Agriculture Reference
In-Depth Information
vermiculite at 10°C, mentioned in the previous section, time to 50% rooting
varied from 8 to 63 days depending on cultivar, and there were large bulb-to-
bulb differences within a cultivar in time to root appearance (Miedema,
1994a). For all cultivars, rooting was most rapid at 10°C and was very slow at
30°C. Therefore, temperature responses of rooting were similar to, but faster
than, those of sprouting.
If root initials are regularly removed from bulbs planted on moist substrate,
sprouting is inhibited. For some cultivars this inhibition can be reversed by
including the cytokinin benzyl adenine (BA) in the solution (see Table 7.1). In
addition, wounding the base plate accelerates sprouting and increases the
effectiveness of BA in promoting sprouting of de-rooted bulbs (Miedema, 1994c).
Roots are initiated from the periphery of the primary thickening meristem (PTM)
in bulbs (see Fig. 2.10). An absence of starch granules in the PTM is associated
with root dormancy. When bulbs are stored at high temperatures (30°C), starch
remains undetectable in the PTM and roots do not initiate. However, if BA is
injected into such bulbs the shoots begin sprout growth, although starch
remains undetectable in the PTM (Ernst
et al.
, 1999). These experiments indicate
that the acceleration of bulb sprouting that is associated with the growth of roots
may be caused by cytokinins produced in the roots.
Bulb respiration rates and carbohydrate metabolism
Dormant onion and garlic bulbs have remarkably low rates of respiration when
compared with other vegetable crops (see Table 7.2). In storage, as time pro-
gresses, the rate of respiration increases (see Fig. 7.10). Respiration rates increase
with temperature (see Figs 7.8b and 7.10) but, for intact dormant bulbs, the rate
of increase with temperature is remarkably low. At a temperature of 40°C
Table 7.1.
The effects of root presence (+) or removal (-) and 10
mol/l benzyl
adenine (BA) on the sprouting in water culture of bulbs of four onion cultivars and
of shallot cv. 'Noordhollandse Strogele'. Values are expressed as the percentage of
bulbs with visible sprouts. Within columns, data followed by the same letter are not
significantly different at
P
< 0.05 (from Miedema, 1994c).
Treatment
Roots
Medium
'Radar'
'Stentor'
'Copra'
'Hyton'
Shallot
Intact
Water
50
b
50
a
50
a
50
a
50
a
Removed
Water
7
d
2
c
0
c
0
b
0
b
Removed
BA
60
a
31
b
16
b
12
b
7
b
Time to 50%
11
13
15
31
16
sprouting
with roots
in water
(days)
