PHYSIOLOGY OF CROP GROWTH, DEVELOPMENT AND YIELD - Onions and other Vegetable Alliums - page 123

Stomatal physiology
Stomatal opening in onion, as with other species, involves the electrochemical
pumping of potassium K + ions into the stomatal guard cells, causing an
increase in their osmotic pressure and turgor. Because the guard cells lack
starch granules, the influx of K + cannot be electrically balanced by the
generation of negative malate ions from starch, as occurs in many species.
Instead, chloride, Cl - enters the guard cell along with the K + and maintains
electroneutrality (Schnabl and Raschke, 1980). Hence, chloride is important
in plant nutrition (see Chapter 6). Potassium uptake by guard cells correlates
with the morning opening of stomatal apertures, but in the afternoon K + levels
decline and an increase in sucrose concentration in the guard cells maintains
turgidity and open apertures (Amodeo et al. , 1996).
The sudden leaf necrosis, termed 'onion blast', involves stomata. This
results from ozone damage to leaves when atmospheric conditions result in high
ozone concentrations. Onion cultivars differ in sensitivity to ozone damage.
Resistant varieties are in fact hypersensitive and, under high concentrations of
ozone, the membranes of their stomatal cells become leaky. This causes rapid
stomatal closure and thereby prevents the entry of ozone inside the leaves and
consequent damage to the mesophyll cells (Eagle and Gableman, 1966).
ONION BULBING
The estimation of bulb initiation
A feature that unequivocally indicates that bulbing has started is the
development of leaf initials into bladeless 'bulb scales'. This can be diagnosed as
the first occurrence of a 'leaf ratio' (leaf blade length:sheath length) of less than
unity (Heath and Hollies, 1965; Fig. 4.26). Where many plants are available
for sampling, scale initiation can be quite quickly estimated from examining
plants sliced longitudinally upwards through the centre of the sheath. Mean
dates of bulb initiation can be estimated from a series of such samples (de
Visser, 1994a).
Normally, a leaf ratio below unity is coincident with marked swelling of the
outer leaf sheaths and a consequent rapid increase in 'bulbing ratio'
(maximum bulb diameter:minimum sheath diameter). This latter ratio has
been more commonly used in bulbing studies, since it can easily be measured
non-destructively, whereas the assessment of minimum leaf ratio involves the
dissection of plants and is more laborious.
However, the occurrence of the first leaf ratio less than unity is not invariably
linked with the attainment of a particular bulbing ratio. Bulbing ratios greater
than 2, commonly used to define bulb initiation, can occur in the absence of bulb