Agriculture Reference
In-Depth Information
TPE. Ideally, managed environments should be
uniform, repeatable, and allow differentiation of
genotypes as well as the assessment of mechanisms
or traits and putative quantitative trait loci (QTLs)
or candidate genes associated with performance
under drought. Site uniformity and control of
water availability should aid in reducing the error
variance and reduce potential for genotype ×
season interaction to increase heritability and
genetic variance. Also, the site should be managed
appropriately to reduce the potential for root
disease. Use of combinations of drip and gravity-
fed irrigation has assisted in the development of
managed droughts for screening wheat lines at
CIMMYT (Trethowan et al., 2005). The potential
also exists to develop large rain-out shelters with
the capacity for controlling water availability.
Such a system has been established using drip
irrigation for large-scale screening of rice in China
(Liu et al., 2006).
2008; Rebetzke et al., 2008a,b). Recognition of
this complexity by some breeding companies is
refl ected in a change from selection for yield
under drought to selection for greater water-use
effi ciency and subsequently greater yield for
each additional unit of water used (Passioura
2006a).
Why should a genetic approach to improving
performance under drought be adopted? Growers
currently implement a range of agronomic prac-
tices, such as improved tillage systems or use of
break crops to increase WUE and yield. However,
these practices may bring extra risk and potential
cost to the grower, particularly as growers target
the maximal WUE (Fig. 11.4). This is because
many of these practices incur fi nancial costs that
may not be recovered if seasonal conditions dete-
riorate in dryland environments. Despite the
potential for greater return, these costs expose
growers to greater risks and the likelihood of
reduced return. Genetic opportunities exist to
increase WUE (Richards et al., 2002) with little
additional cost to the grower.
It is intuitively appealing to consider yield in a
physiological framework. An accumulation of
research knowledge underpins the integration of
biological processes contributing to fi tness or
yield in droughted environments. Further, many
features of physiological traits make them
Physiological breeding
The literature on eco-physiology of natural plant
communities commonly refers to the ability of a
plant to survive under water limitation as drought
resistance or tolerance. Indeed, natural communi-
ties have evolved many mechanisms that allow
plants to survive under conditions of severe water
defi cit (Passioura 2002). Under managed crop-
ping systems, “fi tness” is rarely described in
terms of plant survival. Rather, agricultural fi tness
refl ects productivity across a range of water-
availability patterns. In many production systems
growers make much of their income in favorable
years when yields are greatest. Similarly, in many
cropping regions, breeding companies receive
royalties on the quantity of grain delivered.
Hence, when there is strong incentive to boost
productivity in favorable environments or condi-
tions, there may be little incentive to focus spe-
cifi cally on selection for performance in droughted
environments. As indicated, selection for
improved drought tolerance is challenging. Pro-
ductivity under drought refl ects a culmination of
many complex processes (Tuberosa et al., 2002),
themselves under polygenic and epistatic control
of QTLs with small effects (Maccaferri et al.,
Water-limited yield potential
22 kg ha
-1
mm
-1
10 kg ha
-1
mm
-1
Current status
Investment risk
Fig. 11.4
Hypothesized increase in grower return (gross
margin per hectare) with increasing system input and subse-
quent investment risk. The relationship demonstrates that
increasing agronomic inputs (e.g., fertilizer, break crops to
reduce root disease) produce potential increases in water-use
effi ciency (WUE) and yield but at increasing fi nancial risk to
the grower. Genetic increases in WUE add minimal increase
to the cost to growers (after P. Carberry, pers. comm.).
