Agriculture Reference
In-Depth Information
The stages described in Fig. 4.35 can be grouped into four broad phases: (i)
'Juvenile phase'; (ii) 'Thermo phase'; (iii) 'Competition phase'; and (iv) 'Completion
phase' (van Kampen, 1970). The stages within each phase have similar environ-
mental requirements, but different phases have different requirements. During the
juvenile phase onions cannot be induced to flower; they must reach a certain
critical weight or leaf number before they can enter the thermo phase, in which
they can be induced by low temperatures to initiate inflorescences. Inflorescence
initials, once formed, can be destroyed and are observed to wither and degenerate
in conditions that favour bulb development. This is termed the 'competition
phase', because of the apparent competition between inflorescence development
and bulb development. Inflorescences are particularly vulnerable before they are
visible outside the bulb or pseudostem. Once inflorescences are visible, the
completion phase is favoured by higher temperatures than the previous two
phases. Temperature ranges and optima for the mostly temperate region cultivars
so far investigated, and other important environmental influences at each phase,
are outlined in Table 4.7.
The minimum size for flower initiation
The critical plant size above which inflorescence induction can occur depends
on cultivar. For example, seedlings of the Dutch spring-sown cv. 'Rijnsburger'
can be induced when they have seven or more leaves initiated (counting the
cotyledon as one), whereas the more bolting-resistant, autumn-sown cv.
'Senshyu Semi-globe Yellow' must have ten or more leaves initiated. Shoot dry
weights of 0.06 and 0.45 g correspond to these two leaf stages, respectively.
The transition from a zero to a maximum inflorescence initiation rate occurs
over a narrow weight range in seedlings (see Fig. 4.36).
Fig. 4.36.
The effect of the number of leaves initiated at the start of vernalization
on the proportion of plants with inflorescence initials after 77 days at 9°C and 18 h
photoperiods for cv. 'Rijnsburger Bola' (solid squares, spring-sown cv.) and cv.
'Senshyu Semi-globe Yellow' (open triangles, autumn-sown cv.) (from Brewster,
1985. Courtesy of
Annals of Botany
).
