Agriculture Reference
In-Depth Information
control of development in bulb onions. It therefore provides a powerful con-
ceptual framework for understanding the bulb onion crop and seeing how
many factors interrelate to influence yields and maturity date. With modi-
fication of the functions relating bulb development rate to photoperiod, it could
probably be made applicable to other cvs growing in climatic zones different
from north-western Europe.
The simulation model proved reasonably successful at predicting growth
and bulb development on fertile soils in the Netherlands, where it was developed.
There is no provision for the effects of nutrient and water stress in the model, and
overestimates of yield in some model validation trials indicated previously
undetected influences of such stress at some sites (de Visser, 1994b). Such a
growth model could be combined with simulations for disease development (see
Chapter 5) to make assessments of likely disease impact on yields.
FLOWERING
Flowering is an essential prerequisite of seed production, and its importance is
self-evident for all but the purely vegetatively propagated alliums. When
producing bulbs or growing shoots for food, flowering is generally unwanted,
except in the unusual case of Chinese chives, in which both inflorescences and
leaves are eaten. Apart from this, flowering or bolting downgrades the quality
of the crop and may curtail the growing season, for example in leeks or salad
onions. Therefore, an understanding of flowering, and how it interacts with
vegetative growth and bulbing, is important for successful food crop
production as well as for seed production. Flowering has been most fully
investigated and is best understood in onion. Research on flowering in onion
has been reviewed by Brewster (1997a,b) and Rabinowitch (1990a),
pollination by Currah (1990) and seed development by Rabinowitch (1990b),
and fuller reference lists can be found there.
Onion flowering
The sequence of stages in floral development
Stages in inflorescence initiation and development are shown in Figs 2.6 and
2.17. The sequence of stages in the cycle from seed germination to seed ripening
in the life cycle of an onion is illustrated in Fig. 4.35. Each stage of this cycle has
different environmental requirements. For some stages, different cultivars are
known to have different environmental optima, and to require different lengths
of time to pass through the stage. Also, when inflorescences develop within bulbs,
the rate of initiation is strongly influenced by the size of the bulbs. Some stages in
the life cycle progression can be reversed; for instance, 'devernalization' can
