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Selection of wheat for high early vigour

Michael Zerner1, Gurjeet Gill1 and Greg Rebetzke2

1School of Agriculture, Food and Wine. The University of Adelaide, Roseworthy Campus, SA 5371.
2
CSIRO Plant Industry, PO Box 1600, Canberra, ACT 2601.
Email: michael.zerner@adelaide.edu.au

Abstract

A field experiment was undertaken in 2007 to evaluate plant traits associated with competitiveness against weeds. Wheat germplasm used in this study was identified from lines initially developed through following four cycles of recurrent selection for greater early vigour. Elite high vigour selections were crossed to donors of alternative, gibberellic acid-sensitive dwarfing genes (Rht8, Rht12 and Rht13) before top-crossing to commercial wheats, creating approximately 3000 lines. These lines were assessed for a range of plant traits, including plant height, early vigour, and canopy temperature (CT) during anthesis. Across all lines, mature plant height was normally-distributed across the population, ranging from 25 to 100 cm. Plant height showed a very strong negative linear relationship with CT (r2 = 0.88, P<0.001) indicating taller lines had cooler canopy temperatures. The cooler canopies of the taller lines may reflect a greater ability to extract soil water resulting from a larger root system. Despite the relationship with height, considerable variation in CT was found within different plant height groups. For example, some semi-dwarf lines had CT up to 3oC cooler than corresponding current commercial cultivars of similar height (<70 cm). Height was also strongly related to early vigour score (r2 = 0.88, P<0.001) where taller lines tended to have higher early vigour scores which in turn were strongly but negatively correlated with CT (r2 = 0.93, P<0.001). Genotypic variation in early vigour and CT indicates potential in selection of mid-height lines with higher early vigour and lower CT than barley (check) and current commercial cultivars.

Key Words

Competitive ability, early vigour, canopy temperature

Introduction

Wheat is the most widely grown cereal in southern Australia where herbicide resistance in weeds is increasing at an alarming rate. Comparisons with other winter cereals show wheat to be among the poorest competitors with weeds (Lemerle et al. 2001). Wheat varieties with greater competitive ability would assist to reduce weed growth and seed production and the potential for yield loss. Previous research has highlighted the importance of plant height and traits associated with early crop vigour for weed suppression and tolerance to weeds (Coleman et al. 2001; Rebetzke and Richards 1999; Seefeldt et al. 1999). Increasing plant height for improved competitiveness is not desirable due to the negative effects associated with reduced yield due to lower harvest index and losses due to lodging. The aims of this study were to breed wheats with reduced plant height while maintaining high early vigour and develop methodology to select these desired lines.

Methods

A field experiment was undertaken at Roseworthy, South Australia (34.3οS, 138.4οN) during 2007 to evaluate plant traits associated with competitiveness with weeds. This study used wheat germplasm initially developed through four cycles of recurrent selection for greater early vigour using elite lines from a Chuan-Mai 18/Vigour 18 doubled haploid population. Elite high vigour selections were crossed to donors of gibberellic acid-sensitive dwarfing genes (Rht8, Rht12 and Rht13) before top-crossing to commercial wheat parents. This created approximately 3000 lines, which were grown in 4 m single rows. These lines were assessed for mature plant height, visual early vigour scores (1-5 scoring system based on plant biomass and ground cover, 1-low vigour and 5-high vigour) and canopy temperature (CT) during anthesis. Early vigour was assessed twice, firstly at the 3-leaf growth stage and then at early tillering. CT was measured using an infra-red thermometer directly along the crop row and adjusted to account for temperature changes during the day by subtracting the mean CT of neighbouring barley (check) rows.

Results and Discussion

Large variation for plant traits such as, height, early vigour and CT was found across the population. Plant height showed a normal distribution across the population and ranged from 25 to 100 cm (Table 1). This large range in plant height occurred in response to the varying potency of the dwarfing genes introduced into the population (Rht8, Rht12 and Rht13).

Table 1. Distribution of height across the wheat germplasm and range of adjusted canopy temperatures (CT) and early vigour scores within each height group. CT was normalised by subtracting values of neighbouring barley checks. Commercial cultivars, Wyalkatchem and Yitpi had plant heights of 60 and 70 cm respectively, both averaging CT of -0.2C.

Plant Height

Number of lines

Canopy temperature

Early vigour score

   

Max

Min

Max

Min

(cm)

(oC)

(oC)

   

20

1

0.2

0.2

1.5

1.5

25

5

1.1

-1.1

3

1

30

15

3.5

-1.1

3

1

35

84

6.4

-1.5

3

1

40

218

5.7

-1.9

5

1

45

245

4.0

-3.2

5

1

50

243

7.4

-2.9

4.5

1

55

252

5.1

-3.1

4

1

60

388

10.3

-3.2

5

1

65

498

7.1

-3.0

5

1

70

393

6.0

-3.0

5

1

75

291

4.5

-3.2

5

1

80

119

2.7

-2.7

5

1.5

85

57

4.0

-2.8

5

1.5

90

9

1.9

-1.9

4

2

95

2

1.1

0.6

4.5

3

100

1

0.0

0.0

3

3

Figure 1. Relationship between visual early vigour scores and canopy temperature (CT) measured during anthesis. Mean values of adjusted CT have been used for each mean early vigour score. CT was normalised by subtracting values of neighbouring barley checks.

Plant height showed a strong negative linear relationship with CT (r2 = 0.88, P<0.001). This indicates taller wheat lines had cooler canopies, which may be a reflection of those lines having larger root systems enabling a greater ability to extract soil water. Tall wheat lines have been shown to possess larger root systems than semi-dwarfs (Siddique et al. 1990). Further studies into the root growth of these lines are still required to confirm this. Despite this general trend, there was considerable variation in CT within different plant height groups shown in Table 1. The height of current commercial cultivars ranged from 60-70 cm with an adjusted canopy temperature of -0.20C. Therefore at the equivalent height there were some wheat lines which had a CT up to 3C cooler than the commercial cultivars and possessed early vigour scores of 4.5-5 (Table 1).

Plant height was also strongly positively related to early vigour (r2 = 0.88, P<0.001), indicating taller lines tended to have higher early vigour. Despite this, again there was considerable variation within plant height groups, showing presence of high vigour in some short-mid height lines (Table 1). Early vigour scores were then strongly negatively correlated with CT (r2 = 0.93, P<0.001), shown in Figure 1. This indicates that cooler canopies are associated with high early vigour, indicating CT could be a useful tool for the selection of high vigour wheats. In combination with the genotypic variation observed with early vigour and CT within different plant height groups, there is potential to select mid-height wheat lines with higher early vigour and lower CT than current commercial cultivars. This relationship between early vigour and CT also indicates the benefits of vigorous early growth are carried on to later growth stages providing cooler crop canopies during anthesis, which may reduce crop stress during grain-fill.

Conclusion

Plant height was shown to have a strong negative relationship with CT and strong positive relationship with early vigour. Considerable variation in CT and early vigour was found within the low-mid height lines in the population. This provides opportunity for selection of mid-height lines that possess high early vigour and low CT. Such wheat lines are expected to be highly competitive with weeds and produce high yields.

References

Coleman, RK, Gill, GS and Rebetzke, GJ (2001). Identification of quantitative trait loci for traits conferring weed competitiveness in wheat (Triticum aestivum L.). Australian Journal of Agricultural Research 52, 1235-1246.

Lemerle, D, Gill, GS, Murphy, CE, Walker, SR, Cousens, RD, Mokhtari, S, Peltzer, SJ, Coleman R and Luckett DJ (2001). Genetic improvement and agronomy for enhanced wheat competitiveness with weeds. Australian Journal of Agricultural Research 52, 527-548.

Rebetzke, GJ and Richards, RA (1999). Genetic improvement of early vigour in wheat. Australian Journal of Agricultural Research 50, 291-301.

Seefeldt, SS, Ogg, AJ and Hou, Y (1999). Near-isogenic lines for Triticum aestivum height and crop competitiveness. Weed Science 47, 316-320.

Siddique, KHM, Belford, RK and Tennant, D (1990). Root:shoot ratios of old and modern, tall and semi-dwarf wheats in a Mediterranean environment. Plant and Soil 121,89-98.

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