Agriculture Reference
In-Depth Information
distribution of
b
(G) is known, then the distribution of t(G) can be predicted. If
the distribution of
b
),
then probit analysis can be used to fit values to the constants in Eqn 4.10 using
data on rates of germination in different temperatures, T and water potentials,
b
(G) is normal, defined by a standard deviation of
b
(
b
(G) increases with temperature, as has been found true of some species
(Rowse and Finch-Savage, 2003), then increases in germination rate as a
result of increased temperature are offset by the increasing
. If
b
(G) and
germination rate rises to a maximum at an optimum temperature and then
decreases, with further increase in temperature (Rowse and Finch-Savage,
2003; Finch-Savage, 2005).
An alternative to hydrothermal time models for predicting the response of
germination to temperature and water potential has been developed (Rowse
et
al.
, 1999; Rowse and Finch-Savage, 2003). This approach is based on the idea
that, for visible germination to occur, metabolic activity in the seed must
generate sufficient osmotic pressure within the cells of the radical for the turgor
pressure of these cells to exceed the yield threshold, Y, of the cell walls, thereby
allowing radical expansion and emergence from the seedcoat. In this model it
is assumed that progress in generating osmotic potential occurs if the osmotic
potential around the seeds exceeds a minimum,
min
. The value of
min
is below
the minimum water potential necessary for germination,
b
. The fact that
progress towards germination occurs at
b
in a seed priming (see
Chapter 6, 'The Priming of Seeds'), means that this model includes the full
range of water potentials at which seeds are physiologically active. Based on
these ideas, Rowse and Finch-Savage (2003) derived Eqn 4.11 to predict the
time to germination of percentile G of a seed population:
levels below
t(G) = 1/[k(1 -
/
min
)(T / T
b
- 1)] ln [(
b
(G,T) - Y) / (
b
(Eqn 4.11)
(G,T) -
)]
The constants and variables are defined below and the numbers in
brackets following the definitions are the best fitting values for onion (Rowse
and Finch-Savage, 2003):
t(G) = time for percentile G of the seed population to germinate, days
T = temperature around the seed, °C
T
b
= base temperature for germination, °C (= 1.2)
= water potential around the seed, MPa
b
(G,T) = base water potential above which the Gth percentile of the
population can germinate at temperature T°C MPa (= -0.87 for
b
(50) when T
< T
d
, see Eqn 4.12, below)
min
= the minimum water potential for metabolic advancement of the
seeds towards germination, MPa (= -4.18)
Y = yield threshold of radical (young root) cell walls that opposes cell
expansion by turgor pressure, MPa (= 0.71)
k = rate constant relating germination rate to temperature above T
b
, the
base temperature when there is no water stress (i.e. when
= 0)/day (= 0.014)
