Agriculture Reference
In-Depth Information
through a short stalk with top-sets surrounded by the outer bulb membrane, to a
short emerged stalk crowned by a cluster of top-sets. After a certain point following
inflorescence emergence, rapid stalk elongation and the development of flowers
and top-sets are promoted by warm temperatures and long photoperiods, again, as
is the case with onion.
Kamenetsky
et al.
(2004) investigated the conditions needed to produce
normal flowers in a clone from Russia (see Fig. 2.18). They distinguished four
phases in reproductive development: (i) transition of the apical meristem to a
reproductive state; this occurred most rapidly following cool storage of the bulbs
and growth in long photoperiods; (ii) scape elongation, this was favoured by low
storage and growing temperatures and long photoperiods; (iii) inflorescence
differentiation; (iv) completion of floral development to anthesis. A temperature of
20/12°C (day/night) and a photoperiod of 10 h interrupted by just 1 week of 20 h
photoperiods during scape elongation resulted in normal anthesis. A temperature
just 3°C higher caused young flower buds to abort. Without the 1 week of
long photoperiods the scape aborted and never emerged from the pseudostem.
With more than 1 week of long photoperiods vegetative top-sets formed on the
inflorescence, and these grew and squashed the developing flowers. So, with this
clone, a very precise sequence of temperatures and photoperiods was required to
achieve normal anthesis. Suppression of the whole inflorescence or suppression of
florets by top-sets occurred outside all but a narrow combination of conditions.
Growth and development
In contrast to bulb onions and garlic, leek is harvested as a growing shoot. The
objective in crop production is to produce shoots of marketable size before the
plants bolt. Therefore, it is useful to understand what controls flower stalk
initiation and development relative to vegetative growth. The rate of elongation
of flower stalks, once initiated, clearly shows some seasonality, and visible
bolting is concentrated in the long photoperiods of spring and summer.
However, the main variable in the 'race' to produce a marketable-sized leek
before the plant visibly bolts is not the rate of flowering, but the rate of growth.
In fertile soil, growth rates are primarily determined by temperatures and light
levels. Premature bolting is a problem from very early plantings, which grow
slowly because of low temperatures in the early spring, and from late summer
and autumn plantings, where the low temperature and light levels of winter
restrict growth rates, and the plants fail to achieve marketable size before bolting
occurs in the spring.
Early vegetative growth can be described by fairly simple rules involving
temperature, light and leaf number. For spring-sown leeks in field conditions,
both the number of leaves initiated at the shoot apex, the number of leaves
