American Journal of Plant Sciences, 2013, 4, 2015-2022
http://dx.doi.org/10.4236/ajps.2013.410252 Published Online October 2013 (http://www.scirp.org/journal/ajps) 2015
Bioassay of Winter Wheat for Gibberellic Acid Sensitivity
Alexander D. Pavlista1*, Dipak K. Santra1, David D. Baltensperger2
1Department of Agronomy and Horticulture, University of Nebraska, Panhandle Research & Extension Center, Scottsbluff, USA;
2Department of Soil and Crop Sciences, Texas A&M University, College Station, USA.
Received August 20th, 2013; revised September 20th, 2013; accepted October 8th, 2013
Copyright © 2013 Alexander D. Pavlista et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Increasing winter wheat seedling growth would make it a better winter cover crop. Gibberellic acid (GA3) seed treat-
ment may accomplish this by stimulating stem growth. A bioassay, mimicking field conditions, could determine the
relative sensitivity of conventional and semi-dwarf cultivars. In growth chambers set for cool (10˚C/4˚C) and warm
(21˚C/4˚C) conditions, wheat seeds were treated with 0 and 125 to 16,000 ppm GA3. The cultivars Goodstreak (tall or
conventional) and Wesley (semi-dwarf) were compared as standards. Emergence and plant height were measured.
“Goodstreak” showed a significant growth promotion at 500 ppm GA3 when seeds were dipped and 2000 ppm when
GA3 was applied in-furrow under both temperature regimes. “Wesley” in general required the same or a higher dose of
GA3. Separately, the seeds of nine other cultivars were treated with GA3 as the standards. Based on maximum height
promotion, the most sensitive cultivars under cool conditions were Goodstreak, Harry, Millenium, and Wahoo; under
warm conditions, the most sensitive cultiv ars were Alliance, Goodstreak, Jagalene, and Millenium. In general, the least
GA3 sensitive cultivars were Arrowsmith, Scout66, and Wesley. “Buckskin” and “InfinityCL” were intermediate. The
rye cultivar Rymin also was tested and showed less sensitivity to GA3 than “Goodstreak”. When 6 benzyladenine (6BA)
with GA3 was applied to “Goodstreak” and “Wesley” seed, emergence, plant height and weight, and tiller formation
were reduced. Wheat cultivars will respond to GA 3 and differ in the amount of GA3 needed. The results of this growth
chamber study will guide subsequent field trials.
Keywords: Plant Growth Regulator; Gibberellin; Planting Aid; Cover Crop
1. Introduction
Introducing winter wheat into irrigated cropping systems
is limited by an overlap between optimal planting date of
winter crops such as wheat and rye with optimal harvest
date for summer crops such as dry bean and potato in the
High Plains [1]. Stimulating seedling growth of wheat
under cooler conditions could allow later planting in the
fall and possibly reduce wind erosion. Another aspect is
that when soils are dry before planting winter wheat, it is
recommended to plant seeds deeper, about 7.5 cm, in the
anticipation that the seedling r oots will reach water. How-
ever, in this case, emergence takes longer and is less due
to the greater distance for the coleoptile to reach the sur-
face. In both cases, a stimulation of stem growth would
improve w h e a t health.
A possible method of promoting stem growth of late-
planted and deep-planted winter wheat is to apply a
growth promoter to seeds. This could enhance emergence
and stem elongation. Recently [2], the promotion of seed-
ling growth was accomplished with treating seeds with
growth-promoting bacteria in a greenhouse. However,
this may be accomplished directly by applying the natu-
ral product gibberellic acid (GA3) to wheat seeds. The
promotion of stem growth by gibberellins has been known
since the 1930s when a rice disease was identified to be
due to a pathogenic fungus Gibberella fujikuroi [3]. More
than 130 gibberellins have been identified. GA3 is the
key gibberellin, is highly active and is well known to sti-
mulate stem elongation [4,5]. Greenhouse bioassays for
stem elongation resulting from foliar-applied GA3 on le-
gumes (Phaseolus vu lgaris) have been reported [6,7]. Win-
ter wheat stimulation by GA3 presents the problem that
popular cultivars for irrigated wheat production are semi-
dwarf, that is, they contain genes that may lower produc-
tion of gibberellins or desensitize plants to endogenous
GA3 and other gibberellins [8,9].
The genetics of plant height in wheat is known to be
complex and is determined by more than 20 Rht (“re-
*Corresponding a uthor.

Bioassay of Winter Wheat for Gibberellic Acid Sensitivity 2016
duced height”) genes with different types of genetic me-
chan ism, which ar e loc ated acro ss 17 of 21 ch ro mosomes
in wheat [10-13]. Depending on their reaction to exoge-
nous GA3, the Rht genes are classified into two groups:
GA3-sensitive (synthesis mutants) and GA3-insensitive
[11]. Dwarfism in wheat lines carrying GA3-sensitive
genes is due to either the absence of or a modified spec-
trum of endogenous gibberellins. These genes are invol-
ved in GA3 biosynthesis and normal growth can be re-
stored by exogenous GA3. The most important dwarfing
gene of this category used agronomically in wheat is
Rht8c (formerly known as Rht8), which was first intro-
duced into European wheat in the 1930s and now is
widely used in many wheat cultivars adapted to warm
climates [14,15]. The GA3-insensitive dwarfism in wheat
lines is either due to reduced or lack of response to ex-
ogenous GA3. Major GA3-insensitive Rht genes are at
two loci on the chromosome 4BS and 4DS; each locus
has multiple alleles that induce varying degrees of dwar-
fism. Four major GA3-insensitive genes are Rht-B1b
(formerly Rht1), Rht-D1b (formerly Rht2), Rht-B1c (for-
merly Rht3), and Rht-D1c (formerly Rht10) of which
RhtB1b and RhtD1b are the two most widely GA3-in-
sensitive dwarfing genes in modern wheat cultivars [16,
17]. Wheat cultivars carrying these semi-dwarf genes do
not respond (increase in plant height) to endogenous
GA3.
The objective of this study was to determine whether
winter wheat cultivars, both regular and semi-dwarf types,
would respond to seed-applied GA3 with increased stem
elongation. This information would then be used in field
trials to stimulate growth of late-season planted winter
wheat.
2. Materials and Methods
2.1. Growth Chamber Conditions
Experiments were conducted in two growth chambers
(Conviron model CMP 3023). One chamber was set for
cool conditions, 10˚C day and 4.4˚C night, and the other
for warm conditions, 21.1˚C day and 4.4˚C n ig ht ( Figure
1). Plants were exposed to six hours (11 am to 5 pm)
each day to either 10˚C or 21.1˚C, and six hours (11 pm
to 5 am) to 4.4˚C. The remaining hours were gradual
transitions periods between day and night temperatures.
Daylight was supplied by a bank of florescent light bulbs
set at level 4 for a 12-hour photoperiod corresponding to
day and night temperatures. Lamps were approximately
1.5 m above plants.
2.2. Plant Material and Trial Conditions
The winter wheat cultivars Goodstreak and Wesley were
standards in all tests and in 2006, the cultivars Alliance,
Arrowsmith, Buckskin, Harry, InfinityCL, Jagalene, Mil-
Figure 1. Temperature schedules for growth chamners.
lenium, Scout 66, and Wahoo were included (Table 1).
All seed was certified and obtained locally. In 2007, the
rye cultivar Rymin was tested with winter wheat culti-
vars Goodstreak and Wesley. Seeds were planted 1 cm
deep in 36 cm by 51 cm flats lined with paper towels and
filled with Fafard Superfine Germination Mix (American
Clay Works, Denver, CO). Flats were soaked prior to
planting and watered as needed. Each flat constituted a
replication. Six seeds for each treatment and cultivar
were planted 2.5 cm apart and 1 cm deep in rows t hat were
5 cm apart, 18 rows per flat. Three replications were
placed in each of the two growth chamber.
2.3. Experiments
Gibberellic acid was applied as Release LC, a 4% a.i. by
weight formulation, i.e. 1 g GA3/30 mL, formulation
(Valent BioScience Corp., Long Grove, IL). Release LC
was diluted serially with water from 16,000 to 125 ppm
GA3. Experiments were conducted in 2005, 2006, 2007,
and 2009. In 2009, 6-benzyladenine (6BA) as MaxCel, a
1.9% a.i. by weight formulation (Valent Bioscience) was
added to GA3. In 2005, 36 seeds of “Goodstreak” and of
“Wesley” were dipped for 2 minutes in 3 ml of GA3 at 0,
125, 250, 500, 1000, 2000, 4000, 8000, or 16,000 ppm.
Seeds were air-dried for three days before planting. In a
second test in 2005, seeds were planted in flats and 0.125
ml/seed of GA3 at 0 to 16,000 ppm was applied in-furrow
with a pipette. In 2006, seeds of nine winter wheat culti-
vars (listed above) plu s “Goodstr eak ” and “W esley” wer e
dipped in 0, 250, 1000, 4000, and 16,000 ppm GA3. Treat-
ments were drained and seeds air-dried before planting.
In 2007, the bioassay developed in 2005 using seed dip-
ped in GA3 at 0, 250, 500, and 1000 ppm was used to test
the sensitivity of rye cultivar Rymin in comparison with
the wheat cultivars Goodstreak and Wesley under the
warm growth chamber conditions. In 2009, the bioassay
was conducted under warm conditions to test the influ-
ence of adding 6BA at 0, 125 , 50 0, an d 2000 pp m to G A3
at 0, 500 ppm for “Goodstreak”, and 1000 ppm for
“Wesley”.

Bioassay of Winter Wheat for Gibberellic Acid Sensitivity 2017
Table 1. Description of winter wheat cultivars.
Cultivar Complimentary Grouping1 Height DescriptionColeoptile Length Rht genes2 References
Alliance Chisholm Moderately short Short Rht1Bb (Rht1) [18]
Arrowsmith - Moderately tall Moderately long Rht1Bb (Rht1) [19]
Buckskin Scout Tall Long [20]
Goodstreak Colt Tall Long [21]
Harry Brule Moderately short Short Rht1Bb (Rht1) [22]
InfinityCL - Moderately tall - Rht1Bb (Rht1) [23]
Jagalene Abilene Moderately short Moderately s hort Rht1Bb (Rht1) Syngenta
Millenium - Moderately tall Moderately short Rht1Bb (Rht1) [24]
Scout66 Scout Tall Long [25]
Wahoo Arapahoe Medium Medium Rht1Bb (Rht1) [26]
Wesley Sumner Short Short
Rht1Bb (Rht1) & Rht8c
(Rht8) [27]
1Cultivars within each complimentary group share at least 50% of the same parent lines. 2Source was [28] Guedira et al., 2010.
2.4. Data and Statistical Analysis
In 2005, emergence and plant heights were measured at
18 and 31 days after planting (DAP), and fresh weight
was measured at 66 DAP. In 2006, only plant height was
measured at 30 DAP under warm conditions and 40 DAP
under cool conditions. In 2007, plant height was meas-
ured at 5 DAP under warm conditions. In 2009, measure-
ments of emergence at 14 DAP, plant height at 17 and 28
DAP, and tiller number at 28 DAP were conducted und er
warm conditions. Data from each experiment were ana-
lyzed using SAS Proc ANOVA (version 9.1, SAS Insti-
tute, Cary, NC) and means were separated using least
significant difference (SAS Institute) for each cultivar.
3. Results
3.1. Bioassay Development
In developing a bioassay for winter wheat sensitivity to
GA3, cvs. Goodstreak with a regular growth habit and
Wesley, a semi-dwarf, were treated and planted. These
were placed in one of two growth chambers set to
“warm” and “cool” conditions. Treatment was applied as
a seed dip or as a furrow drip. Under warm conditions in
2005, the emergence of both Goodstreak and Wesley was
unaffected by GA3 applied up to 2000 ppm applied as a
seed dip, but above that, there was a significant decrease
of emergence (Table 2). Under cool conditions, the
emergence was not inhibited until the seed was treated
with 8000 ppm. Furrow application of GA3 had no effect
on emergence even at 16,000 ppm. Plant height at 18
DAP showed a significant increase when Goodstreak
seed was treated with 500 ppm regardless of the chamber
temperature regime (Table 2). With furrow treatment,
height of Goodstreak was promoted as well but 2000 ppm
was required to achieve a significant increase over the
check. Wesley was less sensitive to GA3 than Goodstreak.
For Wesley, plant height was increased by treating seed
with 2000 ppm and placed in the warm chamber and 500
ppm when placed in the cool chamber (Table 2). Furrow
application of GA3 required 4000 ppm in the warm cham-
ber and 2000 ppm in the cool chamber to elicit a signifi-
cant height increase.
3.2. Cultivar Evaluation
The seed of nine additional wheat cultivars were treated
with GA3, planted and placed into the two growth cham-
bers each with a different temperature regime. Under
warm conditions, three cultivars, Infinity CL, Scout 66
and Wahoo, did not show a significant height increase
with GA3 applied at 250 ppm while the other eight of the
11 cultivars did (Table 3). Under cool conditions, three
cultivars, Alliance, Bucksk in and Jagalene, sh owed a de-
creased sensitivity to GA3 and three cultiv ars, Infinity CL,
Scout 66 and Wahoo, showed an increased sensitivity
compared to their performance under warm conditions
(Table 3). The other five cultivars responded to the same
exposure to GA3 under both temperature regimes. Note
Goodstreak, a tall cultivar, reached a maximum response
with 250 ppm GA3 while Wesley, a semi-dwarf cultivar,
reached a maximum response at 1000 ppm (Table 3). Ap-
plication of 4000 ppm GA3 increased plant height for all
cultivars under both temperature regimes. When seeds
were exposed to 16,000 ppm GA3, there was no emer-
gence under cool conditions and 0% to 16% emergence
under warm conditions (data not shown).
3.3. Rye Comparison
A short-term bioassay using seed dips and seeding under
arm conditions was verified in 2007 and the rye cv. w

Bioassay of Winter Wheat for Gibberellic Acid Sensitivity 2018
Table 2. Growth of winter wheat cvs. Goodstreak and Wesley grown in grow th chambers set for 21.1 ˚C/4.4˚C (warm chamber)
or 10˚C/4.4˚C (cool chamber) day/night temperature cycle after seed or furrow application of gibberellic acid (GA3), 2005.
“Goodstreak” “Wesley”
Warm Chamber Cool Chamber Warm Chamber Cool Chamber
Seed1 Furrow1 Seed Furrow Seed Furrow Seed Furrow
GA3 ppm Emergence a t 18 DAP2, %
0 100 A3 100 94 A 100 100 A 94 94 A 100
125 94 A 94 100 A 100 89 A 72 89 A 100
250 94 A 100 100 A 94 94 A 94 94 A 89
500 100 A 100 100 A 100 89 A 100 94 A 89
1000 89 A 100 100 A 89 94 A 89 89 A 94
2000 89 A 100 100 A 100 100 A 100 94 A 89
4000 67 B 94 100 A 94 67 B 100 78 A 89
8000 7 C 94 34 B 100 12 C 94 44 B 94
16000 2 C 94 2 C 100 2 C 83 2 C 94
Plant height4 at 18 DAP, mm5
0 129 B 123 D 54 B 56 C 110 B 115 B 52 B 53 C
125 152 AB 131 CD 71 AB 53 C 128 AB 114 B 66 AB 59 BC
250 156 AB 130 CD 78 AB 52 C 134 AB 113 B 71 AB 59 BC
500 175 A 127 CD 86 A 52 C 133 AB 119 B 79 A 64 BC
1000 195 A 128 CD 90 A 60 BC 133 AB 120 B 78 A 66 BC
2000 188 A 140 C 89 A 67 B 137 A 124 B 72 A 71 AB
4000 167 A 169 B 77 AB 77 A 148 A 137 A 74 A 72 AB
8000 63 C 167 B 52 B 78 A 107 B 139 A 63 AB 82 A
16000 - 197 A - 84 A - 139 A - 79 A
1Seeds were dipped in GA3 and then planted, or seeds were first planted and GA3 was applied with a pipette at 0.125 ml per seed. 2DAP = days after planting.
3Mean separation of GA3 rates for each column (cultivar, thermal period, and application method) using least significant difference at P < 0.05. 4Plant height
was measured from potting soil to the highest leaf tip. 51 inch = 254 mm.
Table 3. Growth of eleven winter wheat cultivars grown in growth chambers set for either at 21.1˚C/4.4˚C (warm chamber)
or at 10˚C/4.4˚C (cool chamber) day/night temperature cycle after seed application1 of gibberellic acid (GA3), 2006.
Warm Chamber at 30 DAP2 Cool Cham b er at 40 DAP
GA3, ppm
0 250 1000 4000 0 250 1000 4000
Cultivar Plant height3, mm4
Alliance 258 B5 321 A 319 A 317 A 163 B 186 AB 216 A 209 A
Arrowsmith 258 C 302 B 318 AB 330 A 178 C 203 B 215 B 231 A
Buckskin 268 C 321 B 417 A 461 A 192 B 212 B 261 A 273 A
Goodstreak 297 B 375 A 410 A 428 A 173 B 235 A 268 A 293 A
Harry 241 C 277 B 300 AB 328 A 145 B 175 A 192 A 187 A
InfinityCL 236 C 253 BC 272 AB 305 A 143 C 175 B 201 A 200 A
Jagalene 214 B 256 A 250 A 267 A 147 B 171 AB 185 A 183 A
Millenium 221 B 259 A 255 A 277 A 161 B 190 A 196 A 206 A
Scout66 308 B 324 B 350 B 440 A 153 B 161 B 217 A 217 A
Wahoo 251 B 272 AB 272 AB 282 A 156 B 197 A 196 A 189 A
Wesley 227 C 274 B 303 A 306 A 154 C 186 B 1 9 4 A B 212 A
1Seeds were dipped in GA3 and then planted in flats. 2DAP = days after plating. 3Plant height was measured from potting soil to the highest leaf tip. 41 inch =
54 mm. 5Mean separation of GA3 rates for each cultivar (row) using least significant differences at P < 0.05. 2

Bioassay of Winter Wheat for Gibberellic Acid Sensitivity 2019
Rymin was tested. Goodstreak showed a significant
height increase at 5 and 13 DAP when treated with 500
ppm GA3 while Wesley did not show a response this
early even with 1000 ppm (Table 4). This agreed with
the test in 2005 when height was measured at 18 DAP,
but in 2007 , a response was ob served for both wheat cul-
tivars at 250 ppm when measurements were taken 30
DAP (Table 3). The rye cv. Rymin did not show a height
increase until seeds were exposed to 1000 ppm (Table
4).
3.4. Addition of 6BA
In 2009, this bioassay was used to determine whether
6BA could supplement GA3 growth promotion and in-
crease tillering of wheat seedlings. Goodstreak seed was
treated with 500 ppm GA3 with and without three con-
centrations of 6 BA, 125, 500 and 2000 pp m. Wesley w as
likewise treated but 1000 ppm GA3 was used. Good-
streak when treated with 500 ppm GA3 alone showed a
significant height promotion at 17 and 28 DAP (Table 5)
as expected from the bioassay. But when adding 6BA at
500 ppm or 2000 ppm, height was decreased as well as
tiller number and plant fresh weight (Table 5). Wesley
height was not affected by 1000 ppm GA3 as in previous
tests but the addition of 6BA at 2000 ppm decreased
plant height, plant weight and tiller number (Table 5).
6BA at 2000 ppm also suppressed emergence of both
cultivars.
Table 4. Plant height of rye cv. Rymin in comparison ti winter wheat cvs. Goodstreak and Wesley grown in a growth cham-
ber set for 21.1˚C/4.4˚C (warm chamber) day/night temperature cycle after seed application1 of gibberellic acid (GA3), 2007.
Rye cv. Rymin Wheat cv. Goodstreak Wheat cv. Wesley
5 DAP2 13 DAP 5 DAP 13 DAP 5 DAP 13 DAP
GA3 ppm Plant height, mm3,4
0 136 B5 173 B 112 B 203 B 98 166
125 153 AB 190 AB 115 B 210 B 105 174
250 154 AB 199 AB 115 B 204 B 103 159
500 158 AB 198 AB 134 A 232 A 107 176
1000 161 A 213 A 148 A 235 A 108 175
1Seeds were dipped in GA3 and then planted. 2DAP = days after planting. 3Plant height was measured from potting soil to the highest leaf tip. 41 inch = 254 mm.
5Mean separation of GA3 rates for each column using least significant difference at P < 0.05.
Table 5. Growth of winter wheat cvs. Goodstreak and Wesley grown in a growth chamber set for 21.1˚C/4.4˚C (warm cham-
ber) day/night temperature cycle after seed application1 of gibberellic acid (GA3) with 6-benzyladenine (6BA), 2009.
Wheat cv. Goodstreak
Emergence Plant height2 Tiller/plant Plant weight
GA3 6BA 14 DAP3 17 DAP 28 DAP 28 DAP 28 DAP
ppm ppm % mm4 # g4/plant
0 0 97 A5 103 B 139 B 3.0 A 0.13 A
500 0 97 A 132 A 176 A 3.0 A 0.15 A
500 125 97 A 125 A 172 A 3.0 A 0.12 AB
500 500 100 A 120 AB 149 B 2.6 B 0.10 B
500 2000 43 B 66 C 77 C 2.1 C 0.04 C
Wheat cv. Wesley
0 0 9 3 A 89 A 131 AB 3.4 A 0.15 A
1000 0 87 A 98 A 149 A 3.1 A 0.15 A
1000 125 87 A 98 A 142 AB 3.3 A 0.15 A
1000 500 90 A 89 A 113 B 2.9 A 0.09 B
1000 2000 17 B 48 B 76 C 1.9 B 0 .09 B
1Seeds were dipped in GA3 with or without 6BA, and then planted. 2Plant height was measured from potting soil to the highest leaf tip. 3DAP = days after
planting. 41 inch = 254 mm; 1 oz = 28.35 g. 5Mean separation of GA3 rates for each column for each cultivar using least significant difference at P < 0.05.

Bioassay of Winter Wheat for Gibberellic Acid Sensitivity 2020
4. Discussion
4.1. Wheat cv. Goodstreak and Wesley
“Goodstreak” is a hard red winter wheat that is well-
adapted to western Nebraska [21]. It grows to a conven-
tional height, tall, and has a long coleoptile (Table 1)
adapted for low moisture conditions. “Wesley” is a hard
red winter wheat, short with a short coleoptile (Table 1)
and has a high yield potential in the north central Great
Plains [27]. It is a semi-dwarf whose height tends to be
about 15% shorter than “Goodstreak” [21]. Semi-dwarf
cultivars tend to emerge erratically due to their slow
growth and short coleoptiles. For comparing conventio-
nal and semi-dwarf sensitivity to GA3, these two culti-
vars were deemed excellent representatives. Table 2 did
not show a difference in emergence between “Goodstreak”
and “Wesley” either between their untreated checks or
their response to GA3 applied as a seed dip or in furrow
up to 2000 ppm under warm conditions and up to 4000
ppm under cool conditions. GA3 applied to the seed at
4000 ppm under warm conditions and at 8000 ppm under
cool conditions significantly lowered emergence and hi-
gher rates nearly eliminated it. Furrow application of
GA3 had no effect on emergence.
4.2. Wheat Bioassay Comparisons
Allan et al. [29] began developing a bioassay for GA3
sensitivity for wheat by consecutively injecting 100 ppm
GA3 into vernalized seedlings transferred into a green-
house. Dwarf and semi-dwarf cultivars were tested. The
shortest cultivars were insensitive to GA3 injection. The
three tallest cultivars did respond to GA3 but did not at-
tain the height associated with conventional cultivars.
Under both warm and cool conditions, GA3 seed- and
furrow-treated “Wesley” attained the height of untreated
“Goodstreak” (Table 2). Pinthus and Abraham [30] grew
wheat plants of two cultivars with lines differing in height-
reducing alleles in vermiculite drenched in 25 ppm GA3
and placed in growth chambers set at continuous 11˚C or
25˚C. They reported that the tall cultivar, containing no
height reducing alleles, responded to GA3 but as dwarf-
ing increased in the lines, the response greatly diminish-
ed. At the higher temperature, height increased more ra-
pidly than at cooler temperatures. Table 2 showed that
for “Goodstreak” temperature did not affect GA3 sensi-
tivity for the stimulation of plant height. For “Wesley”
however, cool (10˚C) conditions increased the sensitiv ity
to both seed- and furrow-applied GA3 stimulation of growth.
Pereira et al. [31] soaked wheat seeds in GA3 for 4 d. At
2˚C, maximum stimulation of coleoptile length was with
500 ppm GA3 for all isolines. At 18˚C, the response to
GA3 was diminished in dwarfing isolines. They conclud-
ed that GA3 insensitivity was highly influenced by higher
temperatures as was observed with “Wesley” (Table 2).
Comparing the two cultivars, “Goodstreak” was more
sensitive to GA3 than “Wesley” under warm (21˚C) con-
ditions. Table 2 suggested that to best highlight differ-
ences between these cultivars is to bioassay for GA3 re-
sponse with seed treatment instead of furrow and possi-
bly to grow plants under the warmer conditions. The hi-
gher exposure to GA3 for height stimulation from Wesley
compared to Goodstreak is expected because Wesley car-
ries Rht-B1b and Rht8c, two GA3-insensitive genes where-
as Goodstreak is not known to carry any such genes. The
response of leaf length to applied GA3 in the Rht-B1b
and Rht-D1b genotypes increased sign ificantly with low-
ering of temperature [32]. We also found similar effect of
lower temperature in Wesley.
4.3. Cultivar Comparisons
Table 3 showed no distinct pattern between cultivars,
convention al or semi-dwarf, in sensitivity to GA3 app lied
to seed at either temperature. Some cultivars were less
sensitive to GA3 under cooler conditions while some
showed no difference. Except for three cultivars, Buck-
skin, Goodstreak, and Scout 66, eight of 11 cultivars in
this study carry the same GA3-insensitive semi-dwarf
gene Rth-B1b. Wesley carries this gene plus the GA3-
sensitive gene Rht8c. The lack of uniform pattern in GA3
response among these eight cultivars may be associated
with additional factors. Both Rht-B1b and Rht8c act in
semi-dominant fashion, where the genetic background
plays an important role in the extent of GA3 response of
these genes [11]. Except for the cultivars, Buckskin and
Scout 66, other cultivars with known complementary
group belong to different complementary groups indicat-
ing significant difference in their genetic background.
Although dwarf phenotype is associated with GA3 func-
tion, another plant hormone, indole-acetic acid (IAA) also
plays a role in cell elongation [33,34]. Differences in GA3
response at warm and cool temperatures among different
cultivars may be related to a temperature effect on leaf
elongation as mediated by the level of endogenous GA1.
The leaf elongation response to endogenous and exoge-
nous GA3 may be restricted by the upper limits set by the
different Rht alleles [32]. Further study will be necessary
to understand basis of such different response of these
cultivars to GA3.
4.4. Winter Rye
Winter rye as wheat may be used as a cover crop [35] as
well as used for livestock feed and pasture [36]. Like
winter wheat, winter rye cultivars may have dwarfing
characteristics related to either GA3 insensitivity and syn-
thesis mutants [11]. “Rymin” registered in 1973 [37] is a
popular rye grown throughout Nebraska. It grows to a
tall or medium height [36,37]. Therefore, GA3 was ap-

Bioassay of Winter Wheat for Gibberellic Acid Sensitivity 2021
plied to seeds of “Rymin” rye and compared to “Good-
streak” and “Wesley” wheat under warm (21˚C) daytime
conditions (Table 4). The sensitivity of “Rymin” to GA3
exposure was less than that of “Goodstreak” but greater
than that of “Wesley”. This may indicate that “Rymin”
rye responded intermediately between “Goodstreak” and
“Wesley” wheat.
4.5. Cytokinin Addition
It is well documented that cytokinins promote the growth
of lateral buds [4,38]. For cereals, this would suggest that
cytokinins could promote tiller formation. One com-
monly used cytokinin is 6BA. Since GA3 promoted stem
growth of wheat, this might be enhanced by adding 6BA
to promote tillering as well. Table 4 showed that adding
6BA to GA3 had no affect on enhancing emergence,
seedling height, tiller number, or seedling weight com-
pared to GA3 alone. 6BA at 2000 ppm had an inhibitory
effect on these growth parameter s when added to GA 3; at
500 ppm, 6BA inhibited some of these parameters.
5. Conclusion
Treating seed of winter wheat cultivars with GA3 and
planting in shallow flats placed under warm daytime
temperature (21˚C) with cold nighttime (4˚C) tempera-
tures is a good short turn-around (1 to 3 w) bioassay for
both conventional and semi-dwarf cultivars. Winter rye
can be tested in the same system. The next study will be
to test GA3-treated winter wheat under field conditions
and determine whether the growth of late-planted wheat
could be stimulated to grow to that size of normally
planted wheat.
6. Acknowledgements
We thank Les Kampbell and Eric Nielsen for their tech-
nical assistance, and the financial support by the Nebras-
ka Wheat Board and Nebraska Crop Improvement Asso-
ciation.
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