Agriculture Reference
In-Depth Information
4
D
EVELOPMENT AND
Y
IELD
This chapter starts with an outline of the principles determining crop yields and
how they apply to onions. This gives a context within which to discuss how
environmental and internal factors control successive phases of development
from seed, through shoot growth, to the harvested bulbs or shoots. Inflorescence
initiation and development and seed formation are then considered, along with
interrelationships between vegetative and floral growth. Models based on
physiological relationships that predict crop growth, development and yield are
then discussed. Onion, the most intensively studied crop, is considered primarily,
followed by sections on each of the other edible alliums, which behave similarly to
onion in many ways.
The physiology of bulbing and crop yield in onion was reviewed by Brewster
(1990a, 1997a, b) and Bosch-Serra and Currah (2002), and research on
flowering in onion has been reviewed by Rabinowitch (1990a) and Brewster
(1987, 1997a, b), pollination by Currah (1990) and seed development by
Rabinowitch (1990b), where more references to the research literature can be
found.
The target of vegetable allium production is a high yield of high-quality
produce. The attainment of such a crop is the end result of many processes of
growth and development. Well-established principles determine the yield of all
crops, and some of these have been investigated for onions and leeks. The yield
of a crop is determined by: (i) the quantity of light absorbed by its leaves while
harvestable dry matter is being produced; (ii) the efficiency with which the
absorbed light is converted by photosynthesis into sucrose; (iii) the conversion
coefficient between photosynthetic sucrose and the biochemical constituents
of the harvested material; (iv) the proportion of photosynthetic output
transferred to the harvested fraction of the plant; and (v) the weight losses due
to respiration and decay after the above photosynthetic and biosynthetic
