Greater early vigour to improve agronomic water-use efficiency
To this point, this review has focused on improving agronomic water-use efficiency by manipulating leaf-level water-use efficiency, primarily through the use of 
13C. However, there are other strategies that also offer promise (reviewed by Richards et al., 2002
). As discussed earlier, an important strategy to improve agronomic water-use efficiency in cropping environments where the soil surface is frequently rewetted is to restrict water lost by evaporation from the soil surface. This maximizes crop transpiration and improves the ratio T/ET (Condon and Richards, 1993; Richards et al., 2002). For cereals, indeed any crop, a reduction in soil evaporation is most easily achieved through the rapid development of leaf area to shade the soil surface from direct solar radiation. Good stand establishment and vigorous early plant growth will both contribute to rapid leaf area development.
Good stand establishment is best achieved by plants that reach the soil surface quicker if seed is sown relatively shallow, and that reach the soil surface much more consistently if seed is sown deep, such as when farmers are seeding into moisture below a dry topsoil. In wheat, the widely-used GA-insensitive dwarfing genes Rht-B1b (Rht1) and Rht-D1b (Rht2) have had a major impact on agronomic water-use efficiency by improving HI and crop standability. The latter feature has been most important in irrigated cropping systems, but the same genes strongly inhibit the expression of long coleoptiles that may be an important attribute for rain-fed systems (Ellis et al., 2004). Alternative, GA-sensitive dwarfing genes exist in wheat that allow the expression of much longer coleoptiles (from tall wheats) in plants with semi-dwarf stature and high HI (Rebetzke et al., 1999).
Early vigour (fast leaf area development) is an important adaptation of barley and durum wheat to terminal drought in Mediterranean environments (Van Oosterum and Acevedo, 1992; Lopez-Castaneda and Richards, 1994; Villegas et al., 2000) because it improves the ratio T/ET and encourages growth when evaporative demand is low, giving higher A/T. Traits important for vigorous early growth in cereals were identified by comparing barley with bread wheat (Lopez-Castaneda et al., 1995). This comparison showed that large embryo size, high SLA, and growth of a large coleoptile tiller were important attributes of barley, which is characterized by very high early vigour, but these traits were lacking in semi-dwarf wheats. Extensive screening of a collection of tall wheats revealed excellent sources of each of these traits (Richards and Lukacs, 2002). In a targeted breeding programme, these were combined to produce a new parental line with early leaf area growth double that of current Australian semi-dwarf varieties (Richards et al., 2002). High-vigour backcross lines with this parent as the vigour donor out-yielded low-vigour lines from the same population by up to 13% in favourable Mediterranean-type environments (c. 280–450 mm in-season rainfall), but there was no difference in yield in drier environments with less than 250 mm in-season rainfall (Table 1; Botwright et al., 2002). The sets of vigour backcross lines grown by Botwright et al. (2002) were still much less vigorous than the tall vigour donor, probably because the recurrent parent contained a GA-insensitive dwarfing gene. The high vigour traits of large embryo, large coleoptile tillers, and greater SLA are now being combined in GA-sensitive semi-dwarf backgrounds so as to achieve early vigour much more like barley (Richards et al., 2002). Field-based screening for early leaf area growth is subject to large genotypexenvironment interaction and hence poor heritability (Annicchiarico and Pecetti, 1998; Rebetzke and Richards, 1999). To counter this, Richards et al. (2002) advocated selection using plants grown in more repeatable conditions and the use of leaf width as an indirect selection tool for embryo size. This is because of the high heritability of leaf width and its strong association with early leaf area.
Combining greater early vigour and higher A/TThe outcomes of the simulation study summarized earlier (
Fig. 5) indicated that combining greater early vigour and higher
A/
T may give substantially greater average yield gains in many rain-fed environments, but especially in environments with large temporal variability in rainfall. Is such a combination of traits possible? High early vigour in cereals appears to be partly associated with high
SLA (Lopez-Castaneda
et al., 1995
; Rebetzke
et al., 2004
). To the extent that high
SLA is reflected in low photosynthetic capacity (via less N per unit leaf area), then high
SLA may also be reflected in higher
13C. This suggests that if high vigour and low-
13C are to be successfully combined, then a tendency to higher
SLA will need to be avoided during selection. This may be desirable for other reasons.
SLA has relatively low heritability in cereals (Rebetzke
et al., 2004
), so its value as a selection tool for high early vigour may be limited. Also, simulation modelling of the impact of selecting for higher
SLA in wheat indicates that maintaining N supply may be an important prerequisite for sustaining yield gains achieved via this trait (Asseng
et al., 2003
). The associations between early vigour in cereals and traits other than high
SLA, such as embryo size and coleoptile tiller appearance (Liang and Richards, 1994
), indicate that there may be other ways to achieve higher early vigour that could allow co-selection for lower values of
13C.
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