Agriculture Reference
In-Depth Information
The first stage in the production of a good onion crop is to sow viable seed at an
appropriate density and achieve a high level and uniformity of seed
germination and seedling emergence. Many of the processes fundamental to
this aim have been studied in onions and many of the principles established
apply to other crops, particularly the small-seeded vegetable crops.
Seed storage and viability
The viability of onion seeds can decline rapidly if they are stored in warm, moist
conditions, a feature common to most orthodox seeds (Ellis, 1991), and which
presents difficulties for maintaining seed quality in tropical areas. Orthodox
seeds, like the alliums and most crop species, can tolerate desiccation, and
storage longevity is increased thereby. At sufficiently low moisture contents
such seeds can tolerate sub-zero temperatures and this can further increase
longevity. In view of the importance of the long-term storage of viable seeds for
genetic conservation (Astley, 1990), equations for predicting the effects of
storage conditions on the loss of seed viability have been developed and applied
to onions among other crops (Ellis and Roberts, 1980a, 1981; Ellis, 1991).
If the percentage of seeds that germinate is plotted against time of storage in
constant, controlled conditions, a sigmoidal curve that conforms to a negative
cumulative normal distribution results (Ellis, 1991; Fig. 4.5a). This implies that
different seeds die at different times and that the frequency distribution of seed
deaths is normal. If the percentage germination scale is transformed to a
probability scale known as probits, where percentage germination is in units of
standard deviation of the distribution of seed longevity, the relation between
percentage germination and time becomes linear (see Fig. 4.5b).
The standard deviation of the frequency distribution of seed death in time
is also a time and hence a period of longevity and, from the properties of the
normal distribution, it represents the time taken for longevity to fall from 97.7
to 84.1% or from 84.1 to 50%. The straight line relationship between probit
percentage viability and time can be represented as:
V = K
i
p/
(Eqn 4.3)
where
V = probit percentage viability
p = days in storage
= the standard deviation of the distribution of time until seed death in the
storage environment
K
i
= a constant for the seed-lot representing the initial viability of the seed-
lot at the start of storage.
