Germination and post-germination growth to carrot seedling emergence: predictive threshold models and sources of variation between sowing occasions

• Published in: The New phytologist Vol. 139; no. 3; pp. 505 - 516
• Main Authors: FINCH-SAVAGE, W. E., STECKEL, J. R. A., PHELPS, K.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 01-07-1998; Blackwell
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Carrot (Daucus carota L.); Carrots; crop establishment; Economic plant physiology; Embryo development. Germination; Fundamental and applied biological sciences. Psychology; Germination; Growth and development; hydrothermal time; Modeling; Seed germination; Seedbeds; Seedling emergence; Seeds; Soil water; Sowing; thermal time; threshold models; Water temperature; Carrot (Daucus carota L.); germination; crop establishment; seedling emergence; threshold models; thermal time; hydrothermal time; Temperature; Daucus carota; Growth; Germination; Plant juvenile growth stage; Environmental factor; Plant emergence; Forecast model; Water potential; Seed bed; Vegetable crop; Dicotyledones; Angiospermae; Spermatophyta; Umbelliferae
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1046/j.1469-8137.1998.00208.x

Cumulative germination curves were recorded for carrot
(Daucus carota L.) seeds at a range of constant
temperatures (T) and water potentials (Ψ) in the laboratory
and under variable soil conditions in 15 seed-bed
environments in the field. A single base temperature (Tb),
a distribution of base water potentials (Ψb(G))
for
percentiles (G) of the population and the hydrothermal time
constant (Hg) were determined from laboratory data.
Although less effective at low Ψ, it was possible, using
these germination parameters, to satisfactorily describe the
effect of T and Ψ on germination rates under constant conditions
according to the threshold models of thermal
and hydrothermal time. These models were applied to field data
with the condition that the germination process
ceased if T<Tb for thermal time and
additionally Ψ<Ψb(G) for hydrothermal time. Neither model accurately predicted germination patterns in
the field. However, the pattern of germination was
adequately described in most situations by a modified threshold
model in which the predicted progress towards
germination was unaffected by soil Ψ, provided it remained above
Ψb(G), and was therefore more rapid under
variable seed bed conditions than hydrothermal time. In this
modified threshold model, the condition Ψ<Ψb(G)
had to be fulfilled at the initiation of radicle extension
before germination occurred. This result implies that the
initiation of radicle growth operates as a moisture-sensitive
step that can determine germination and seedling
emergence timing under variable soil-moisture conditions. Seedling emergence was also recorded in the field and used
to determine, separately, the impact of germination
and post-germination growth on the variation in seedling emergence
patterns. The analysis suggests that delays
in seedling emergence occur largely in the germination phase, but
that seedling losses and variation in the spread
of seedling emergence times within the population occur largely
during the post-germination growth phase.