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Weed Science,
54:316–320. 2006Influence of environmental factors on slender amaranth
(
Amaranthus viridis) germinationWalter E. Thomas
Ian C. Burke
Janet F. Spears
Crop Science Department, Box 7620, North
Carolina State University, Raleigh, NC 27695-7620
John W. Wilcut
Corresponding author. Crop Science Department,
Box 7620, North Carolina State University, Raleigh,
NC 27695-7620; john
pwilcut@ncsu.eduGermination response of slender amaranth to temperature, solution pH, moisture
stress, and depth of emergence was evaluated under controlled environmental conditions.
Results indicated that 30 C was the optimum constant temperature for
germination. Germination of slender amaranth seed at 21 d was similar, with 35/
25, 35/20, 30/25, and 30/20 alternating temperature regimes. As temperatures in
alternating regimes increased, time to onset of germination decreased and rate of
germination increased. Slender amaranth germination was greater with acidic than
with basic pH conditions. Germination declined with increasing water stress and
was completely inhibited at water potentials below
20.6 MPa. Slender amaranthemergence was greatest at depths of 0.5 to 2 cm, but some seeds emerged from as
deep as 6 cm. Information gained in this study will contribute to an integrated
control program for slender amaranth.
Nomenclature:
Slender amaranth, Amaranthus viridis L. AMAVI.Key words:
Light, temperature, solution pH, moisture stress, burial depth, weedbiology.
Slender amaranth is widely distributed in over 80 countries
and most prevalent in plantation agriculture. It is becoming
more evident in the southeastern United States, including
North Carolina, Tennessee, and Mississippi, but is
not currently found in the central and western regions of
the United States or Canada (Holm et al. 1997). More recently,
data have indicated that slender amaranth can be
found as far west as Arizona and as far north as Michigan,
New Jersey, and New York (USDA, NRCS 2004).
Kigel (1994) has compiled much of the available information
on
Amaranthus seed germination, including researchon effects of light (Gallagher and Cardina 1998; Gutterman
et al. 1992; Oladiran and Mumford 1985), temperature
(Baskin and Baskin 1977; Ghorbani et al. 1999; Oladiran
and Mumford 1985; Weaver 1984; Weaver and Thomas
1986), osmotic potential and salinity (Ghorbani et al.
1999), hormones (Holm and Miller 1972; Kepczynski et al.
1996; Weaver 1984), and burial depth (Ghorbani et al.
1999; Oryokot et al. 1997; Webb et al. 1987). Kigel (1994)
indicated that most
Amaranthus spp. respond to light, butthe response level varies among
Amaranthus species. Furthermore,alternating temperatures and light have been reported
to provide optimal germination conditions.
Even though extensive germination data on many of the
Amaranthus
spp. are widely available, observational datafrom several southeastern states including Mississippi, North
Carolina, and Tennessee suggest differences in survival of
slender amaranth compared to other commonly occurring
Amaranthus
spp. (R. Hayes, University of Tennessee, personalcommunication; D. Reynolds, Mississippi State University,
personal communication). These observed differences
may be due to several factors, including differences in
germination and emergence requirements. Therefore to optimize
weed control and maximize the efficiency of management
tactics, biological and ecological information, specifically
germination requirements, should be examined. The
ives of these studies were to examine the effects of
constant and alternating temperature regimes, solution pH,
moisture stress, and burial depth on slender amaranth seed
germination.
Materials and Methods
Slender amaranth seed was harvested from fields near
Clayton, NC in August, 2000. Seed was allowed to dry for
2 weeks at 25 C and then stored at 5 C until use in experiments.
Seed was sieved to remove any extraneous plant or
floral material. The sieved seeds were divided in an air column
separator
1 and separated into light and heavy fractions.The heavy fraction, the majority of which were fully developed
seed, was used in germination and emergence experiments.
Seed was tested for viability with the use of 1%
tetrazolium chloride solution before each trial (Peters 2000).
Light was provided for 8 h to coincide with the length
of the high-temperature component of the temperature regime
for all experiments conducted in growth chambers.
Observations were made during the 8-h light period.
Temperature
The first ive was to find the optimum germination
temperature. Twenty slender amaranth seeds were evenly
spaced in 50-ml Erlenmeyer flasks containing three pieces
of filter paper
2 and 8 ml of deionized water. The flasks werearranged on a gradient table (Larson 1971) in six lanes corresponding
to a constant temperature of 15, 20, 25, 30, 35,
and 40 C, with six flasks per lane. These experiments precluded
randomization, as the zones of temperature were
fixed in position (Larson 1971). Each flask was representative
of one replication. Flasks were sealed with parafilm to
hold in moisture. Light was provided by fluorescent overhead
bulbs set for an 8-h–light, 16-h–dark regime with a
light intensity of 30.2
mmol m22 s21. Daily germinationThomas et al.: Germination of slender amaranth
x 317counts were made for the first 7 d, then every 3 d until no
seed germination was observed for seven continuous days.
Each seedling was removed when a visible radicle could be
discerned (Baskin and Baskin 1998). The experiment was
conducted twice and the data were combined.
Additional experiments were conducted in growth chambers
to determine the germination response to alternating
temperature. A randomized complete block design with four
replications was used. Each replication was arranged on a
different shelf within the respective germination chamber.
Blocks were considered study replication over time. Fifty
slender amaranth seed were evenly spaced in 110-mm-diameter
by 20-mm petri dishes containing two pieces of germination
paper
3 and 10 ml of deionized water. Four temperatureregimes were selected to reflect typical seasonal variation
in North Carolina. The regimes, 25/10, 30/15, 30/
20, and 35/20 C, correspond to mean daily high and low
temperatures for the months of May, June, July, and August,
respectively, in Goldsboro, NC (Owenby and Ezell 1992).
These regimes also correspond to a range of effective day
and night temperatures for June, July, and August for diverse
locations throughout the United Sates (Patterson 1990).
The high-temperature component of the regime was maintained
for 8 h. Light was provided by fluorescent overhead
bulbs set for an 8-h–light/16-h–dark regime with a light
intensity of 34.9
mmol m22 s21. Burke et al. (2003) haveshown the light quality for the germination chambers. Daily
germination counts were made for 7 d, then every 3 d until
no seed germination was observed for 7 d. Each seedling
was removed upon germination as previously mentioned.
The experiment was conducted twice and the data combined
for analysis.
Solution pH
An experiment with a randomized complete block design
with four replications of treatments was used to examine the
effects of solution pH on slender amaranth germination.
Each replication was arranged on a different shelf within the
respective germination chamber. Blocks were considered
study replication over time. Buffered pH solutions were prepared
according to the method described by Gortner
(1949), with the use of potassium hydrogen phthalate in
combination with either 0.1 M HCl or 0.1 M NaOH to
obtain solution pH levels of 3, 4, 5, and 6. A 25 mM
sodium tetraborate decahydrate solution was used in combination
with 0.1 M HCl or 0.1 M NaOH to prepare solutions
with pH levels of 7, 8, or 9. Fifty slender amaranth
seeds were placed in petri dishes containing 10 ml of the
appropriate pH solution, and the petri dishes were placed
in 25/10, 30/15, 30/20, and 35/20 C germination chambers.
Germination was determined as previously mentioned.
The experiment was conducted twice, and the data were
combined for analysis.
Water Potential
An experiment with a randomized complete block design
and four replications of treatments was used to examine the
effects of water potential on slender amaranth germination.
Each replication was arranged on a different shelf within the
respective germination chamber. Blocks were considered
study replication over time. Solutions with osmotic potentials
of 0.0,
20.3, 20.4, 20.6, 20.9, and 21.2 MPa wereprepared by dissolving 0, 154, 191, 230, 297, or 350 g of
polyethylene glycol
4 (PEG) in 1 L of deionized water (Michel1983). Fifty slender amaranth seeds were placed in petri
dishes containing 10 ml of appropriate PEG solution, and
the petri dishes were placed in 25/10, 30/15, 30/20, and
35/20 C germination chambers. Germination was determined
as previously mentioned. The experiment was conducted
twice, and data were combined for analysis.
Burial Depth
The experimental design was a randomized complete
block with treatments replicated four times in a glasshouse
at an average day temperature of 33
6 5 C and a nighttemperature of 23
6 5 C. Natural light supplemented withfluorescent lamps at a light intensity of 300
6 20 mE m22s
21 were used to extend the day length to 14 h in glasshousestudies to simulate field conditions.
A Norfolk loamy sand soil (fine loamy, siliceous, thermic,
Typic Paleudults), a typical coastal plain soil in North Carolina,
was used in burial studies. Twenty slender amaranth
seeds were placed on the soil surface or covered to depths
of 0.5, 1.0, 2.0, 4.0, and 6.0 cm with the same soil. Pots
were subirrigated initially to field capacity, and then surface
irrigated daily to field capacity. Emergence counts were recorded
daily for the first 7 d, then every 3 d thereafter.
Plants were considered emerged when a cotyledon could be
visibly discerned. The experiment was conducted three
times, and data were combined for analysis.
Statistical Analysis
Data variance was visually inspected by plotting residuals
to confirm homogeneity of variance before statistical analysis.
Both nontransformed and arcsine-transformed data
were examined, and transformation did not improve homogeneity.
ANOVA was therefore performed on nontransformed
percent germination. Linear, quadratic, and higherorder
polynomial effects of percent germination over time
were tested by partitioning sums of squares (Draper and
Smith 1981). Regression analysis was performed when indicated
by ANOVA.
ANOVA indicated higher-order polynomial effects for
germination resulting from alternating temperature regimes,
solution pH treatments, and moisture stress treatments.
Thus, the germination response for each treatment was
modeled with the use of the logistic function:
y
5 M(1 1 exp[2 K(t 2 L)])21 [1]where
y is the cumulative percentage germination at time t,M
is the asymptote or theoretical maximum for y, L is thetime-scale constant or lag to onset of germination, and
K isthe rate of increase. When a nonlinear equation was fit to
the data, an approximate
R2 value was obtained by subtractingthe ratio of the residual sum of squares to the corrected
total sum of squares from one (Askew and Wilcut 2001;
Draper and Smith 1981). Following nonlinear regression,
ANOVA was conducted on parameter estimates.