Vol. 51—1997 - NorthEastern Weed Science Society

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90
Since weed germination/emergence patterns are mostly affected by temperature (l,
5, 18), he response of germination under different thermal conditions is often expressed in
terms f degree-days. Ghera and Holt (8) give a good review of models using thermal time
(degre -days) as predictor in weed science literature. Because those models include time
vector, they provide more accessible information for weed management. However, there are
some eoretical aspects ignored by this approach. First, the degree-day concept is based on
the lin arity between organism development rate (here seed germination) and temperature
(16). ck of linearity, which is normally the situation in seed germination, could result in an
undere timate near the low temperature threshold and an overestimate near the optimum
tempe ture. Secondly, extrapolating the response curve to the temperature axis to obtain
thresh ld temperature is not justified. Thirdly, those models only predict mean germination
time f m mean time versus temperature relationships. Excluding the variation in the
popul ion loses the information of emergence pattern.
The above drawbacks can be avoided by integrating both rate versus temperature and
the ge ination distribution. Wagner et al, (15) reviewed the progress in temperature versus
organi m development time and constructed a protocol for insect development time
predic ion. So far, there is only one study using the approach in perennial weed prediction
(9). T e justification of the temperature versus plant development approach could provide a
biolog cally and statistically sound method for weed emergence pattern prediction.
The objective of this study was to evaluate the suitability of the protocol of Wagner
et al, (15) for modeling annual weed germination patterns, and to highlight the procedure of
emerg nee pattern prediction for three annual weed species that are common in the Northeast
regio .
MATERIALS AND METHODS
Seeds of redroot pigweed (Amaranthus retroflexus L.), lambsquarters (Chenopodium
L.), and large crabgrass (Digitaria sanguinalis (L.) Scop.) were collected from the
cience Research Facility in Mansfield, CT and stored at room temperature for about a
.or to the study.
Five hundred homogeneous (same size and color) seeds of each weed species were
germi ated in growth chambers under constant temperatures from 10 to 340C with an
interv I of 3 0C. These temperatures were selected because they represent the temperatures in
this r gion of the country during spring and early summer. Sponge (16.0 em x 16.0 ern x 1.5
cm) as placed in a 24.5 em x 24.5 ern x 2.0 em bioassay dish. The sponge was saturated
with .I. H20 and covered by one Whatman No. 1 filter paper onto which seeds were
place . Bioassay dishes were kept in the dark in a growth chamber set at the desired
tern rature. Room light exposures during daily germination evaluations were presumed
adeq te for light requirements. Before setting up germination tests, the surface sterilization
of se d was performed by soaking with 10% commercial Clorox for ten minutes and then
rinsi with distilled water. The germinated seeds (radicle emerged at least lmm) were
coun d and removed daily. Experiments were terminated when no seed germinated in two
succ sive days after a minimum 14- day test. The experiment was repeated once.