Grain Yield Improvement in Water-Limited Environments - Wheat Science and Trade - page 218

be limited to traits controlled by fewer QTLs and
to the likelihood of identifying genotypes contain-
ing all or most QTLs in smaller populations com-
monly used by breeders (Bonnett et al., 2005).
Enrichment strategies may aid in overcoming
large numbers of QTLs in biparental- and back-
cross-based breeding programs.
Marker-assisted selection is likely to provide
the greatest benefi t to breeders targeting novel
traits from distantly related germplasm. For
example, markers are extremely useful in identi-
fying recombinants when targeting positive genes
linked in repulsion to genes of negative effect.
Several QTLs have been identifi ed with potential
for selection of improved performance in water-

limited environments (Table 11.2). Some of these
refl ect major genes with known effects on
improved root growth (e.g., saline tolerance or
root disease resistance) to increase water use,
while the functional basis of others is not well
understood (e.g., Kirigwi et al., 2007).
Critical to successful implementation of linked
markers in breeding are well-defi ned QTLs for
improved productivity. Experimental popula-
tions under evaluation should be representative of
the target populations under selection. Genotyp-
ing should be adequate in development toward a
good molecular map with even marker coverage
and quality mapping (good markers and map
development); phenotyping should be sound and

Table 11.2 Traits with potential for improving wheat performance in water-limited environments. Details are also provided
on genetic control for each trait.

Ease of
Screening

Chromosomal Location of
Genes Trait Heritability Source

Root health

Aluminum tolerance Simple High 4D Delhaize et al. (2004)

Boron tolerance Diffi cult High 7B, 7D Jefferies et al. (2000)

Cereal cyst nematode Diffi cult Low 2B, 2D Lagudah et al. (1997)

Salt stress

Sodium exclusion Simple High 2A, 4D Huang et al. (2006)

Sodium tolerance Diffi cult High 5A Byrt et al. (2007)

Drought stress

Phenology Simple High 2A, 2B, 2D, 3A, 3B, 5A, 5B,

5D, 6A, 6B, 7A, 7B

Snape et al. (2001)

Osmotic adjustment Diffi cult Moderate 7A Morgan and Tan (1996)

Carbon isotope disc. (leaf) Diffi cult High 1B, 1D, 2D, 3B, 4A, 4B, 4D,

5A, 7A, 7B

Rebetzke et al. (2008b)

Carbon isotope disc. (grain) Diffi cult High 1D, 2A, 2D, 4B, 4D, 6D, 7B Rebetzke et al. (2008b)

Canopy temperature Simple Moderate 1B, 2B, 3B, 4A Pinto et al. (2008)

Stem carbohydrates Diffi cult Moderate 1A, 2B, 2D, 3B, 4B, 5B, 6B,

7A, 7B

Rebetzke et al. (2008a)

Stomatal conductance Diffi cult Low 1B, 2A, 2B, 2D, 4A, 4B, 4D,

7A, 7B

G.J. Rebetzke, unpublished data

Glaucousness Simple Moderate 2B, 2D Tsunewaki and Ebana (1999)

Staygreen Diffi cult Moderate 2B, 2D Verma et al. (2004)

Leaf rolling Simple High Unknown Sirault et al. (2008)

Early vigor Simple High 2D, 4B, 4D, 5A Rebetzke et al. (2001a)

Coleoptile length Simple Moderate 2B, 2D, 4A, 4B, 4D, 5D, 6B Rebetzke et al. (2007b)

Harvest index Diffi cult High 2B, 2D, 4B, 4D Ellis et al. (2002)

Photosynthetic capacity a Simple Moderate 1B, 1D, 2D, 3B, 4A, 4B, 4D,

5B, 6B, 7A, 7B

G.J. Rebetzke, unpublished data

Restricted-tillering Simple High 1A Spielmeyer and Richards (2004)

Rate-of-grain-fi lling Diffi cult Uncertain Unknown Whan et al. (1996)

Root biomass Diffi cult Low 1B Waines and Ehdaie (2007)

a Surrogates for photosynthetic capacity (SPAD, SLW, SLN, leaf N content).