Agriculture Reference
In-Depth Information
early-season, non-stored onion, they have thin skins, low dry-matter content,
soft flesh and only short dormancy (Currah and Proctor, 1990).
Onion cultivars are distinguished and characterized by a range of features.
These have been systematized by IBPGR and are incorporated in their system for
classifying cultivar collections (Astley
et al.
, 1982). Distinguishing features of the
foliage include the colour (i.e. the depth of green), leaf length and leaf erectness.
Bulb features include the bulb shape, the uniformity of bulb shape and the bulb
skin colour. Bulb shapes can be globe, a flattened globe, sometimes with a flat top,
spindle-like or almost cylindrical. Sometimes there is a high 'shoulder' to the
bulb. Skin colours can be white, yellow, light brown, brown, dark-brown, red,
purple or green. Features of the inflorescence include its fertility, the flower
number in the umbel, the tepal and anther colour, the presence or absence of
bulbils (see Fig. 4.41) in the inflorescence and whether an inflorescence is indeed
produced or whether reproduction is normally vegetative.
The above features are strongly heritable and easily seen by eye, although
many are subject to environmental variability, as has been shown for bulb
shape, which is influenced by the population density at which plants are grown
(Dowker and Fennell, 1974). Ideally, bulb shape and many other properties
important in characterizing a cultivar should be judged by comparisons with
standard varieties grown in the same conditions, thereby discounting the
effects of environment. Cultivar features characterizable in such trials include
bulb skin thickness, the extent of bulb doubling (i.e. how many obvious
'centres' exist when the bulb is sliced transversely), firmness of bulbs, dry-
matter percentage in bulbs, sweetness and flavour strength of bulbs, potential
storage life and bulb flesh colour. Bulb flesh colours can be white, white-flecked
green, yellow, red or purple.
The day-length requirement for bulbing can be judged from the location
where the cultivar is grown and the time of year it bulbs. Also, in trials at one
location, an indication of day-length response can be gained by comparing the
rates of bulbing and maturing of different cultivars (e.g. Magruder
et al.
, 1941).
Features of flowering that are characterized in comparative trials include the
length of the scape, the time of flowering relative to standard varieties and the
cold requirement for bolting, usually measured as percentage of bolting
relative to a standard variety. Features of great practical significance, and
which again require comparative screening against standard varieties, include
susceptibilities to pests and diseases and ability to tolerate soil or climatic
stresses. Organs not selected for by humans - for example, the flower and seed
capsule - have been little affected by domestication and exhibit no great
variation (Fritsch and Friesen, 2002).
In the catalogues of seed companies that trade over a wide geographical area
onion cultivars are frequently classified into 'short-day' (SD), 'intermediate-day'
(ID), 'long-day' (LD) and 'very long-day' (VLD) day-length types. This refers to the
minimum day-length (photoperiod) needed to stimulate bulb development. LD
types are characterized by long-storing cultivars from the north-eastern USA and
