Agriculture Reference
In-Depth Information
The response of germination of onion cv. 'Hyton' to a range of constant
water potentials and temperatures was well characterized by Eqn 4.11, with
the optimized parameters given with the definition of terms under Eqns 4.11
and 4.12 (see Fig. 4.12a). Figure 4.12b shows how
b (50) is constant until T d ,
and then increases linearly. The temperature at which
b (50) reaches zero is
the ceiling temperature for germination for the 50th seed percentile - predicted
to be 33°C - quite close to the value in Fig. 4.8b from Ellis and Butcher (1988).
A hydrothermal time model (Eqn 4.10), with
b increasing above a T d of 17°C,
gave similar results to Fig. 4.12a and similar values to the constants that were
common to both models, namely T b ,
b , T d and m.
Although based on the idea that germination results from osmotic
potential decreases that drive turgor pressure to exceed a critical yield
threshold, Y in the root cell walls, Rowse and his colleagues have termed their
model the 'Virtual Osmotic Potential' (VOP) model. This is because the
assumption of a constant Y is unproven, and some evidence from other species
indicates that Y decreases to allow germination. Nevertheless, the equations
derived from this postulate provide a model that includes the effects of
temperature and water potential on both the germination rate and percentage
germination. Above T b and below T d the germination rate increases linearly
with temperature (see Fig. 4.12a). At high temperatures, the percentage
germination at any water potential decreases as temperature increases above
T c , the critical or maximum for germination, for progressively lower percentiles
of the population. Figure 4.12a shows that T c decreases as
b ,
decreases, as does
optimum temperature for germination, T opt . These effects of temperature on
T opt
b as temperature
increases above T d (see Fig. 4.12b). The lack of a sharp peak for germination
rate at T opt corresponds with some of the results of Ellis and Butcher (1988),
and has been observed in a number of other species. The VOP model also allows
progress towards germination at water potentials lower than
and T c
are indirect, and are caused by the increase of
b , as occurs
during seed priming. Hydrothermal time models require a separate treatment
for these conditions and the introduction of a further concept of 'hydrothermal
priming time' for water potentials between
b .
The VOP model can be formulated to sum progress towards germination in
the varying temperatures and water potentials that occur under field con-
ditions. Increases in the virtual osmotic pressure,
min and
πv , in the seed for successive
steps in time at varying temperature, T and water potential,
can be summed
until
b is exceeded and germination is predicted to occur. Eqn 4.13 is the
appropriate formulation (Rowse and Finch-Savage, 2003), and this is now
used in computer simulations of field emergence of onion seedlings (Rowse and
Finch-Savage, 2005):
d
πv (G)/dt = k(1 -
/
min )(T / T b - 1)[
b (G) - Y -
πv (G)]
(Eqn 4.13)
with symbols as defined above for Eqn 4.11 for
b
constrained as in Eqn
4.12.
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