Agriculture Reference
In-Depth Information
Effects of water stress on germination
The responses of germination and emergence to temperature described above
derive from studies of seeds on moist filter paper or in moist soil. Under
conditions of water stress, rates and percentages of germination decrease as
stress increases (see Fig. 4.11a, b; Rowse and Finch-Savage, 2003).
Responses of germination rate to temperature and water potential have
been modelled by extending the concept of thermal time for germination to
hydrothermal time for germination. Here, the effectiveness of a temperature for
advancing germination in the suboptimal range depends on the water
potential around the seed, as defined by its level above a base water potential
below which germination cannot occur (Rowse and Finch-Savage, 2003),
thus:
HT
= (
b
(G))(T
T
b
)t(G)
(Eqn 4.10)
HT
= hydrothermal time for germination, MPa°Cd
= water potential around seed, MPa
b
(G) = base water potential below which the germination cannot occur
for the Gth percentile of the population, MPa
T = temperature, °C
T
b
= base temperature for germination, °C
t(G) = time for Gth percentile to germinate (days)
HT
and T
b
are assumed constants, while
b
(G) varies with a percentile (G)
and this determines the germination time for that percentile. If the frequency
Fig. 4.11.
Effects of water stress on onion seed germination. (a) Cumulative
germination with time at 15°C for onion seeds subjected to water potentials of 0
(
). Symbols are
experimental data and lines are calculated from a 'virtual osmotic potential' (VOP)
model. (b) Germination rates at 15°C (reciprocal of germination time) for the 20th
(
), -0.175 (
●
), -0.28 (
▫
), -0.39 (
), -0.51 (
) and -0.67 MPa (
) germination percentiles of onion seeds as determined by
the surrounding water potential. Lines are calculated from the VOP model (from
Rowse
et al
., 1999. Courtesy of
New Phytologist
©
, 1999).
), 50th (
●
) and 80th (
▫
