Agriculture Reference
In-Depth Information
This relationship has been combined with Eqn 4.3 to give an equation that
that predicts the percentage germinable (i.e. viable) seeds in a seed-lot after
storage at known moisture content, m and at temperature T (Ellis and Roberts,
1980a):
p/ 10
(6.975
3.470log
10
m
0.04T
0.000428T.T)
V = K
i
(Eqn 4.5)
where
V = probit percentage of germinable seeds
K
i
= seed-lot constant
p = period of storage, days
m and T as for Eqn 4.4.
The constants 6.975 (K
E
) and 3.470 (C
W
) are characteristic of the species,
i.e. onion. The constants 0.04 (C
H
) and 0.000428 (C
Q
) have been found similar
for all species investigated (Ellis, 1991). K
i
, the seed-lot constant, varies
depending on the seeds' environment before entering controlled storage. K
i
is
the probit percentage viability at the beginning of storage; it is best estimated
by an initial rapid ageing test of the seeds, which involves keeping them moist
at warm temperatures and measuring their decline in viability. Simple
estimates of initial percentage viability from germination tests are subject to
large sampling errors, and small differences have a large effect on K
i
(Ellis and
Roberts, 1980b).
Germination and seedling emergence
The fact that seeds emerge more quickly from moist, warm soil rather than
from cold, dry soils must have been known to the earliest agriculturalists.
However, it is only relatively recently that scientists have developed a
quantitative understanding of the relationships between soil temperature and
moisture and the germination and emergence of seedlings. The effects of soil
strength (mechanical impedance) on emergence and germination are also
beginning to be understood quantitatively. Onions and leeks have frequently
been used in the studies, and their responses typify patterns common to most
crop species while having quantitative responses particular to them. These can
be compared and contrasted with other crops and they point to particularities
of response that can give useful insights for establishing these crops in the field.
Two aspects of germination are important - first, the percentage of seed to
germinate and, secondly, the rate of germination. The rate of germination can be
quantified as the reciprocal of the time for a certain percentage, usually 50%, of
the total number of seeds to germinate (Hegarty, 1972). Figure 4.7 shows the
response of both rate and percentage germination of onion seeds to temperature
in the absence of moisture stress. Above 25°C both rate and percentage germi-
nation decline, but between 5 and 25°C the rate of germination increases almost
linearly.
