Published January, 2009
Occurrence and Proposed Cause of Hollow Husk in Maize
ABSTRACT
In 2007, a maize (Zea mays L.) ear abnormality that we term here as “hollow husk” occurred in research trials designed to alter
the level or the sensing of plant ethylene. The unique experimental conditions of 2007 enabled us to document the occurrence of
hollow husk and propose a physiological mechanism for its cause. Ears exhibiting hollow husk have normal appearing husks that
feel hollow due to an abrupt cessation in ear development and a concomitant lack of silk emergence. Hollow husk occurred when
the foliage of actively growing plants was sprayed before the VT growth stage with a chemical treatment that should either lower
the level of plant ethylene (a strobilurin fungicide), or one that should decrease the plant’s sensitivity to ethylene (1-MCP). An
attempt to increase ethylene status (via ethephon) led to virtually no hollow husk symptoms. The percentage of plants exhibiting
hollow husk symptoms depended on the hybrid, the stage of plant growth when sprayed, and the combination of management
conditions that promoted plant growth. Plants sprayed at V15 generally exhibited greater symptoms than those sprayed at V11,
and hollow husk successively increased with increases in N supply and decreased with increases in plant population. Based on
our data, we speculate that hollow husk is a physiological ear abnormality related to a perturbation in the level or the sensitivity
of the plant to ethylene.
E
ar abnormalities occur in maize due to a myriad of
reasons that include mutations, pathogen infections, environmental perturbations, and chemical applications. There are obvious implications from a commercial standpoint, since malformed
ears produce lower yields. Because of maize’s whorled phylotaxis
and determinant reproductive development, many ear deformities
are caused by events which occur during vegetative growth. One
of the more common ear deformities found in production fields
is the so-called “blunt ear” (also referred to as “beer-can” or “hand
grenade” ear), which is smaller in length than normal ears, but
often has a greater circumference due to larger individual kernels
(Nielsen, 1996). This condition results from a perturbation in cell
division in the apical meristem of the embryonic ear shoot when
the primordia for kernel ovules are being formed (Lejeune and
Bernier, 1996). Blunt-ear originates from chemical or environmental stresses that occur between the V7 and V15 growth stages
(Nielsen, 1996), and is distinctly different from stresses that disturb pollination, or that cause kernel abortion, which are typically
manifested as kernel tip back (Thomison and Geyer, 2007). Bluntear occurred as a result of using certain combinations of soil insecticides and postemergence herbicides, cold temperature shock, or
wide fluctuations in temperature during ear formation (Nielsen,
2007). Circumstantial evidence in some situations links the
blunt ear symptoms to postemergence applications of herbicides,
University of Illinois, Crop Science Dep., 1201 W. Gregory Dr. Urbana, IL
61801. Received 9 May 2008. *Corresponding author ([email protected]).
Published in Agron. J. 101:237–242 (2009).
doi:10.2134/agronj2008.0158N
Freely available online through the author-supported
open access option.
Copyright © 2009 by the American Society of Agronomy,
677 South Segoe Road, Madison, WI 53711. All rights
reserved. No part of this periodical may be reproduced
or transmitted in any form or by any means, electronic
or mechanical, including photocopying, recording, or
any information storage and retrieval system, without
permission in writing from the publisher.
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fungicides, insecticides, and/or assorted additives, while in other
situations no obvious connection can be identified with postemergence applications (Nielsen, 2007).
Many growers noted a new widespread ear abnormality that
occurred in a uniform manner in maize plants in 2007 that we
term here as “hollow husk.” Because hollow husk was widespread
in 2007, we believe that it is not just a localized or a random
event within any one field (Nafziger, 2007) and the significance
of when and where it occurs is of importance to growers. Hollow
husk ears are characterized by a lack of silk elongation along with
a reduction in the growth of the earshoot (i.e., the husk contains
no ear or in the instances of ear growth, the ear is small and does
not form grain, see the results section for visual estimations).
We believe that the hollow husk abnormality is similar to the
“remnant ear” observed in 2006 (Nafziger, 2007), or the baby
or arrested ears that were widely observed in some areas in 2007
(Nielsen, 2007), but is distinctly different from blunt ear as blunt
ear is only a decrease in stature of the ear and kernel number and
does not involve a lack of silk emergence.
A high degree of hollow husk symptoms occurred in 2007 in
two of our ongoing research trials where the role of ethylene in the
response of maize to growth conditions and stress were evaluated.
Because of our unique experimental conditions in 2007, we were
able to accurately document, pinpoint, and characterize the occurrence of hollow husk in maize and we propose here that ethylene
may play a role in the physiological mechanism for its cause.
MATERIALS AND METHODS
Research Approach
Two separate but adjacent experiments in the same field
in 2007 were designed to alter the level or the perception of
Abbreviations: 1-MCP, 1-methylcyclopropene; ACC, 1-aminocyclopropane1-carboxylic acid; a.i., active ingredient; DKC, DeKalb; DOY, day of year; R,
reproductive; UTC, untreated control; V, vegetative.
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237
Notes & Unique Phenomena
Fred E. Below,* Kateri A. Duncan, Martin Uribelarrea, and Thomas B. Ruyle
plant ethylene. One experiment consisted of a single hybrid
and combinations of N level and plant population to achieve
varying degrees of cultural stresses (hereafter referred to as the
cultural stress experiment), while another examined 10 different commercial hybrids grown at the optimal N level and plant
population (hereafter referred to as the hybrid comparison
experiment). To both experiments, foliar treatment applications were made to either decrease or increase the plant’s ethylene status, or alternatively to decrease the plant’s sensitivity
to ethylene. We used the strobilurin-based fungicide Headline
(containing the active ingredient pyraclostrobin; BASF,
Florham Park, NJ) to decrease ethylene levels, since strobilurin
fungicides decrease the activity of ACC (1-aminocyclopropane1-carboxylic acid) synthase (Grossmann and Retzlaff, 1997;
Ypema and Gold, 1999), the rate limiting enzyme in ethylene synthesis. We enhanced ethylene levels with ethephon
(2-chloroethylphosphonic acid; Rhone-Poulenc Agriculture
Company, Research Triangle Park, NC; Warner and Leopold,
1969), and we decreased the plant’s sensitivity to ethylene by
using the competitive ethylene inhibitor 1-MCP (Sisler and
Serek, 1999; Sisler et al., 1996; AgroFresh Inc., Spring House,
PA). In both experiments, an untreated control (UTC) that
was not sprayed with any compound was used for comparison.
For both experiments, Headline was applied at the commercial rate of 100 g/ha with the inclusion of the non-ionic
surfactant, Intimidator (Crop Production Services, Galesburg,
IL) at 350 mL/ha. 1-MCP was applied at a rate of 25 g a.i./ha
with the crop oil concentrate, Dyne-Amic (Helena Chemical
Company, Collierville, TN) at 0.38% by volume (as per recommendation from AgroFresh Inc.), and ethephon was applied
at 250 g a.i./ha without any additional additives (Langan and
Oplinger, 1987). All applications were made between 1030 and
1300 central standard time using a CO2 pressurized back-pack
sprayer at a rate of 200 L/ha using XR TeeJet (Wheaton, IL)
1102 vs. nozzles on a 114 cm 4-nozzel spray boom which delivered a 152 cm swath. Applications were made to the cultural
stress experiment on 29 June 2007 (V11 growth stage), and to
the hybrid comparison experiment on 2 July 2007 (V15 growth
stage; Ritchie et al., 1992). Plants were considered at a particular growth stage when greater than half of the plants in a plot
were exhibiting characteristics of that stage.
Field Experiments
Both experiments were located on the Fisher Farm at the
Crop Sciences Research and Education Center in Savoy, IL.
The soil at this site is a Drummer-Flanagan soil association
(fine-silty, mixed, superactive, mesic Typic Endoaquolls) typical
of East-Central Illinois, with the proper pH and drainage, and
with adequate levels of P, K, and other nutrients for high productivity. Precipitation and daily temperature extremes were
measured on site. Both experiments were overplanted with a
machine cone planter on 3 May 2007 and thinned at V6 to
achieve the desired plant populations. Fertilizer N treatments
were hand applied in a diff use band between the center of the
rows as ammonium sulfate, and followed by cultivation, when
the crop was between the V2 and V3 growth stages.
For the cultural stress experiment, treatments consisted of a factorial combination of three N levels (45, 90, and 180 kg N/ha),
three plant populations (74, 99, and 123 thousand plants/ha),
238
and the three ethylene treatments (strobilurin fungicide,
1-MCP, and ethephon) arranged in a randomized complete
block experimental design with four replications. An untreated
control (nonsprayed) was included for each N by plant population combination. An experimental unit was a four row plot
(rows 5.3 m long spaced 0.76 m apart) with the center two rows
receiving the treatments. A high-yielding commercial hybrid
(DKC 63-74) was used.
For the hybrid comparison, the four ethylene treatments
were the main plots and the 10 hybrids the subplots of a split
plot experimental design with four replications. The hybrids
represented a range of brands, biotechnology trait combinations, relative maturities, and endosperm end-use segments and
included (with relative maturity in parenthesis): DKC60-18
(110), DKC 63-74 (113), DKC 57-79 (107), DKC 61-66 (111),
DKC 58-13 (108), Pioneer 33N11(114), Asgrow RX837 (113),
Agrigold 6467 (110), Becks 5827 (111) and FS 6996 (115). Plots
were two rows (5.3 m long and 0.76 m apart) with each row
receiving the ethylene treatment. A constant plant population
of 74,000 plants/ha with an N level of 180 kg N/ha was used
for all hybrids.
Plant Measurements and Statistical Analysis
Flowering dynamics were estimated in the cultural stress
and hybrid variation experiment by counting the proportion
of plants with visible silks and anthers daily starting on 10 July
2007 which was when the crop reached tasseling. The occurrence of hollow husk ears in these trials was quantified at the
R3 growth stage (grain milk stage) by counting the number of
plants that had ears lacking any visible silks, yet still had normally developed husks.
The corresponding statistical analysis was done using PROC
MIXED in SAS (SAS Institute, 2000). Plant population, N
rates, and product applications were considered fi xed effects,
and mean separation was performed using the PDIFF option in
the MIXED procedure. The significance level used in all of the
experiments was 5%.
RESULTS
The early part of the 2007 growing season was dry with
only 41 mm of precipitation between planting and 1 wk before
the V11 treatment application (Fig. 1). An especially dry and
warm period (0 mm of precipitation and maximum temperatures up to 4°C higher than the 10-yr average for that month)
began 30 d after planting [day of year (DOY) 148] when the
crop exhibited severe leaf rolling during the day. This condition persisted until DOY 172 when an 8-d rainy period began
totaling 95 mm of precipitation. Afterward, the crop exhibited
extremely rapid growth progressing through five leaf stages
(from V11–V15) in only 5 d. This highly unusual growth spurt
occurred directly before a V11 or a V15 ethylene-treatment
application (Fig. 1).
The first visual indication of the hollow husk phenomenon
in 2007 was the detection of fully developed ear husks with
no silk emergence (Fig. 2A). These husks had a hollow feeling
due to arrested ear and silk development. Excision of the husk
revealed ears with varying degrees of arrested development
ranging from no visual ovule development to a decrease in the
number of ovules per kernel row (Fig. 2B). Kernel row number,
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Table 1. The impact of treatments designed to alter the status
(ethephon or strobilurin) or the sensitivity (1-MCP) of plants
to ethylene on the percentage of plants with hollow husk
symptoms in 2007. The UTC represents untreated control
plants. Values for V11 are averaged over the three plant populations and three N levels of the cultural stress experiment,
while V15 values are averaged over the 10 hybrids of the hybrid variation experiment.
Treatment
V11
V15
% of plants exhibiting hollow husk
41.2 a
51.0 a
1.3 b
8.9 b
0.6 b
1.0 c
0.1 b
1.2 c
Strobilurin
1-MCP
Ethephon
UTC
Different letters indicate significant differences (P < 0.05) within application time.
Fig. 1. The dates, amounts of precipitation, and daily
temperature extremes at the experimental site during the
2007 growing season. The daily high temperature is indicated
by a solid line and the low temperature by a dashed line.
Bars represent the total precipitation on a given day. The
occurrence of the growth stages and when the treatments
were applied are shown at the top of the figure.
Fig. 2. (A) Comparison of a normal ear (top) and an ear
exhibiting hollow husk (bottom) and (B) husked ears showing
the potential differences in the severity of hollow husk
symptoms where ovule development is not visible (far left) to
varying degrees of ovule formation. Pictures were taken on 21
July 2007 at the R2 growth stage. Scale is indicated to the left,
where one bar is equivalent to 1 cm.
however, was not affected (data not shown). Kernel ovules that
failed to develop exhibited a staminate-like appearance, and
did not produce silks. Hollow husk symptoms were most abundant on strobilurin-treated plants (sprayed at V11 or V15), but
also occurred with 1-MCP when treated at V15 (Table 1). In
neither experiment did increasing the ethylene levels with ethephon cause more hollow husk plants than in the UTC.
Associated with the hollow husk symptom was a drastic
reduction in silk emergence and in the total number of plants
with exposed silks (Fig. 3). This effect was more pronounced
for plants grown at the optimal (74,000 plants/ha) than the
supra-optimal plant population (123,000 plants/ha), although
high population delayed the onset of silking in UTC plants.
High plant population also delayed the onset and the degree of
pollen shed, but pollen shed was not altered by the strobilurin
fungicide (Fig. 3) or any of the other ethylene treatments (data
not shown).
The magnitude of hollow husk was closely associated with
cultural conditions conducive to maximum plant growth;
symptoms increased with increasing N rate and decreased
with increasing plant population (Table 2). Sixty-two percent
of strobilurin-treated plants exhibited hollow husk symptoms
when grown at the lowest population and with the highest N
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rate (i.e., 74,000 plants/ha with 180 kg/N), compared to 23%
of plants grown at the highest population and with the lowest N (123,000 plants/ha with 45 kg N/ha). Although much
lower in magnitude, a similar pattern was observed for 1-MCPtreated plants with 5% of ears exhibiting hollow husk under the
best N/plant population combination, and only 1% under the
poorest (data not shown).
Hybrids exhibited considerable variation in their sensitivity
to hollow husk and in their response to the ethylene treatments
(Table 3). Although the strobilurin fungicide generally resulted
in the highest levels of hollow husk (range of 9–89%), 1-MCP
also caused substantially more hollow husk ears (range 2–19%),
compared to the UTC, with some hybrids being particularly
susceptible. There was no relationship between a hybrid’s
susceptibility to hollow husk induced by 1-MCP and its susceptibility to hollow husk induced by the strobilurin fungicide
(Table 3). There was also no relationship between a hybrid’s
relative maturity and the occurrence of hollow husk. A very
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239
Table 2. The effects of plant population and fertilizer N rate
on the percentage of plants with hollow husk symptoms induced by strobilurin fungicide application at the V11 growth
stage in 2007.
Plant
population
plants (× 1000)/ha
74
99
123
Average
N rate, kg N/ha
45
90
180
Average
% of plants exhibiting hollow husk
43.3
64.7
61.5
56.5 a
31.4
43.3
50.0
41.6 b
22.5
20.5
33.1
25.4 c
32.4 c
42.9 b
48.2 a
Different letters indicate significant differences (P < 0.05) within populations or
N rates.
Table 3. Hybrid variation in the percentage of plants with hollow husk symptoms induced by treatments designed to alter
the status or sensitivity of the plant to ethylene in 2007. The
UTC represents untreated control plants. Treatments were
applied at the V15 growth stage.
Hybrid
Fig. 3. The effect of plant population and an application of
strobilurin fungicide at V11 on the time course of (A and
B) silking and (C and D) pollen shed in 2007. Bars indicate
standard error and stars indicate significant differences from
the untreated control (UTC) at the 5% confidence level.
The UTC is represented by black circles, while strobilurin
fungicide treated is represented by white circles.
small percentage of the hybrids exhibited hollow husk when
treated with ethephon (Table 3).
DISCUSSION
The hollow husk symptoms observed in our study occurred
in plants that were actively growing following relief from a
prolonged period of vegetative water stress (Fig. 1); a condition under which ethylene is an important factor (Young et al.,
2004; Morgan and Drew, 1997). The unique growth conditions during 2007 are unlikely to be exactly repeatable in a field
setting and therefore we believe that hollow husk symptoms
may be hard to repeat in a replicated fashion in subsequent
years. However, we believe our replicated trials in 2007 serve to
help us understand hollow husk and its impact, as ear deformities have severe implications for grain yield.
Actively growing plants produce or perceive optimal ethylene
levels (Davies, 1987; Morgan and Drew, 1997), and as such, a
sudden decrease in level (i.e., from the strobilurin fungicide) or
perception (i.e., from 1-MCP) would be inhibitory to growth.
The higher occurrence of hollow husk ears in chemically
treated plants grown with high N and low plant population
(the cultural stress experiment, which should be exhibiting the
most rapid growth and should have higher endogenous ethylene levels before treatment; Morgan and Drew, 1997) demonstrates that ethylene levels and perception are important to
maintain proper growth and reproductive viability. Conversely,
the plants that were chemically treated under high population
and low N levels (which we believe have less ethylene produced
endogenously before treatment) show less susceptibility to hollow husk, again demonstrating that proper ethylene levels are
required for optimal growth and when altered by a chemical
agent, can severely affect reproductive capacity.
240
Agrigold 6467
Asgrow RX837
Becks 5827
DKC 57-79
DKC 58-13
DKC 60-18
DKC 61-66
DKC 63-74
FS 6996
Pioneer 33N11
LSD (0.05)
Strobilurin
78.8
35.0
88.8
50.6
35.5
15.0
9.2
64.0
71.6
62.6
20.6
1-MCP
Ethephon
% of plants
18.7
0.9
12.0
2.7
6.6
0.9
13.0
2.0
1.9
0.0
14.7
1.9
1.9
0.9
8.6
0.0
9.4
0.9
2.8
0.0
4.1
ns†
UTC
0.0
1.9
0.0
0.0
0.0
0.0
0.0
0.0
1.0
0.0
ns
† ns indicates nonsignificant differences among hybrids.
Other evidence is mounting showing that ethylene does
not act strictly as a growth inhibitor, but under some circumstances can also stimulate plant growth (Pierik et al.,
2006). Various growth aspects for several plant species follow
a biphasic response model with both stimulatory and inhibitory responses depending on the ethylene concentration and
the tissue (Pierik et al., 2006). Ethylene stimulates internode
or hypocotyl elongation in germinating seedlings of wheat
(Triticum aestivum L.) and Arabidopsis, respectively (Smalle et
al., 1997; Suge et al., 1997), and may be needed for silk elongation which did not occur in hollow husk ears (Pinthus and
Jackson, 1994; Fig. 3). Ethylene has also been shown to change
the sexual orientation of individual flowers (Davies, 1987) or of
whole dioecious plants (Byers et al., 1972), in agreement with
our observation of staminate-like flowers at the tip of hollow
husk ears (Fig. 2B).
Hollow husk conditions were exacerbated with strobilurintreated plants, and strobilurin fungicides inhibit the activity of ACC synthase, a key enzyme in ethylene production
(Grossmann and Retzlaff, 1997; Ypema and Gold, 1999).
Strobilurin fungicides also have a multitude of other noted
effects besides ethylene production, including impacting
photosynthesis and chlorophyll degradation (Grossmann and
Retzlaff, 1997) and we cannot discount that strobilurin fungicides may impact other key regulatory processes in plants that
affect yield under unique weather conditions. Also, we cannot
discount the possibility that adjuvants included in the strobilurin fungicide application may also play a role in affecting ethylene levels as well as producing hollow husk.
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In addition to strobilurin fungicides, 1-MCP also decreases
the plant’s sensitivity to ethylene (by inactivating ethylene
receptors), we believe we lowered the effective ethylene status
of the plant, explaining the hollow husks that were observed
from 1-MCP application at V15. There are clearly growth-stage
and environment specific instances where plants produce excess
ethylene that is detrimental to growth (Cheng and Lur, 1996;
Sun et al., 2005; Pierik et al., 2006), and in some cases this
can be protected or alleviated with 1-MCP (Hays et al., 2007).
However, in our experiments, we believe that the 1-MCP treatments occurred under a unique set of weather conditions and
at a crucial time for continued ear development. In contrast
to the chemical treatments designed to lower ethylene status
with strobilurin fungicides and ethylene perception with
1-MCP, we observed virtually no hollow husk symptoms where
the ethylene level was likely enhanced by ethephon, although
these plants were noticeably shorter (data not shown). This
observation that high rates of applied exogenous ethylene with
ethephon did not contribute to the occurrence of hollow husk
demonstrates that it is the lowering of ethylene levels that leads
to the incidence of hollow husk, although further experimentation is needed to verify and validate this hypothesis.
The large variation among hybrids exhibiting hollow husk
(Table 3) is indicative of genetic variation in the response to
plant growth regulators. Varieties of rice (Oryza sativa L.) and
wheat differ in their production and/or sensitivity to ethylene (Klassen and Bugbee 2002) and we speculate that maize
hybrids also differ in their responses to ethylene. Moreover, the
lack of a relationship between a hybrid’s hollow husk symptoms
induced by the strobilurin fungicide (a decrease in synthesis)
or by 1-MCP (a decrease in sensitivity) shows that these two
mechanisms can act independently to alter plant ethylene
status (i.e., at the enzymatic level or the receptor level) and
variation differs among hybrids (i.e., hybrids may have varying numbers of ethylene receptors). Collectively, the variation
among the hybrids in response to the treatments demonstrates
that ethylene perception differs among commercial hybrids and
may contribute to fitness and/or susceptibility to hollow husk
as well as other stresses.
Ear development of maize is also clearly associated with the
integration of the other plant hormones, such as abscisic acid
and gibberellin, which can directly interact with ethylene (Ross
and O’Neill, 2001). We do not discount that other hormones
may play a role in the occurrence of hollow husk in addition
to that of ethylene. The ease with which growth regulators can
hinder, rather than enhance ear development was well documented by Earley et al. (1974) who showed the hybrid, the stage
of plant growth, the part of the plant treated, and the type of
growth regulating chemical all affected the degree of earshoot
response. Similarly, we believe the specific environmental conditions experienced at our site in 2007, in conjunction with
the application of chemicals that should alter ethylene level or
sensitivity caused hollow husk.
Midwest growing season in 2007 may have caused perturbations in maize ethylene levels and that coupled with the spraying of ethylene altering compounds caused hollow husk to
occur. Although further molecular and biochemical evidence
is needed to verify the alterations in the ethylene biosynthesis
and perception pathways in plants exhibiting hollow husk, the
documentation of its occurrence and causal factors is worth
examining due to its negative impact on grain yield. We could
benefit by understanding how ethylene plays a role in maize
growth conditions as well as understanding when best to apply
ethylene altering compounds.
ACKNOWLEDGMENTS
This study is part of project 15-0390 of Agriculture Experiment
Station, College of Agricultural, Consumer, and Environmental
Sciences, Univ. of Illinois at Urbana-Champaign. It was supported
in part by AgroFresh, Inc. The authors express their gratitude to
Honeywell Inc. for supplying the ammonium sulfate, BASF for the
Headline fungicide, and the various companies for the hybrid seed.
We would like to thank Peter R. Thomison for careful reading of the
manuscript. We would also like to thank Mark Harrison, Jim Kleiss,
Brad Bandy, and Juliann Seebauer for technical assistance.
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SUMMARY
Our unique set of experiments during the summer of 2007
allowed us to speculate that ethylene is a potential causal
agent in the occurrence of the hollow husk ear abnormality.
We believe that the environmental conditions during our
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Agronomy Journal
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Volume 101, Issue 1
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2009