per plant of multiple-ear hybrids and decreases the size
of ears on single-ear hybrids, even under favorable cul-
tural conditions. These decreases are ascribed largely to
reduced amounts of light per plant, reduced stalk diam-
eter (Eisele, 1938), reduced leaf area (Eisele, 1938;
Earley, 1961; Nunez and Kamprath, 1969; Brown et al.,
1970), and probably reduced amounts of growth-pro-
moting substances. The interval of time, moreover, be-
tween initiation of the first and the lower earshoots per
plant is probably increased with decreased land area per
plant.

On the other hand, the beneficial effects of large land
areas per plant are associated with more light per plant,
which aids production of maximum leaf area and stalk
diameter; large leaf areas, in turn, intercept more light
and synthesize more of the materials needed for ear
production. Spacing the plants more widely also probably
decreases the interval between initiation of earshoots.

It is interesting to note that the effects of rate of plant-
ing on the number and size of ears per plant are much
the same as from shading of plants (see pages 22-24).
The decrease in number and size of ears per plant when
plants are shaded apparently substantiates the statement
of Thimann and Skoog (1934) and Avery et al. (1937b)
that plants produce growth substances only in the pres-
ence of light.

Table 3. Genetic Influence on Number of Mature Ears per
Plant of Four Single-Cross Hybrids, 7953



Number of
plants with


WF9 x
C103


L317
x R4


2110
x K64


2110
x 540


One ear


27


5


1





Two ears


8


315


49


13


Three ears





4


103


27


Four ears








25


23


Five ears











5


Six ears











1



GENETIC INFLUENCE

Hybrids shown in Figure 7 were grown at the rate of
one plant per 40"x40" hill (4,000 plants per acre) in
1953. Tillers were removed at the four-leaf stage. The
number of ears per plant for each hybrid is given in
Table 3. Apparently, these hybrids have a genetically
controlled mechanism for making possible the production
of one to six ears per plant. A similar conclusion was
reached by Collins and Russell (1965), who reported
that, at 8,000 plants per acre, "all single crosses of the
two-ear inbreds produced some harvestable second ears,
which suggests that a homeostatic mechanism exists in
the two-ear x two-ear crosses that was not present in the
one-ear x one-ear crosses and that was not present to as
great an extent in the one-ear x two-ear crosses. Second-
ear development is proposed as a mechanism by which
Corn Belt maize plants can exhibit developmental
homeostasis."

At 16,000 plants per acre, Leng (1954) as well as
Collins and Russell observed no consistent genetic influ-
ence on the number of ears per plant that had been ob-
served at lower rates of planting.

The maximum number of ears per plant that a given
hybrid can produce is genetically controlled. The expres-
sion of the genetic potential for the number of ears per
plant depends upon many environmental factors, how-
ever, including water (Robins and Domingo, 1953; Howe
and Rhoades, 1955; Denmead and Shaw, 1960; Card
et al., 1961) ; light (Stinson and Moss, 1960; Barley et al.,
1966 and 1967; Pendleton et al., 1967; Schmidt and
Colville, 1967) ; and soil fertility and rate of planting
(Long, 1953; Krantz and Chandler, 1954; Lang et al.,
1956; Zuber et al., 1960). Therefore, genetic control of
the number of ears per plant appears to be quantitative,
not qualitative.



WF9XCI03 L3I7XR4 2110 *K64 2110x540




Figure 7. Genetic influence on number of ears per plant.
These plants were grown at one plant per 40" x 40" hill.



10



BULLETIN NO. 747



SIMULTANEOUS POLLINATION
OF ALL SILKING EARSHOOTS

As shown in Figure 5, the two upper earshoots of the
two-ear hybrid L3 1 7 x R4 developed at more nearly the
same rate than the two upper earshoots on the single-ear
hybrid WF9xC103. Moreover, the length of time be-
tween silking of the first and second earshoots of LSI 7 x
R4 was less than for WF9 x C103. It is quite likely, there-
fore, that the first and second earshoots of L3 1 7 x R4
were pollinated at more nearly the same time than were
the first and second earshoots of WF9xC103. This
could perhaps account for the production of two ears per


1 3 4 5 6 7

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