Agriculture Reference
In-Depth Information
Between the base temperature, T
b
, and T
d
, a temperature somewhat below
the optimum for germination (see Fig. 4.12b),
b
is constant for a particular
percentile G. Above T
d
,
b
increases linearly with temperature with slope m (=
0.051 MPa°/C) (see Fig. 4.12b).
The above conditions are expressed as:
T
b
< T
T
d
;
b
(G,T) =
b
(G)
d
(Eqn 4.12)
T
b
< T
T
c
;
b
(G,T) =
b
(G)
d
+ m(T - T
d
)
Here, T
c
is the ceiling or maximum temperature for germination of the
percentile concerned.
The spread of germination for different percentiles of the population is due
to variation in
b
(G), the base water potential for germination for each
percentile. In the model of Rowse and Finch-Savage (2003), the distribution of
b
(G) is assumed normal and is characterized by the standard deviation
b
(MPa), the optimised value of which was 0.21MPa. Although
b
increases
above T
d
,
b
is assumed not to change.
Fig. 4.12.
Combined temperature and water potential effects on onion seed
germination. (a) The effect of temperature and water potential on the germination rate
(reciprocal of the time to 50% germination, days) of onion seeds. Water potentials 0
(
). The lines are fitted according
to Eqns 4.11 and 4.12. (b) The effect of temperature on the base water potential,
b
(50), below which germination will not occur for the 50th percentile in onion seed
germination. The values of
●
), -0.18 (
), -0.28 (
), -0.39 (
▫
) and -0.51 MPa (
b
(50) were calculated according to the 'virtual osmotic
pressure' VOP model of Eqn 4.11. Vertical lines are the standard deviation of
b
(50)
(from Rowse and Finch-Savage, 2003. Courtesy of
New Phytologist
©
, 2003).
