Narcissus and Daffodil

The biology of Narcissus 11
flowered, and those that did not produced four or more leaves; in contrast, few
laterals flowered, usually if there were three or four leaves.
There are no experimental data on the factors which trigger floral initiation
(Rees, 1972). Experimentation is difficult as initiation takes place before bulb lifting,
and would be complicated by the perennial habit and, hence, possibly the effects
of earlier years. However, bulb units appear to reach a critical weight before they
are likely to contain flowers. In many bulbous ornamentals, flower initiation occurs
at a fixed time in the normal annual cycle of development, and often, for example
in tulip and iris, after a minimum number of leaves have been initiated, perhaps
related to apex size (Rees, 1985a). A similar situation may exist in narcissus, as
there appears to be some relationship between leaf numbers and flowering, albeit
different in terminal and lateral bulb units (see above). Rees (1986) determined
the critical weight for flowering, based on bulb unit weights: using bulbs dissected
in August, the critical weight was about 1.15 g, although there was some overlap in
the weight distribution of flowering and non-flowering units. The critical weight
for flower initiation of bulb units, and hence critical weight for clusters, may vary
from year to year (Dickey, 1940; Rees, 1986), with growing conditions (Roh et al.,
1978; Kim and Lee, 1982), and between bulbs propagated by chipping and ‘ordinary’
bulbs (ADAS, 1987).
The number of flowers per bulb or per weight of bulbs is important for commercial
bulb producers. Rees (1986) examined the relationships between the
number of bulbs and flowers per tonne of bulbs, the number of bulb units per
bulb, and the number of flowers per bulb unit. Variations in flower yields might
arise for a number of reasons, such as a large number of bulb units being below
critical size, the occurrence of a few very large bulb units, or variations in the critical
size itself. Over four years, the number of flowers obtained varied from 27.5 to
32.9 thousands/tonne in ‘Carlton’ and from 21.0 to 31.8 in ‘Golden Harvest’. A
major cause of variation in flower yield was the number of bulb units per unit
weight in larger bulbs, which was only partly compensated by an increase in the
number of bulb units per bulb, although the number of bulb units per tonne was a
relatively consistent statistic. The number of flowers per bulb unit varied between
cultivars and years, and the critical bulb unit weight for flowering varied from year
to year. To obtain high numbers of flowers per tonne of bulbs, mean bulb unit
weight should just exceed the critical weight, and cluster weights should be multiples
of the critical weight, as illustrated by the analyses of Alkema and van Leeuwen
(1978) and Kruyer (1981) (Figure 1.8). At present the factors controlling these
responses are not known, and flower yields can be manipulated only crudely
through changing the grade of bulbs planted, planting density and the duration of
the crop (growing the crop for one or more years).
Experiments carried out by Gerritsen and van der Kloot (1936) and Hartsema
(1961), involving lifting bulbs early (around or before floral initiation) and excising
leaves, suggested that, under normal conditions, the presence of green leaves was
essential for floral initiation. However, in bulbs cold-stored for 6 months, new
flowers were sometimes initiated in the lateral bulb units in the absence of green
leaves (Hartsema and Blaauw, 1935). This suggested that the effects of light are
not essential for flower initiation to take place (Hartsema, 1961). Although the
effects of temperature on floral initiation are largely unknown, there is much
information on the effects of temperature on flower development and production