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qCTB-4-2 for 3 traits (spike length, full grains per spike and
spikelet fertility), qCTB-5-1 for 5 traits (plant weight, panicle
exsertion, blighted grains per spike, full grains per spike and
spikelet fertility). The variance explained by a single QTL
ranged from 0.80 to 16.80%. Three QTLs (qCTB-1-1, qCTB-4-1
and qCTB-4-2) were detected in two or more traits. Then, it set
a foundation for cloning cold-tolerance genes and provided
opportunities to understand the mechanism of cold tolerance
at the booting stage (Zeng et al. 2009).
Low temperature stress is common for rice grown in
temperate regions and at high elevations in the tropics. The most
sensitive stage of this stress is booting, about 11 days before
heading. Japonica cultivars are known to be more tolerant
than indicas. It had been constructed a genetic map using
191 recombinant inbred lines derived from a cross between a
temperate japonica, M-202 and a tropical indica IR50 in order
to locate qualitative trait loci (QTLs) conferring cold tolerance.
The maps with a total length of 1,276.8 cM and an average
density or one marker every 7.1 cM was developed from 181
loci produced by 175 microsatellite markers. Cold tolerance was
measured as the degree of spikelet sterility of treated plants at a
12ÂșC temperature for 5 days in the growth chamber. QTLs on
chromosomes 1,2,3,5,6,7,9 and 12 were identifi ed to confer cold
tolerance at the booting stage. The QTL contribution to the
phenotypic variation ranged from 11 to 17%. The two QTLs
with the highest contribution to variation, designated qCTB
2a and qCTB3 were derived from the tolerant parent M-202,
each explaining approximately 17% of the phenotypic variance.
Two of the eight QTLs for cold tolerance were contributed by
IR50.
In control condition, cold tolerance was measured as the
percentage of reduction in panicle exsertion and in spikelet
fertility. Evaluating cold tolerance through the reduction in