Agriculture Reference
In-Depth Information
impact damage can be estimated. Using this analysis in handling machinery and
systems, features causing unacceptably high impact forces or frequencies can be
identified and improved. In bulk stores, onions at the bottom, which may be 5 m
deep, are subjected to a greater static load than those at the top and this can also
lead to damage, as indicated by a greater respiration rate (Herold
et al.
, 1998).
Bulb treatments to prevent sprouting in storage
Apart from decay due to disease, internal sprout growth is the primary cause of
the changes that lead to the deterioration of stored bulbs. Therefore, if sprout
growth can be delayed or prevented, bulb storage life can be extended. Pre-
harvest applications of sprout-inhibiting chemicals and postharvest exposure
of bulbs to ionizing radiation have both been developed to prevent sprout
development in stored onion bulbs.
PRE
-
HARVEST APPLICATION OF MALEIC HYDRAZIDE
,
A SPROUTING INHIBITOR
Maleic
hydrazide (MH) is a growth-regulatory substance that disrupts cell division
(Isenberg
et al.
, 1974). If applied to onion leaves while they are green and actively
exporting photosynthates, MH is translocated to the shoot apex, where it
prevents cell division. In stored bulbs this suppresses sprout and root growth. The
increase in cytokinin and other growth-promoting substances, and decrease in
growth inhibitors associated with sprout development, does not occur. In
untreated bulbs respiration rates increase rapidly as they approach sprouting,
whereas this increase is much lower in MH-treated bulbs (Ward and Tucker,
1976).
A minimum concentration of 20 ppm MH at the centre of bulbs is required
reliably to suppress sprout growth. To achieve this, the timing of applications in
relation to the stage of crop development is important. The MH needs to
accumulate at the growing point when bulbs are nearly mature, and have
therefore completed the cell division required to form bulb scales and sprout
leaf initials, but it must be applied while the leaves are still green and actively
translocating. Application too early results in the development of bulbs with
abnormal, soft, 'spongy' centres. Application rates in the range 1.1-5.7 kg/ha
in aqueous solution have been tested, and the lower to middle part of this range
has proved effective (Komochi, 1990).
Because the amounts absorbed and translocated from foliar sprays will vary
with weather conditions and the state and activity of the leaves, the optimum
quantity to apply is subject to some uncertainty. In the UK it is recommended to
apply 8.4 l/ha of 36% active ingredient (a.i.) MH when the crop has reached the
stage of 10% fall-down and, in The Netherlands, application at a similar rate
and stage is recommended. Studies from the USA and Poland indicate that
applications at 50% fall-down are best. The difference between these two sets of
recommendations may be because bulbs are harvested at a later developmental
stage in the USA and Poland. A period following spray application of at least 10
h without rainfall is needed for MH absorption.
