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Results
The spikelet number for the unvernalized long day (ULD) treatment
ranged from 20.0 (CS/ Marquis 5A) to 29.0 (CS/Capelle Desprez
5A and 5D) for the lines (Table
1). In all lines
and Chinese Spring the vernalized long day (VLD) treatment resulted
in a reduction in spikelet number with a range of 14.7 ('Chinese
Spring') to 18.8 (CS/Capelle Desprez 5D) (Table
1). In the ULD
treatment ten of the lines were significantly lower, while three
lines were significantly higher in spikelet number than normal
Chinese Spring (Table
1). In the VLD
treatment all lines had higher spikelet number than normal Chinese
Spring with nine significantly so (Table
1).
Discussion
In all of the substitution lines and normal Chinese Spring there
were substantially fewer spikelets per ear in the VLD than the ULD
treatment. These differences reveal a component of spikelet number
expression that is associated with vernalization response. Since
previous work has indicated that vernalization genes in wheat
influence development only up to flower initiation (Flood and
Halloran 1984b; Griffiths et al 1985) the apparent association of
vernalization response with spikelet number may reflect pleiotropy of
these genes or their close linkage to genes influencing spikelet
number.
The significant differences from Chinese Spring in spikelet number of
the nine lines under VLD indicates the likelihood that the
substituted chromosomes involved, carry genes with more direct
effects on spikelet number than on vernalization response. The
substitution lines could be grouped into five classes on the basis of
their effects on spikelet number compared to normal Chinese Spring
under the ULD compared with the VLD treatment: two chromosomes,
Capelle Desprez 5A and 5D significantly increased spikelet number
under both treatments; chromosome Kenya Farmer 5D caused a
significant increase under the ULD treatment but no effect under the
VLD treatment. Hope 5D and Thatcher 5A caused significant increases
under VLD but no effect under the ULD treatment. Chromosomes Hope 5A
and 5D, Red Egyptian 5A, Marquis 5D and Capelle Desprez 5B caused
significant reductions in spikelet number under the ULD treatment and
significant increases under the VLD treatment. Chromosomes Timstein
5B, Kenya Farmer 5A and 5B, Thatcher 5B and Marquis 5A also caused
reductions in spikelet number under the ULD treatment but there was
no significant effect under the VLD treatment. The inconsistent
effects between the two treatments (ULD and VLD) on spikelet number
of the substitution chromosomes, indicates a certain independence of
the components of spikelet number influenced by the presence and
absence of vernalization response. This suggests that genes are
present on these. chromosomes which influence the rate and/or
duration of spikelet initiation and whose effects are not necessarily
pleiotropic expressions of vernalization response genes. Genetic
control of the rate of spikelet initiation in the absence of
vernalization and photoperiod influences has been reported previously
(Rahman et al 1977). Such effects, whether increasing or reducing
spikelet number, in the presence or absence of vernalization response
indicate that in crosses between different wheats it would be
feasible to select for increased spikelet number from transgressive
segregation between vernalization genes and those with direct effects
on spikelet number.
Four of the six 5A chromosomes in Chinese Spring ( i.e., from Hope,
Timstein, Red Egyptian and Capelle Desprez) caused significant
changes in spikelet number in the absence of vernalization response.
Chromosome 5A possesses a major gene for vernalization response
(Halloran 1986, Law et al 1976) and the Q gene conferring the
"vulgare" head character (Sears 1954) and floret
fertility (Frankel et al 1969). Chromosome 5A appears therefore to
have made a significant contribution to the floral biology of
hexaploid wheat and is likely to have contributed substantially to
its spikelet number determination. Triticum monococuum, the A
genome donor to wheat, possesses substantially higher spikelet number
than the proposed B genome donors, T. longissimum and
T.
speltoides (Bamakharamah et al 1984), T. sharonensis
(Aegilops sharonen-sis) (Kushnir and Halloran 1928b) and D genome
donor, T tauschii (Bamakhramah et al, loc cit; Lagudah 1986).
It is likely, therefore, that the superiority of T. monococcum
for this character resides largely with effects of chromosome
5A.
Further study of the variation for, and the physiological basis of,
high spikelet number in T. monococcum would appear to be
relevant for the possible incorporation of increased spikelet number
in hexaploid wheat.
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