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Weed Science, 54:316–320. 2006

Influence of environmental factors on slender amaranth

(Amaranthus viridis) germination

Walter 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; johnpwilcut@ncsu.edu

Germination 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 amaranth

emergence 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, weed

biology.

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 research

on 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, but

the 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 data

from 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, personal

communication; 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

separator1 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 paper2 and 8 ml of deionized water. The flasks were

arranged 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 germination

Thomas et al.: Germination of slender amaranth x 317

counts 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

paper3 and 10 ml of deionized water. Four temperature

regimes 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) have

shown 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 were

prepared by dissolving 0, 154, 191, 230, 297, or 350 g of

polyethylene glycol4 (PEG) in 1 L of deionized water (Michel

1983). 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 night

temperature of 23 6 5 C. Natural light supplemented with

fluorescent lamps at a light intensity of 300 6 20 mE m22

s21 were used to extend the day length to 14 h in glasshouse

studies 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 the

time-scale constant or lag to onset of germination, and K is

the rate of increase. When a nonlinear equation was fit to

the data, an approximate R2 value was obtained by subtracting

the 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.