Root traits of sweet potato that assist with phosphorus uptake

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Ariel Bennett
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2 Root traits of sweet potato that assist with phosphorus uptake David Minemba PhD candidate School of Agriculture and Environment 208 Postgraduate Showcase: Frontiers in Agriculture UWA Institute of Agriculture

3 Sweet potato in Papua New Guinea Important staple: Staple food for 80-90% of the total popn. in PNG 500 kg/person/year 33 cultivars/household PNG has over 2000 cultivars Cash income: AU$20 million Food and nutrition security

Gleeson Arthur Villordon (LSU, USA) Technical support: Evonne

4 Support from Australia.. Supervisors: Australia Awards Megan Ryan Erik Veneklaas Deirdre Gleeson Arthur Villordon (LSU, USA) Technical support: Evonne Walker Daniel Kidd Rob Jeffery Bill and Rob Greg Cawthray Gatton Research Station Australia Plant Nutrition Trust award

5 Sweet potato.. Origin and movement Originate in South America large gene pool and richness in wild sp.. Papua New Guinea Moved to other parts of world around 00 to 200 AD. It followed three routes, Kumara line (Polynesian sailors), Camote line (Portuguese explorers), Batata line (Spainards) All three routes converged in PNG (2rd largest collection of genotypes outside S. America) South America (Source: Ballard et al., 2005, after Yen, 982a).

6 Global production Location Average yield (t/ha) World 3 Asia 20 Africa 5 Globally 00 million t China 70% area and 85% prodn. Yield potential 20 t/ha Oceania 6 South America 4 North America 25 Europe 3 Source: FAO 204 Sweet potato in Oceania and Africa are grown on marginal soils => lowest average yield compared to world average.

7 Queensland, Australia Why focus is on Phosphorus? Distribution of staple crops in PNG Sweet potato Sweet potato Source: Bourke and Harwood 2009

8 Distribution of soil taxonomy (USDA) Volcanic soil (Andisol) Highly weathered soil (Inceptisol) Source: Bourke and Harwood 2009

9 Soil chemical properties (Volcanic soil) Soil ph Acidic Olsen P (mg/kg) 2-4 Plant available P Colwell P (mg/kg) 2-5 Plant available P P buffering index Higher capacity to retain P P retention (%) Total P retained Soil composition Clay andloam High cation exchangecapacity Adsorption sites Allophane Stronglyreacts with Alions Sourced: Moody and Redcliff 986 P strongly reacts with Al hydrous oxide and form inorganic compounds fixed that are unavailable to plant roots

10 Common production system Compost mounding in high altitude (2800 m) areas of PNG Sweet potato is cultivated in PNG for over 500 years under P impoverished soil?.

11 Research questions. Do characters that influence P uptake, such as root morphology, arbuscular mycorrhizal fungal (AMF) colonisation, change among sweet potato cultivars? 2. Does low P affect the composition and amount of organic acids and AMF colonisation?

12 Research approach... Sourcing and multiplication of diverse sweet potato cultivars Australia/International Beauregard Northern Star PNG Whagi Besta Tambul Mai Marasunda Maraso

13 Glasshouse experiment. Low P soil from UWA Future farm P source; KH2P04 (soluble) Added based nutrients N, K, Cu, Zn and S 2. Two glasshouse experiments 7 P rates (soluble,) x 3 cultivars x 3 reps 2 P rates (soluble) x 6 cultivars + chickpea x 6 reps 3. Variables measured Root morphology (Winrhizo) Arbuscular mycorrhiza fungi (AMF) Carboxylates Tissues P concentration

14 The results Positive growth responses 0P 0P 20P 40P 60P 20P 360P Tambul Mai 35 days

15 Ability to explore soil for plant available P Root morphology (Exp) All cultivars had thick roots. All cultivars had few short root hairs Not consistent with good ability to explore soil for plant available P

16 Ability to explore soil for plant available P Dense colonisation by AMF (Exp) AMF colonisation of root length (%) Beauregard Tambul Mai Whagi Besta Phosphorus applied (mg P kg - soil)

17 Mechanism to release fixed P Exudation of carboxylates (Exp) First report of organic acid exudation in sweet potato Decrease in total carboxylate exudation with P supply Cultivar differences noted Carboxylate composition (nmol cm/hr) 8 7 Beauregard 8 7 Tambul Mai Phosphorus applied (mg P/kg)

18 Significant differences among cultivars (Exp2) Carboxylate composition (nmol/cm) Significant cultivar effect Beauregard vs PNG cultivars

19 Unexpected find Low shoot P concentration (Exp) Shoot P concentration remain steady until excess P applied Similar trend in root and stem P concentration This likely indicates good internal P use efficiency (mg P/g DM) Phosphorus applied (mg P/kg)

20 Consistent low shoot P concentration at zero P (Exp2) Shoot P concentration (mg/g DM) No cultivar difference

21 Key findings... Root morphology not consistent with good soil exploration ability for all cultivars. 2. All cultivars were colonised by AMF 3. All cultivars exuded carboxylates and notable difference between cultivars were observed 4. All cultivars showed low tissue P concentration at low P consistent with good internal P use efficiency

22 Main message This study identified three mechanisms that sweet potato uses to grow in soils that are inherently low in P. AMF colonisation (Plant available P) Exudation of carboxylates (Fixed P) Good internal P use efficiency Focus for future should be on selection and breeding for improved carboxylate exudation and modify root morphological traits

Concepts in Soil Fertility Jonathan Deenik Assistant Specialist, Soil Fertility Department of Tropical Plant and Soil Sciences

Soils of Hamakua Concepts in Soil Fertility Jonathan Deenik Assistant Specialist, Soil Fertility Department of Tropical Plant and Soil Sciences Outline Importance of Soils Concepts in Soil Fertiltiy -