Agriculture Reference
In-Depth Information
drought has been reported in other crops includ-
ing sorghum (Harris et al., 2007). Genotypic
variation has also been described in wheat for
leaf-glaucousness (Kulwal et al., 2003), leaf rolling
(Color Plate 25) (Sirault et al., 2008), and photo-
synthetic stay-green (Spano et al., 2003; Verma
et al., 2004; Christopher et al., 2008). Reduced
leaf glaucousness (greater waxiness) reduced leaf
temperature and increased photosynthetic rate,
increasing transpiration effi ciency in wheat (Rich-
ards et al., 1986). Durum stay-green mutants
showed a 10% increase in grain yield when water-
stressed in pots (Spano et al., 2003), while the
yield advantage of cultivar Seri was related to its
stay-green phenotype (Christopher et al., 2008).
Verma et al. (2004) also reported an association
between maintenance of green leaf area and grain
yield for wheat sister lines evaluated under optimal
and water-limited conditions.
Plants experiencing severe water stress com-
monly show symptoms of leaf rolling (Sirault
2007). Similarly, genotypes with a greater capac-
ity to roll leaves reduce leaf area to intercept less
sunlight. Reduction in radiation load should
reduce leaf warming and transpiration, slowing
further leaf dehydration. Leaf area is subsequently
maintained to allow photosynthesis to continue
after drought has abated (G.J. Rebetzke, unpub-
lished data; Sirault 2007). The effect of leaf-
rolling has been demonstrated in wheat by Clarke
(1986), who showed leaf rolling reduced effective
leaf area by up to 48% and water loss by up to
84%. Similarly, excised leaves of leaf-rolling gen-
otypes showed reductions in leaf area of 50% in
16 minutes compared with nonrolling genotypes,
where reduction in leaf area was much slower at
42 minutes (Sirault 2007). Genotypic variation in
leaf rolling has been positively associated with
kernel size (Sirault 2007) and yield under drought
(Clarke et al., 1991; Nachit et al., 1992). Despite
evidence for genotypic differences in leaf rolling,
not all variation is of adaptive value, with the
potential for rolling in some genotypes to be asso-
ciated with poor root growth or other hydraulic
resistance (Sirault 2007).
Little is known of genetic control of leaf rolling
in wheat. Rebetzke et al. (2001c) demonstrated
that selection for leaf rolling in wheat can be
readily undertaken under well-watered condi-
tions. Sirault et al. (2008) subsequently demon-
strated that leaf rolling under well-watered and
water-limited conditions was under strong addi-
tive genetic control and showed high narrow-
sense heritability (
h
2
= 0.80 to 0.97).
FUTURE PERSPECTIVES
Drought refl ects a dynamic process that is not
always consistent in the timing or amount of rain-
fall for a target region. Hence empirical selection
for yield in improving adaptation is a challenge
for breeders. A better understanding of the fre-
quency and nature of target environments through
modeling, coupled with a strong physiological
understanding of adaptation, has potential to
identify useful traits for selection in breeding pro-
grams. The challenge now is the identifi cation of
genetic sources of variation for such traits, devel-
opment of inexpensive high-throughput screen-
ing tools, and appropriate evaluation of selected
lines in target or related-constructed environ-
ments. Molecular markers will assist in indirect
selection of improved water uptake through
development of better root systems. However, the
polygenic control of many yield-related, physio-
logical traits will reduce effectiveness of marker-
assisted selection. Similarly, the value of transgenes
needs to be assessed in fi eld environments.
Further, evaluation of transgenes should focus
less on survival as this is unlikely to be of value
in wheat production systems, and efforts directed
at assessment for productivity or yield with water
limitation. Future efforts should focus on effi -
cient marker-implementation strategies for alleles
of small genetic effect, and development of inex-
pensive phenotyping methods for use in early-
generation selection where screening may be
among thousands of lines. Finally, efforts should
be directed at further assessment of candidate
traits singly or in combination for use in
breeding.
REFERENCES
Alvarez, S., E.L. Marsh, S.G. Schroeder, and D.P.
