Agriculture Reference
In-Depth Information
The peak of histone synthesis associated with a peak of cell division in the
shoot apex of stored bulbs occurred at about the same time (150 days after
harvest) in cultivars that visibly sprouted at that time or in those that took a
further 100 days to visibly sprout (Carter
et al.
, 1999). So, sprouting time was
not necessarily associated with cell division at the shoot apex, since the interval
between this meristematic activity and the elongation of the sprout leaves
differed between cultivars.
There has been some debate as to what constitutes dormancy in onion bulbs,
since metabolic changes continue through the period from bulb maturity to
sprouting (Gubb and MacTavish, 2002). An absence of visible growth has been
proposed as a clear-cut criterion for dormancy, and a method to determine
whether sprout leaves have started elongating has been devised (Bufler, 2001;
Yasin and Bufler, 2007; Bufler, in press, a). The technique is based on the
observation that the blades of sprout leaves elongate well before a slow elongation
of the sheath. The initial mean and standard deviation of leaf blade length in
sprout leaves with sheath lengths in the range 2.0-3.5 mm dissected from bulbs
following harvest and a 2-week curing are used to define sprout leaf blade length
in dormant bulbs. In subsequent samples, sprout leaves with sheaths in the range
2.0-3.5 mm are measured, and those with leaf blades longer than the mean
value for dormant bulbs, plus its 95% confidence interval, are assumed to have
started elongating and, therefore, to be sprouting and to have exited dormancy.
There is variation in the time to commencement of sprout leaf elongation
between individual bulbs within a cultivar, and hence to get a clear picture of
metabolic changes occurring at the shoot apex of bulbs as they emerge from
dormancy it is important to make measurements on sprout leaves and their
enclosed apices from individual bulbs, not on samples bulked from several
bulbs (Bufler, in press, a; Yasin and Bufler, 2007).
The skin of onion bulbs
The skin of an onion is the first thing a potential purchaser sees, and attractive,
intact skins are essential for high-quality onions. Therefore, knowledge of how
to produce and maintain sound and attractive skins is important for the onion
industry. The outer, dry skin of the onion bulb protects the inner, fleshy scales
from damage and disease, and prevents them drying out. During the maturation
of bulbs the outer scales lose water and form thin, dry skins that completely
envelop the bulb. Chemical changes to the flavonoid pigments occur during skin
drying, and these determine the final external colour of the bulbs, which is
important in many markets. Also, phenolic substances formed in this process
make skins resistant to invasion by microbial pathogens.
Changes in the structural carbohydrate composition, as fleshy skins dry to
form papery scales, give rise to greater strength and the water-absorbing proper-
ties of the outer scales. Skin flexibility versus brittleness, and also the permeability
to water, vary with skin water content. These physical properties influence the
resistance of skins to damage and loss during handling, and also the rate of water
