Sweet corn is a herbaceous, annual, warm-season
monocot with an unknown place of origin. Each undeveloped kernel is
a single-seeded, dry indehiscent fruit composed of a small embryo
and large endosperm. One of the features distinguishing sweet corn
from field dent corn is the presence of the recessive form of the
sugary gene su. Sweet corn requires temperatures greater than or equal
to 50 °F (10 °C) for seed germination and proper seedling
growth and development. In the early 1950’s, optimal soil temperatures
for sweet corn germination were established ranging from 60 to 90
°F (15.5 to 35 °C) with 95 F considered optimal and 50 °F
and 105 °F (40.5 °C) considered the minimum and maximum temperature
extremes, respectively. Since then, numerous hybrid cultivars have
been released along with two additional genes which have been incorporated
into commercial cultivars, shrunken-2 (sh2) and sugary enhancer (se),
with increased sugar levels and extended shelf life. Germination of
these endosperm types can be more erratic and these seeds are difficult
to plant using conventional equipment. Although these higher sugar
sweet corn phenotypes are favored among consumers, sweet corn growers
have found that cold soils slow emergence, and reduce stands, earliness,
and profitability. Four kernel properties thought to be associated
with poor stands are 1) insufficient seedling energy reserves due
to reduced starch concentration; 2) membrane damage associated with
high osmotic potential from elevated kernel sugar concentration; 3)
excessively rapid influx of water during the germination process due
to pericarp and 4) susceptibility of the seeds to infection by fungal
seed rot pathogens associated with delayed seed germination. Seed
companies have suggested that sh2 and se phenotypes should be planted
shallower and at soil temperatures higher [at least 10 °F (5.6
°C)] than required for the normal (su) phenotype.
It is important for corn growers to understand the relationships among
temperature, germination and vigor to plan the schedule planting dates
that will not significantly reduce plant stand and plant performance.
Seed vigor is defined as those properties that determine the potential
for rapid, uniform emergence, and the development of normal seedlings
under a wide range of field conditions [Association of Official Seed
Analysts (AOSA), 1983]. Seed companies are required by law to provide
final germination percentages on seed containers, but they do not
need to provide any indication of seed vigor. At present, a lot of
sweet corn seed can only be sold in interstate commerce if it has
a minimal seed germination of 75% or greater. Therefore, in our study,
we defined minimum standard as our lowest level of acceptability for
sweet corn germination in thermo-gradient testing
AOSA (1998) has stated that sweet corn germination should be tested
as 77 °F (25 °C) with first germination counts taken after
4d and final counts taken on the 7th d. AOSA also gives the lower
and upper temperature limits for sweet corn germination from 68 °F
to 86 °F; however, soil temperatures in early spring may be much
lower than 68 °F. In order to rank germination of sweet corn seed
lots at lower temperatures in early spring, more work is needed to
detemine if currently grown sweet corn cultivars behave differently
from previously published standards. A thermo-gradient test could
be used to predict final germination percentages and also seed vigor
under a wide range of temperatures. These results could provide an
indication of how new cultivars of se, sh2, and su sweet corn phenotypes
respond to varying temperatures, especially cooler temperatures, over
a given time period and to provide growers with general scheduling
tools for field planting in cold soils for acceptable seedling emergence.
The objectives of this research were 1) to determine the minimum temperatures
to germinate to 75%, (the minimum germination percent for interstate
commerce) for 27 new sweet corn su (sugary), se (sugary enhanced),
and sh2 (super sweet) cultivars; 2) to determine vigor differences
among the most promising se, su, and sh2 cultivars within each phenotype
for cold tolerance and vigor and possible use in wearly spring plantings
in cold soils. Seeds of each cultivar (Table 1) were placed along
a temperature gradient on a thermo-gradient table, type 5001 (Seed
Processing Holland, Enkhuizen, The Netherlands), and allowed to germinate
over a 7-day period. The gradient treatments were[+ or - 2 F (1.1
C)] 52, 56, 60, 64, 68, 72, 76, 80, 84, and 86 °F (11.1, 13.3,
15.6, 17.8, 20.0, 22.2, 24.4, 26.7, 28.9, and 30.0 °C). Germination
data from thermo-gradient testing were used to determine the minimum
temperatures and time required for su, se, and sh2 cultivars to germinate
at greater than or equal to 75%, defined as minimum acceptable germination
percentage (MAGP); and the minimum temperature to reach the maximum
germination rate (MGR) for a cultivar, defined as the ability to germinate
to MAGP at the same rate equally at low and high temperatures. Generally,
su phenotypes germinated to MAGP within 4 days, with sh2 requiring
6 days, but with se requiring 5 days. We found that within each phenotype,
however, cultivars reacted uniquely to temperature. The most vigorous
and cold tolerant su cultivars were ‘NK 199’ and ‘Merit’
which germinated to MAGP at 52 °F with ‘NK 199’ more
vigorous than ‘Merit’. The su cultivar ‘Sweet G-90
was vigorous at warm temperatures, but the least cold tolerant and
desirable for planting under cold conditions. Within the se cultivars,
‘Precious Gem’, ‘July Gold’, and ‘Imaculata’
germinated to MAGP at 52 F with ‘Precious Gem’ requiring
6 days and ‘July Gold’ and ‘Imaculata’ requiring
7 days. ‘Accord’ was the least cold tolerant se cultivar,
requiring at least 60 F for MAGP with a slow MGR, even at warm temperatures.
None of the sh2 cultivars reached MAGP within 7 days at 52 °F,
as was also observed for certain su and se cultivars.
Table 1. Seed specifications for 27 cultivars
of sweet corn representing three kernel phenotypes
Cultivar |
Pheno-
type |
Color |
Lot No. |
Source |
Germination
(%)z |
Seeds
(no./lb)y |
Country
Gentlemen |
su |
White |
113 (P2) |
Souther States Coop. (Richmond, Va) |
97 |
3314 |
Silver Queen |
su |
White |
OC 5353 LF |
Rupp Seeds, Inc. (Wauseon, Ohio) |
95 |
2270 |
Trucker’s
Favorite |
su |
White |
101 (P1) |
Souther States Coop. (Richmond, Va) |
95 |
1362 |
Bi-Queen |
su |
Bi-color |
NC 4202 MR |
Rupp Seeds, Inc. (Wauseon, Ohio) |
90 |
2406 |
Butter & Sugar |
su |
Bi-color |
69708 R |
Rupp Seeds, Inc. (Wauseon, Ohio) |
90 |
1816 |
Sweet G-90 |
su |
Bi-color |
68624 ELF |
Jeffery’s (Goldsboro, N.C.) |
90 |
1498 |
Jubilee |
su |
Yellow |
NC7103 LR |
Rupp Seeds, Inc. (Wauseon, Ohio) |
95 |
1861 |
Merit |
su |
Yellow |
14007 MR |
Rupp Seeds, Inc. (Wauseon, Ohio) |
95 |
2542 |
NK 199 |
su |
Yellow |
30403804 TH |
Rupp Seeds, Inc. (Wauseon, Ohio) |
90 |
1725 |
Avalanche |
se |
White |
19020 MR |
Mesa Maize, Inc. (Olathe, Colo.) |
92 |
3133 |
Imaculata |
se |
White |
21028 ELR |
Mesa Maize, Inc. (Olathe, Colo.) |
90 |
2633 |
Sparkle |
se |
White |
19056 ELF |
Mesa Maize, Inc. (Olathe, Colo.) |
95 |
2724 |
Accord |
se |
Bi-color |
22053 LR |
Mesa Maize, Inc. (Olathe, Colo.) |
95 |
2542
|
Precious Gem |
se |
Bi-color |
2207 ELF |
Mesa Maize, Inc. (Olathe, Colo.) |
92 |
3360 |
Yankee Gem |
se |
Bi-color |
22031 ELR |
Mesa Maize, Inc. (Olathe, Colo.) |
95
|
2179 |
July Gold |
se |
Yellow |
21034 ELF |
Mesa Maize, Inc. (Olathe, Colo.) |
92 |
2588 |
Merlin |
se |
Yellow |
22023 LR |
Mesa Maize, Inc. (Olathe, Colo.) |
95 |
2588 |
Tuxedo |
se |
Yellow |
21005 ELR |
Mesa Maize, Inc. (Olathe, Colo.) |
90 |
2724 |
Ice Queen |
sh2 |
White |
133827019 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
4268 |
Silver Dollar |
sh2 |
White |
141370009 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
2724 |
Snow White |
sh2 |
White |
143460006 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
3632 |
SCH 55146 |
sh2 |
Bi-color |
40905 |
Harris Moran Seed Co. (Modesto, Calif.) |
90 |
3541 |
Summer Sweet 8102 |
sh2 |
Bi-color |
69534 N |
Abbott & Cobb (Feasterville, Pa.) |
94 |
3087 |
Twin Star |
sh2 |
Bi-color |
143124009 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
4449 |
Bandit |
sh2 |
Yellow |
143141017 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
3632 |
Day Star |
sh2 |
Yellow |
131805014 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
3268 |
Morning Star |
sh2 |
Yellow |
133927032 |
Harris Moran Seed Co. (Modesto, Calif.) |
95 |
3859 |
z- Percent germination given by the various seed sources given on
the seed packet as required by law under the procedures outlined by
the Association of Official Seed Analysts.
y-1000 seeds/ lb = 2204.6 seeds/ kg.
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