Agriculture Reference
In-Depth Information
Above 3.5% of seed moisture, the standard deviation,
, i.e. longevity,
declines as seed moisture content increases, such that the log
declines linearly
with increase in log moisture content (see Fig. 4.6). Below 3.5% moisture content
there is no further increase in longevity with drying (see Fig. 4.6). Storage at or
below this moisture content, known as 'ultra-dry' seed storage, results in
maximal longevity (Ellis
et al.
, 1996). At 3.5% moisture content the onion seeds
are in equilibrium, with a relative humidity of 10% in the surrounding
atmosphere equivalent to a water potential of -311 Pa, a value around which
many crop seeds reach their maximum longevity and which corresponds to the
remaining water in the seeds being in a strongly bound state (Ellis
et al.
, 1990).
Seed longevity decreases at an accelerating rate as temperature increases.
Over the range
20 to 90°C a wide range of species show the same
relationship, with log
10
(i.e. longevity) decreasing as a quadratic function of
temperature. Effects of moisture content and temperature on longevity of
onion seeds have been quantified by Equation 4.4 (Ellis and Roberts, 1980a):
0.000428T
2
Log
10
= 6.975
3.470log
10
m
0.04T
(Eqn 4.4)
K
E
C
w
C
H
C
Q
where
= standard deviation of the distribution of seed longevity (days)
m = seed moisture content as a percentage of moist weight
T = temperature °C.
Fig. 4.6.
The relation between seed moisture content of onion seeds, shown as the
percentage of fresh weight on a logarithmic scale, and the standard deviation of
seed deaths in time,
(a measure of seed longevity expressed as days on a
logarithmic scale), after hermetic storage at 65°C. The critical moisture content is
where the lines intersect (from Ellis
et al
., 1990. Courtesy of
Annals of Botany
).
