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Wu et al. (1984) proposed
that a gene pool with improved resistance to scab could be developed
through recurrent selection by using the single dominant male-sterile
gene Ta1 (ms2), and it was expected that superior
resistant cultivars integrated with high-yielding capacity or
excellent germplasm with improved resistance would be obtained from
the improved population. After more than ten years of experiment, the
population of the gene pool has been significantly improved in
resistance and some new resistant strains with improved resistance
and desired agronomic characteristics have been developed jointly by
this method and conventional selection (Jiang and Wu 1996). Four
resistant genotypes selected from the gene pool and the well-known
resistant cultivar Sumai 3 were used to cross with a set of testers
and a genetic study was made. The objective of this study was: (1) to
evaluate the combining ability of these resistant genotypes; (2) to
explore if their resistance can be transmitted to offsprings; and (3)
to analyze the relationships between parents, F1's and
F2's.
Materials and methods
Host and field trial
Nine winter wheat genotypes
with different levels of resistance to scab were selected and a
factorial design was adopted to investigate the inheritance of the
resistance. Four resistant genotypes TFSL037, Changjiang 8809, W14
and Nantai 7 bred at Nanjing Agricultural University through
recurrent selection and conventional selection during the development
of scab-resistant gene pool in wheat (Jiang and Wu 1996) and the
well-known resistant cultivar Sumai 3 were selected as resistant
parents or paternal ones. Four cultivars with different genetic
backgrounds from Yangtze River Valley, Yangmai 5, Mianyang 11,
Changjiang 8853 and Aiganzao representing high-yielding capacity,
early maturity, good quality and from moderate to high susceptibility
to scab,' respectively, were used as maternal parents or testers.
During 1992-1993, all possible crosses were made in a 4 x 5 factorial
design or NCII mating design. In 1993-1994, a field trial including 9
parents, 20 F1's and 20 F2's was established in
a 3-replicate randomized block design at Nanjing. Each plot, with the
row length of 1.5m and a space of 0.2m, consisted of 1 row for each
of parents and F1's and 5 rows for each of
F2's. 40 seeds were sown per row.
Assessment of resistance to scab
In general, scab resistance in wheat consists of two components:
resistance to initial penetration and resistance to fungal spreading
within plant tissue after infection (Schroeder and Christensen 1963),
and the expression of resistance is attributable mostly to the latter
(Wu 1990). Therefore, the single-floret inoculation was made to
evaluate the fungal-spread resistance. The experimental inoculation
was conducted by injecting 20 microliter of conidiospore suspension
of Fusarium graminearum into a single basipetal floret in the
middle part of heads which were to flower or had just
flowered. The
concentration of the inoculum was about 1 x104
conidia/ml. About 15 plants for the parent and F1, only
one spike per plant, and about 80 plants for the F2 were
inoculated per plot, respectively. When the symptoms were clearly
identified and the differences between genotypes were quite distinct,
head blight, expressed as number of diseased spikelets, was observed
25 days after inoculation. The experimental data were recorded based
on the fungal spread and symptoms on the inoculated heads as
follows:
0.5: only
inoculated floret showed symptom;
1.O: inoculated spikelet showed symptom;
1.8: one spikelet and the main axis of inoculated spike showed
symptom;
2, 3, ..., n: the number of
total diseased spikelets on the inoculated spike.
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