Agriculture Reference
In-Depth Information
effect following flower induction (see Fig. 4.39) and delays bulbing in plants grown
from stored bulbs (see Figs 4.33 and 4.39). It would be interesting to know
whether there is any underlying connection for these various remarkable effects of
temperatures around 25-30°C on the physiology of onion bulbs.
Once sprouting has occurred in rooted bulbs, sprout growth rate increases
progressively with temperatures in the range 0-25°C, typical for a growth
response in onion (see Fig. 4.22). Therefore, during rooting and sprouting
there must occur a transition from the remarkable growth-inhibitory effect of
temperatures around 30°C to the usual increase of growth rate with tempera-
tures in the range 10-30°C. As dormancy declines during bulb storage,
probably at different rates in different individual bulbs, the transition between
these two contrasting temperature responses may give confusing transitional
temperature responses.
High temperatures applied to bulbs immediately after harvest can increase
their subsequent rate of sprouting. Compared with cooler temperatures, 30 or
35°C for 3 weeks after harvest accelerated sprouting in subsequent dry storage
at 15°C, and also rooting and sprouting in bulbs planted on moist vermiculite at
15°C. In bulbs that were not mature at harvest, a postharvest temperature as
low as 25°C had such an effect (Miedema, 1994a). Such high temperatures are
often used to cure freshly harvested bulbs, and this evidence indicates that this
practice may shorten storage life. A similar effect of high temperatures around
harvest time occurs in garlic (see 'Garlic Dormancy and Storage', below).
In summary, onion bulbs pass through three phases in relation to the effects
of warm temperature on sprouting (Miedema, 1994a). Immediately after
harvest, periods of a few weeks at 25-35°C reduce dormancy and result in earlier
sprouting. Subsequently, long-term storage at 25-30°C retards sprouting. Then,
once bulbs are rooted and growing, such temperatures are optimal for sprout
growth, typical of a vegetative growth process in onion.
Cultivar effects
In a comparison of ten cultivars planted on moist vermiculite at 10°C, time to
50% sprouting ranged from 49 to 156 days, depending on cultivar, and time to
50% sprouting in dry storage at 10°C ranged from 149 to 310 days. There was
a large bulb-to-bulb variation in time to sprouting within long-storing cultivars
(see Fig. 7.9).
Differences between cultivars also occur within the range of 0-56 days in
the time when sprout leaves begin to elongate at the shoot apex (Bufler, 2001; in
press, a; Yasin and Bufler, 2007). From observations on bulbs dry-stored at 18°C,
cultivars divided into two classes, with several intermediate types (Bufler, in press,
a). Type 1 cvs, which were all of the type 'American Globe', had a period of up to
8 weeks after curing with no sprout leaf growth, and could therefore be defined as
truly dormant. Type 2, which were all 'Rijnsburger' (Dutch) cvs, started sprout
leaf growth immediately after curing, and hence showed no period of true
dormancy. Interestingly, the 'Rijnsburger' types include some of the longest-
