Agriculture Reference
In-Depth Information
Bulb development in Rakkyo occurs in response to long photoperiods and is
favoured by temperatures in the range 15-25°C (Toyama and Wakamiya,
1990). In Japan, inflorescences differentiate in Rakkyo in late May-June, and
they elongate and bolt in the autumn, with the flowers opening in September.
Rakkyo is therefore unusual among the vegetable alliums in initiating its
inflorescences in the warm season. Plant size is important for flowering: plants
grown from mother bulbs weighing 3 g or less do not flower and a mother bulb
weight of about 11 g results in 50% flowering. Flowering is inhibited by
exposing plants to short photoperiods from mid-April to mid-June, and
accelerated by exposure to longer than natural photoperiods during this period
(Toyama and Wakamiya, 1990).
Unlike the bulb-forming crops, chives are winter dormant and the onset of
dormancy is triggered by the declining temperatures and shortening photo-
periods of autumn. In dormant plants there is no leaf growth even under ideal
growing conditions - for example, at 20°C under good lighting. The extent of
growth on transfer to such standard conditions can be used to measure the
degree of dormancy. Research on German cultivars has shown that the onset of
dormancy is stimulated by exposure over a period of 4-6 weeks to photoperiods
of less than about 14 h. The optimal mean temperature for inducing dormancy is
about 14°C, and a wide day/night fluctuation around this mean is more effective
than a more constant temperature (Poulsen, 1990). During the induction of
dormancy, carbohydrates accumulate in the roots and at the base of the shoots.
Also, growth inhibitory substances - particularly abscisic acid (ABA) -
accumulate and the concentrations of growth promoters - like auxins and
gibberellins - decrease.
Dormancy can be broken by prolonged exposure to low temperatures, and
also by a fairly brief exposure to extremely high temperatures. In Germany,
high temperatures are used to break dormancy before planting chives in
glasshouses for winter leaf production. The standard technique is to immerse
the plants in water, initially at a temperature of 40°C, and allow them to cool to
25°C over a 16 h period. A temperature of 40°C for 3 h is sufficient to break
dormancy, but temperatures above 44°C are lethal.
Chives normally flower at the end of May in the field. They do not usually
flower in the first year following a spring sowing, so probably a period of cool
temperature and short photoperiods is required for flower induction, but
controlled experiments on chive flowering are needed to prove this (Poulsen,
1990).
