Vol. 51—1997 - NorthEastern Weed Science Society

92
The mechanistic model (equ. I) adequately described the rate versus temperature data
(R2's (lose to 0.999). Least-square parameter estimates for the model are given in Table 2.
The germination rate of pigweed shows low temperature inhibition. The rate of
lambscuarters shows no inhibition over the temperature range used in this study. This may
accour t for the reason lambsquaters usually germinates throughout the season, while
pigwesd germinates only when the weather becomes warm. Crabgrass showed both high and
low te nperature inhibitions. A TH value of 309.77 and a TL value of 285.41 indicated that
seed g rmination rate was one-half the expected rate at 36.62 and 12.26 0C, respectively.
Table 2. Parameter estimates of the poikilotherm model fitted to median germination rates (l/median
time) for three annual weed species germination held under 10 to 340C with 3 0C increment.
Species RH025 HA TH HH TL HL Description
AMAR::: 0.371 100097.67 284.64 -50255.41 Low temp. inhibition
----'l -------------------------------------------------
CHEA. 0.189 9599.87 No inhibition
----,. -------------------------------------------------
DISGA 0.279 14136.06 309.77 59272.43 285.41 -81808.67 High & low temp.
inhibitions
By normalizing the germination time, the distributions at all temperatures for each
specie generally overlapped, as indicated by their coefficient of variation at 1, 30, 70, and
100% of the cumulative germination (Table 3).
Table Coefficient of variation (%) at 1, 30, 70, and 100% of cumulative seed germination for three annual
weed s ecies,
Specie! 1 30 70 100
AMARE 37.6 7.3 10.5 45.1
CHEA 17.5 3.1 5.3 21.2
DISGA 19.2 4.9 4.2 31.4
The weighted mean times when 1,5,10,15, ... ,90,95, and 100% of seeds
germinated were used to identify a single temperature-independent distribution of
norm lized germination times for each species. The cumulative Weibull distribution (equ, II)
described the data well. Parameter estimates are given in Table 4. The onset of germination
occurred sooner for pigweed than for lambsquarters. Crabgrass was the slowest. The
differ enceamong 11's indicated that the time needed to approach asymptote was shortest for
lamb quarters, followed by crabgrass and pigweed. It should be noted that 11represents the
progr ssion of germination for the population, and doesn't represent a temperaturedeper
dent rate. Lower values of 13represents distributions skewed toward longer
germ nation time, signifying the seed population was less homogeneous. The results
indic ted that lambsquarters was more homogeneous than pigweed and crabgrass. It is
generally accepted that lambsquarters seed population is heterogeneous. But in this study, the
seeds were selected for uniform size and color.
Table t. Parameter estimates of a cumulative Weibull distribution fitted to a standard normalized distribution
for se d germination of 4 annual weed species.
Species
y
AMA rn 0.367
1.390
0.851
0.999
CHEAL 0.463
2.237
0.646
0.999
DISG f\ 0.582
1.589
0.536
0.999