Molecular Mapping of Complex Traits - Genetics, Genomics and Breeding of Crop Plants - page 165

ranging from 6.4 to 7.5. The resolution of these QTLs was increased by
crosses between two highly embryogenic lines (LR35 and C40) with a poorly
embryogenic line (C149) and the construction of two F 2 /F 3 linkage maps.
The author showed that somatic embryogenesis in sunflower is highly
genotype-dependent and quantitative in nature. Reducing the QTL interval
into a much shorter region resulted in the identification of four AFLP markers
on linkage groups 5 and 13 closely linked to somatic embryogenesis potential.
5.2.7 Abiotic Stress Tolerance
5.2.7.1 Plant Water Status and Osmotic Adjustment
Sunflower offers a rich source of genetic diversity for abiotic stress tolerance,
particularly drought and salt. Drought tolerance has been considered as a
valid breeding target to partially compensate for the loss in yield. However,
this has yet to be exploited. The first QTL analysis of drought tolerance-
related traits has been reported by Poormohammad Kiani et al. (2007a, b,
2008; Table 5-1 ) using a saturated linkage map. QTL mapping of water
status traits (relative water content, leaf water potential, osmotic potential
and turgor potential) as well as osmotic adjustment was investigated in an
RIL population derived from a cross between PAC2 and RHA266 under
well-watered and water-stressed conditions. Using a saturated linkage map,
three to eight QTLs were detected for each trait explaining from 6% to 29% of
the phenotypic variance of the traits; and LOD scores varied from 3.05 to
9.86 (Poormohammad Kiani et al. 2007a). Among the 24 QTLs detected
under well-watered conditions, 5 (21%) were also detected under water-
stressed conditions (stable QTLs). Eight QTLs were detected for osmotic
adjustment on LGs 2, 4, 5, 12 and 13, and four of them were overlapped with
the QTLs of water-status traits on LGs 4, 5 and 12 (Poormohammad Kiani et
al. 2007a).
The most important chromosomal region for osmotic adjustment is
located on LG 5, where the QTL for osmotic adjustment ( OA.5.2 ) is overlapped
with QTLs for relative water content in both water treatments as well as
with the QTLs for leaf water potential under water-stressed conditions.
This region, which is linked to an SSR marker (ORS523-1), appears very
important in water stress tolerance, as it is involved in the expression of
osmotic adjustment as well as in the expression of osmotic potential at full
turgor, leaf water potential and relative water content under water-stressed
condition. This QTL explains 29% of the phenotypic variance in OA and
also 10%, 8% and 18% of phenotypic variance in leaf water potential, relative
water content and osmotic potential at full turgor under water-stress,
respectively. It explains also 25% of relative water content phenotypic
variance under well-watered conditions.