Agriculture Reference
In-Depth Information
Fig. 4.26.
The appearance of developing leaf initials during leaf blade growth (a),
early-stage bulbing (b) and established bulbing (c). p is the pore through which the
blade of the next leaf emerges from its encasing sheath during leaf growth. Bulbing
is characterized by a decrease in the ratio of blade length, B to sheath length, S,
termed the 'leaf ratio'. Initials with B/S below unity are termed 'bulb scales' (Heath
and Hollies, 1965).
scale development in N-deficient plants (Brewster and Butler, 1989). In field-
grown plants grown at densities of 25 and 400/m
2
, bulbing started 8 days sooner
at 400/m
2
when measured by bulbing ratio, whereas bulb scales were initiated
23 days earlier in the plants at this high density than at 25/m
2
(Mondal
et al.
,
1986a).
Conditions favouring carbohydrate accumulation within the plant can
lead to thickened leaf sheaths and increased bulbing ratios without formation
of bulb scales. In addition, leaf initials differentiate into bulb scales on lateral
shoots before this occurs on the main shoot axis. As a result, in cultivars and in
conditions that produce many side shoots, a high bulbing ratio can occur long
before the main axis stops producing green leaves (Wiles, 1989).
Because of this variability in the linkage between increases in bulbing ratio
and decreases in leaf ratio on the main shoot, and because a decrease in leaf
ratio is the first indication of the initiation of the storage scales that ultimately
results in the cessation of leaf blade growth and the ripening of bulbs, a
decrease in minimum leaf ratio below unity on the main shoot axis is the
preferable method of defining bulb initiation. Since leaf initiation rates are
temperature dependent (see Fig. 4.22b), it is possible to use thermal time to
estimate leaf initiation date from leaf number, and hence estimate the date of
initiation of the first bulb scale from a sample taken after the event (Brewster,
1997b).
