The quest for acid-tolerant lucerne Graham Centre Sheep Forum Richard Hayes, Research Scientist
Why do we want Acid-tolerant (AT) lucerne? Soils of SNSW are generally acidic – 74% of ~ 4700 soils in SW NSW had pH Ca ≤ 5.0 (Scott et al . 2007) Lucerne is the only perennial legume broadly adapted across this region Lucerne is sensitive to acid soils NSW represents ~ 50% of domestic lucerne sales Acid soil tolerance has proven to be useful in other forage and crop species More than a dozen scientific articles exist internationally describing the need for AT lucerne Still, no AT lucerne cultivar exists today
Lucerne biomass (kg/ha) on an acid soil with and without surface applied lime (L) 6000 Lucerne Luc + L 5000 4000 3000 2000 1000 0 Spr 04 Aut 05 Win 05 Spr1 05 Spr2 05 Aut 06 Win 06 Spr 06 Sum 07 Aut 07
Challenges in developing acid soil tolerance Soil is highly variable – Up to 1.5 pH unit difference in 1 m 2 soil A range of acid soil-related factors – H + toxicity – Al toxicity – Mn toxicity – Δ nutrient availability (eg Mo deficiency) Other factors affecting plant response eg topography, soil depth, water holding capacity etc
Challenges in developing AT lucerne We are starting off a low base – Devine et al . (1976) estimated that Al tolerance existed in only 2% of a lucerne population Lucerne is sensitive to a Photo: B. Scott range of acid soil factors – Eg Mn & Al toxicity
Challenges in developing AT lucerne Lucerne genetics – Is an autotetraploid – Is almost an obligate out-crosser – Need to maintain genetic diversity to avoid in-breeding suppression Therefore, rate of genetic Photo: B. Dear improvement is slower
Challenges in developing AT lucerne Lucerne is a legume Root nodule bacteria (RNB) is commonly more sensitive to acidity than the plant Therefore, for genuine acid soil tolerance you need: – AT plant – AT RNB – The AT plant to be compatible with the RNB
Summary – why doesn’t AT lucerne exist? A range of acid soil factors to consider Sensitivity to multiple acid soil factors Starting from a low base Need to select a large number of elite individuals to develop a robust population Need to consider the RNB Need to ensure the elite RNB is compatible with the elite plant germplasm In a context where public plant breeding programs are not fashionable
Recent Australian research Screening in high Al solution culture; (Scott et al . 2008)
Recent Australian research Screening in high Al solution culture; (Scott et al . 2008) Sensitive Tolerant
Source; (Scott et al . 2008)
Population equal to or greater than nominated root length (%)
Population equal to or greater than nominated root length (%)
Source; (Scott et al . 2008)
Recent Australian research Seedling validation in high Al soil; (Hayes et al . 2011)
Recent Australian research a) Experiment 2; pH 4.34 c) Experiment 3; pH 4.48 100 Proportion of population with tap root length equal to or greater than nominated length (%) 80 60 40 20 0 b) Experiment 2; pH 5.26 d) Experiment 3; pH 5.36 100 80 60 40 20 0 0 50 100 150 200 0 50 100 150 200 Tap root length (mm) Fig. 2 . The distribution of length of seedling tap roots exhibiting evidence of increased Al tolerance ( ● ) and 3 populations exhibiting evidence of increased seedling vigour ( ○ ) compared with individuals from 6 control populations ( ▲ ).
a) Experiment 2; pH 4.34 100 Proportion of population with tap root length equal to or greater than nominated length (%) 80 60 40 20 0 b) Experiment 2; pH 5.26 100 80 60 40 20 0 0 50 100 150 200 Tap root length
a) Experiment 2; pH 4.34 pH 4.34 Ca 100 Proportion of population with tap root length equal to or greater than nominated length (%) 80 60 Al tolerant populations 40 20 0 b) Experiment 2; pH 5.26 100 80 60 40 20 0 0 50 100 150 200 Tap root length
a) Experiment 2; pH 4.34 pH 4.34 Ca 100 Proportion of population with tap root length equal to or greater than nominated length (%) 80 60 Al tolerant populations 40 20 0 b) Experiment 2; pH 5.26 100 pH 5.26 Ca 80 Increased seedling vigour 60 40 20 0 0 50 100 150 200 Tap root length
Table 1 . Mean seedling root length (mm) for a pair comparison under low and high lime rates Population pH 4.34 pH 5.26 Sardi 7 58.7 110.6 CRCSA 34- 81.9 107.6 36
Table 1 . Mean seedling root length (mm) for a pair comparison under low and high lime rates Population pH 4.34 pH 5.26 Sardi 7 58.7 110.6 CRCSA 34- 81.9 107.6 36 40% increase
Rhizobia research
Rhizobia research Photo: R. Ballard
Rhizobia research Photo: R. Ballard
Fig. 3. Effect of inoculation treatment on the percentage of SARDI ten lucerne seedlings forming nodules in solution culture (pH 4.8), at 11 days after inoculation. 100 80 Plants with nodules (%) 60 40 20 0 NO RHIZOBIA RRI 128 SRDI 672 SRDI 722 SRDI736 Courtesy: R. Ballard, SARDI, Adelaide
Fig. 3. Effect of inoculation treatment on the percentage of SARDI ten lucerne seedlings forming nodules in solution culture (pH 4.8), at 11 days after inoculation. From Book Book, NSW 100 80 Plants with nodules (%) 60 40 20 0 NO RHIZOBIA RRI 128 SRDI 672 SRDI 722 SRDI736 Courtesy: R. Ballard, SARDI, Adelaide
Progress to date: Developed a methodology for mass recurrent selection of seedlings in high Al solution culture Demonstrated up to 40 % increased tap root growth at pH 4.3 Have selected a replacement RNB strain for lucerne adapted for acid soils Have made further plant selections to develop an elite lucerne/RNB combo, for commercial release from 2015
Challenges ahead Mn toxicity No selection for Mn toxicity has yet taken place within elite Al lucerne Populations selected for enhanced growth under high Al are unlikely to be tolerant of Mn toxicity
Binalong Mn concentration (mg/kg) of lucerne herbage Plot B2 7.1 2500 20.2 Plot B3 5.4 Plot B4 14.3 44.1 Mean Gerogery Critical level 50.4 2000 11.8 Plot G1 4.4 2.5 Plot G2 6.3 Plot G3 Critical threshold Plot G4 1500 47.7 22.3 53.6 26.1 1000 22.7 34.3 42.9 500 29.9 51.3 12.7 13.3 10.5 38.7 19.7 13.3 5.7 4.9 0 33.8 23.2 37.3 11.8 Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan (Hayes et al . 2012)
Mn toxicity Reduced shoot growth by 20% Reduced lucerne seedling survival by up to 35% Reduced root length by 40% Therefore, has the potential to offset all the existing gains delivered through enhanced Al tolerance
Opportunities 1. Select for Mn tolerance in lucerne populations 2. Greater use of gene markers to further enhance selection techniques
Conclusions Al tolerant lucerne is coming Likely to be accompanied by RNB with enhanced performance on acid soils New products unlikely to be tolerant of Mn toxicity When those products become available, be sure to use them in conjunction with lime
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