Vol. 51—1997 - NorthEastern Weed Science Society

91
Sine germination time curves are normally skewed, median time is a better
estimator th mean. Median time to germinate was used to parameterize the poikilotherm
rate equatio (14):
r T)=
RH025.T/298.15exp[HAIR(l/298.15-1/T)]
1+exp[HLIR.(I/TL-l/T)]+exp[HHIR(l/TH-I/T)]
(I)
where r(T) i the mean germination rate at temperature T (OK),and R is the universal gas
constant (1. 87 cal degreer! mole-I), RH025 is the germination rate at 250e, assuming no
enzyme inac ivation. HA is the enthalpy of activation of the reaction that is catalyzed by a
rate-controll ng enzyme. TL is the Kelvin temperature at which the rate-controlling enzyme
is half activ and half low-temperature inactive. HL is the change of enthalpy associated with
low tempera ure inactivation of the enzyme. TH is the Kelvin temperature at which the ratecontrolling
me is half active and half high-temperature inactive. HH is the change of
enthalpy ass ciated with high-temperature inactivation of the enzyme. All of their values
were not cal ulated by measures of enzyme substrate reactions at different temperatures.
Instead, the were estimated by non-linear regression.
Cu lative germination distributions with time for each temperature was normalized
at their med an time in order to provide data suitable to obtain a temperature-independent
distribution. The distribution was fitted using the Weibull function, with the form of
F(t) = I - exp {- [(t - Y)/l1]~}
(II)
where F(t) i cumulative germination at normalized time 1, y is the lag in the start of
germination 11is the germination time constant, and ~ is a shape parameter.
RESULTS AND DISCUSSION
Te erature influenced the duration of seed germination of three weed species, with
the relation ips of median germination time and constant temperature forming a curve
(Table 1) w ich refutes the degree-day concept. Median times ranged from 17.67 d at 100e
to 1.52 d at 40C for redroot pigweed, from 14.20 d at 100e to 3.80 d at 310C for
lambsquarte s, and from 9.88 d at 130C to 2.47 d at 340C. For all species, most meahs are
greater than median, thus indicating the distributions skewed to the right. No germination
was observ at 100e for crabgrass and 340C for lambsquarters.
ary statistics for weed seed germinationtime (days) under constant temperatures.
Temperature(0C)
10 13 16 19 22 25 28 31
AMARE 17.67 9.58 5.85 4.62 3.66 2.30 2.38 2.23
17.14 9.92 6.64 5.23 4.19 2.64 2.76 2.83
34
1.52
1.66
CHEAL 14.20 11.00 9.28 7.00 6048 5.15 4.50 3.80
13.43 HAl 9041 7.14 6.76 5.56 4.73 4.34
DISGA 16.38 9.88 5043 5.22 3.63 2.79 2.66
16.37 10.26 5.83 5.58 3.95 3.16 3.01
2047
2.75