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Establishment and root morphology of eight diverse Lucerne populations in a low rainfall, South Australian, environment

Erica Marshall, Alan Humphries, Eric Kobelt, Trevor Rowe and Geoff Auricht

South Australian Research and Development Institute (SARDI), PO Box 397 Adelaide, SA 5001
Email: marshall.erica@saugov.sa.gov.au

Abstract

A field trial was established in 2007 at Coomandook, SA, to look at the impact of drought conditions on the establishment and survival of 8 diverse lucerne (Medicago sativa spp.) populations. Winter active and semi-dormant populations L775 and SARDI Five established with a greater density than the highly winter active cultivar Siriver. The root morphology of each population was assessed by carefully excavating surviving plants to determine if root morphological traits can be attributed to survival over the first summer. SARDI Five and varia populations displayed an increased taproot diameter, taproot area, fine root area, number of laterals, and number of new crown buds, leading to larger plants. Conversely, the falcata and caerulea populations previously thought to be drought tolerant had poor establishment, a very low root dry weight, taproot diameter, and number of new crown buds in this study.

Key Words

Alfalfa, caerulea, varia, falcata, dryland

Introduction

Lucerne (Medicago sativa L.) is a drought tolerant perennial pasture legume that can prevent the occurrence of dryland salinity by extracting water from deep in the soil profile, reducing deep drainage and the movement of mobilised salts in the landscape. Lucerne is valuable in many farming systems due to its ability to produce high quantities of summer forage. As a pasture it can be grown in rotation with cereal crops (Holford and Doyle 1978; Whitfield et al. 1992) to increase forage (Latta et al. 2002), livestock (Crawford and Macfarlane 1995), and grain production (Latta et al. 2001, 2002) relative to rotations with annual pastures.

In South Australia there are approximately 300,000 ha of lucerne grown each year, with the majority of this in the Upper SE region of the state (ABARE 2006). Improving production of lucerne in this area has been a focus of the SARDI lucerne breeding program for over 3 decades (Auricht 2001), with recent research on the adaptation and potential of a diverse range of Medicago sativa spp. germplasm (Humphries et al. 2008). In this study, populations thought to have varying levels of drought tolerance and drought sensitivity (unpublished data) were sown in a low rainfall environment to monitor differences in establishment and identify root morphological traits associated with those differences.

Methods

A trial was sown at Coomandook SA, on the 6th of July 2007, on a duplex soil with sand over limestone (at 1m), to look at the establishment and root morphology of a diverse group of lucerne populations. Seed of the entries was inoculated by coating with Sinorhizobium meliloti, and sown in 10m2 plots (5x2m) at 0.15m row spacings at 5kg/ha through a small plot cone seeder. The trial consisted of 3 commercial lucerne varieties and 5 experimental populations sown in a row-column design with 4 replicates. The winter activity and background of the entries are shown in Table 1.

Plant density was assessed by counting the number of plants in 3, 0.75m2 square quadrats permanently positioned with fixed pegs. An ‘initial’ plant density was measured in September 2007 and a ‘final establishment’ density was measured in March 2008. These pegs were used as marker points so plant counts could be taken from the same place each time to measure the persistence over time for each variety.

Table 1. The winter activity and background of lucerne entries sown at Coomandook, South Australia, in 2007.

Population

Winter Activity

Background

falcata

1

M.s. spp. falcata accession, SA43502

caerulea

1

M.s. spp. caerulea accession from low rainfall environment in Azerbaijan, SA42382

varia

2

M.s. spp. varia accession with creeping rooted trait, SA43158/ PA25

A7

3

Breeders line with long-term persistence

SARDI Five

5

Improved cultivar with broad adaptation

Hunter River

5.5

Australian landrace

L775

7

Breeders line developed from plants surviving drought and erosion on a sand hill at Tintinara, SA

Siriver

9

Highly winter active cultivar, potentially less tolerant to drought

In April 2008, 15 plants from each plot were excavated to a depth of 30cm for measurements. The taproot diameter (1cm below the crown) of each plant was measured using digital electronic callipers, and the number of lateral roots greater than 1mm in diameter was counted. The number of new crown buds was also counted, which included buds and new shoots arising from the crown. Finally, the plants were photographed, and the top 20cm of the root system was removed and oven dried for determination of dry weight (DW).

MultiSpec W32 software was used to calculate the area of the root system for each individual plant. MultiSpec is a program used for raster based image analysis, and can calculate the number of pixels in a photograph of different clusters (i.e. different colour classes-in this case separating taproot from fine roots and background.) The pixels could then be calculated into an area. From this, we can compare the root areas for each plant.

Statistical analysis was performed on all data using linear spatial mixed models to determine the relationship between plant density and root morphological traits.

Results

The Coomandook site received below average rainfall for 2 of the first 4 months after sowing, with a total of 152 mm rainfall in July-October, compared to an 18-year average of 177 mm (pers.comm. Andrew Hansen, 2008). The month before sowing also received substantially less rainfall, with only 26.5 mm compared to the average of 90 mm. Over the first summer, there was one significant rainfall event, with 39 mm on the 22nd of December.

The initial plant density ranged from 55-140 plants/m2 as shown in Figure 1. By March 2008, the plant densities had decreased for all trial entries, to between 19-34 plants/m2. The highly winter active control, ‘Siriver’ had a lower initial density than caerulea and A7 populations, and lower final establishment density than all other populations except for caerulea and falcata. The caerulea population declined from an initial density of 136 to 12 plants/m2, partially due to burial from moving sand. There was an infestation of spotted alfalfa aphid, (Therioaphis maculata Buckton) which caused some damage to these plots.

SARDI Five and the varia populations had the highest number of new crown buds, and the greatest taproot diameter (Figure 2). The falcata and caerulea populations both had very low root DW, taproot diameter, and number of new crown buds. The number of laterals >1mm in diameter was highest for L 775, varia, SARDI Five and Siriver (Figure 2). The larger root systems of these plants accounted for the high correlation between these traits and final establishment density measured in March 2008, as shown in Table 2. Table 2 also shows the correlations between the other root morphological traits measured.

Figure 1. Comparison of initial (dot points, September 2007) and final, plant densities (bars, March 2008) of 8 lucerne populations sown at Coomandook, South Australia in 2007. Error bars show the 5% lsd.

Root DW was strongly positively correlated with all other root morphological traits measured, particularly taproot diameter, fine root area and taproot area. Whilst these traits aren’t completely independent, larger root systems were also strongly correlated with a high number of crown buds. Therefore, plants with a large root system developed in the first 6 months of establishment are more likely to respond quickly to rainfall and be more productive in their first year.

Figure 2. Root morphological characteristics of 8 lucerne populations sown at Coomandook, South Australia, including average root DW in the top 20cm of soil (grams, solid bar), taproot diameter (mm, light grey bar), the number of laterals >1mm (open bar), and the number of crown buds (hatched bar) for each population. The error bars show the 5% lsd.

Root DW in the top 0.2m of soil had the highest correlation (r=0.72) with plant density. This could mean that the varieties that had better plant establishment (i.e. more plants per square metre) also had a greater ability to produce root biomass in the drought conditions.

Table 2. Correlations (r) between various root morphological characters and establishment density of 8 lucerne populations sown at Coomandook, South Australia.

 

Root
DW

Taproot
Diameter

Laterals
>1mm

Crown
Buds

%
Fine Roots

Fine Root
Total Area

Taproot
Area

Total
Root Area

Density
Mar 08

Root DW

1.00

0.98

0.84

0.72

0.76

0.91

0.91

0.92

0.72

Taproot Diameter

0.98

1.00

0.92

0.85

0.79

0.93

0.90

0.93

0.69

Laterals >1mm

0.84

0.92

1.00

0.85

0.84

0.95

0.94

0.97

0.54

Crown Buds

0.72

0.85

0.85

1.00

0.84

0.93

0.79

0.90

0.55

% Fine Roots

0.76

0.79

0.84

0.84

1.00

0.92

0.77

0.87

0.69

Fine root Total Area

0.91

0.93

0.95

0.93

0.92

1.00

0.92

0.98

0.56

Taproot Area

0.91

0.90

0.94

0.79

0.77

0.92

1.00

0.97

0.53

Total Root Area

0.92

0.93

0.97

0.90

0.87

0.98

0.97

1.00

0.55

Density Mar 08

0.72

0.69

0.54

0.55

0.69

0.56

0.53

0.55

1.00

Conclusion

SARDI Five and varia populations had higher root weights and numbers of crown buds, and these traits were highly correlated with establishment (final density). In this dry environment, larger plants have better establishment. Plants with a high root DW also have a higher taproot diameter and root surface area, as well as a greater number of crown buds with potential to develop new shoots. Medicago sativa sp. caerulea and falcata plants, which were collected from dry environments, have smaller root systems, and have not established well at this site.

References

ABARE (2006). Australian Crop and Livestock Report-October 2006, pp 1. online, URL: www.abareconomics.com/publications_html/crops/crops_06/cr_drought_06.pdf accessed April 2008, ABARE economics.

Auricht GC and. Kobelt ET (2001). Advances in lucerne breeding: 42nd Annual Conference Proceedings, Grassland Society of Victoria Inc., Mount Gambier, South Australia.

Crawford MC and Macfarlane MR (1995). Lucerne reduces soil moisture and increases livestock production in an area of high groundwater recharge potential. Australian Journal of Experimental Agriculture 35: 171-180.

Holford ICR and Doyle AD (1978). Effect of grazed lucerne on the moisture status of wheat growing soils. Aust. J. Exp. Agric. 18: 112-117.

Humphries AW and Zhang XG, et al. (2008). Persistence of diverse lucerne (Medicago sativa sspp.) germplasm in a range of acidic and alkaline soils in South and Western Australia. Australian Journal of Agricultural Research 59(2): 139-148.

Latta RA, Cocks PS and Matthews C. (2002). Lucerne pastures to sustain agricultural production in south-western Australia. Agricultural Water Management 53: 99-109.

Latta RA, Blacklow LJ and Cocks PS. (2001). Comparative soil water, pasture production, and crop yields in phase farming systems with lucerne and annual pasture in Western Australia. Australian Journal of Agricultural Research 52: 295-303.

MultiSpec W32 (2007) online, URL: http://cobweb.ecn.purdue.edu/~biehl/MultiSpec/

Whitfield DM, Newton PJ, et al. (1992). Comparative Water Use of Dryland Crop and Pasture Species: Proceedings of the 6th Australian Society of Agronomy, Armidale, The Australian Society of Agronomy.

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