Differences in nutrient foraging among Trifolium subterraneum cultivars deliver improved P-acquisition efficiency
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Background and aims
The growth and root morphology responses to soil phosphorus (P) fertility by five cultivars of Trifolium subterraneum (a temperate annual pasture legume) were examined to assess whether differences in root morphology and/or root acclimation to P stress influenced P-acquisition by the clover varieties, or their critical P requirements (i.e. the rate of P supply needed for maximum shoot yield).
The clovers were grown as microswards in soil with P stratified in a topsoil layer to mimic growth conditions and soil P availability in a pasture. Yield and P content of shoots, and roots from the topsoil and subsoil layers was determined after 5 weeks growth in a controlled-environment cabinet. The lengths, diameters, and root hair lengths of nutrient foraging roots from the topsoil layer were quantified.
The shoot yield of the cultivars was similar when grown with high soil P fertility. However, the cultivars varied up to 1.5-fold in their ability to yield at low levels of soil P supply, and by 1.6-fold in their critical P requirements. All cultivars acclimated to low P soil by increasing root length density in the topsoil but those that yielded relatively well did so by maintaining a large root hair cylinder volume (i.e. they explored more soil) under low soil P conditions. This was associated with maintenance of dry matter allocations to topsoil roots and higher specific root lengths. Both factors assisted development of high root length density for nutrient foraging. Root acclimation responses to P were compared among the cultivars at equivalent relative shoot yields to assess the influence of plant P stress as a trigger for nutrient foraging. The least P-efficient cultivars slowed their allocation of dry matter to foraging roots at lower levels of P stress.
The results suggest variation within T. subterraneum for root proliferation and specific root length could be targeted in breeding for improved P-acquisition efficiency.
KeywordsCritical phosphorus requirement Root diameter Root hair length Subterranean clover
The authors thank Branka Culvenor for technical assistance. The work was predominantly funded by Meat and Livestock Australia (MLA) and Australian Wool Innovation Limited (AWI) as part of “Phosphorus-efficient legume pasture systems” (B.PUE.0104) with data analysis continuing under “RnD4P-15-02-016 Phosphorus Efficient Pastures” a project supported by funding from the Australian Government Department of Agriculture and Water Resources as part of its Rural R&D for Profit programme and from MLA, Dairy Australia, and AWI. M.R. was funded by an ARC Future Fellowship (FT140100103). L.B. was funded by an International Society of Root Research travel grant and supported by the Scottish Government through RESAS workpackage 3.3.
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