Agriculture Reference
In-Depth Information
vigorous seed following priming (Bray
et al.
, 1989). There is evidence, therefore,
that priming allows both repair processes (i.e. DNA repair) and preparation for
accelerated DNA, RNA and protein synthesis in leek seeds when they are set to
germinate.
In the years 1970-1990 there was much experimentation with sowing
pre-germinated onion and leek seed. Seeds were germinated in bulk in aerated
water and sown with pre-emerged radicles in a gel carrier; this was termed 'fluid
drilling' (Finch-Savage, 1984; Gray, 1991). Fluid drilling requires a special seed
drill, whereas drum-primed seed can be sown using the conventional drills used
for untreated seed. Fluid drilling also involves the transport and storage of seeds
in a vulnerable germinated state and the preparation of bulky gels. The potential
benefits were largely achieved using drum-primed seeds, and fluid drilling was
abandoned commercially.
Transplanting
Despite the development and improvement of direct sowing, transplanting is
still widely used in both temperate and tropical regions. Direct sowing will
probably always be subject to a greater risk of failing to establish a uniform crop
at the desired spacing than transplanting. In cool, temperate areas the growing
season for bulb onions can be lengthened by raising seedlings for transplanting
under protection. Trial crops grown from transplants in Lincolnshire in
eastern UK consistently produced bulb yields of 45 t/ha and were 2 weeks
earlier maturing than direct-drilled crops, which yielded 30 t/ha on average. In
Ontario, Canada it is possible to produce the large, mild, 'Sweet Spanish' type of
onion using transplants. In the tropics, small-scale growers can maintain good
weed control, a high level of soil fertility using organic manure, well-controlled
irrigation and, if necessary, shading to prevent excessive soil temperatures on
an intensively managed nursery bed for onion transplants, making this the
most practical way of starting the crop (Currah and Proctor, 1990).
In parallel with the developments in direct sowing, new techniques of
growing from transplants have also evolved in recent years. Onions are
traditionally transplanted as bare-rooted seedlings dug up from nursery beds.
More recently, multi-seeded modular transplants have also been used. Here, six
to eight onion seeds are sown into a small cell or soil block of a suitable growing
medium and, after emergence, each module is transplanted to give a small
cluster of plants at each planting station.
BARE ROOT TRANSPLANTING
In Ontario, onions for transplanting are sown
under glass using 4500 seeds/m
2
, either broadcast or in rows about 10 cm
apart. Day temperatures of about 17°C and night temperatures of 10°C are
maintained. The aim is to produce seedlings with pseudostems 3.5 mm thick
about 11 weeks after sowing. Before planting out, the plants are hardened by
reducing the frequency of watering and exposing them to temperatures of
4-7°C for a week or so (Riekels
et al.
, 1976). In many tropical regions onions
