Adam Stefanski

Root morphological traits that determine phosphorus-acquisition efficiency and critical external phosphorus requirement in pasture species

Annual pasture legume species can vary more than 3-fold in their critical external phosphorus (P) requirement (i.e. P required for 90% of maximum yield). In this work we investigated the link between root morphology, P acquisition and critical external P requirement among pasture species. The root morphology acclimation of five annual pasture legumes and one grass species to low soil P availability was assessed in a controlled-environment study. The critical external P requirement of the species was low (Dactylis glomerata L., Ornithopus compressus L., Ornithopus sativus Brot.), intermediate (Biserrula pelecinus L., Trifolium hirtum All.) or high (Trifolium subterraneum L.). Root hair cylinder volumes (a function of root length, root hair length and average root diameter) were estimated in order to assess soil exploration and its impact on P uptake. Most species increased soil exploration in response to rates of P supply near or below their critical external P requirement. The legumes differed in how they achieved their maximum root hair cylinder volume. The main variables were high root length density, long root hairs and/or high specific root length. However, total P uptake per unit surface area of the root hair cylinder was similar for all species at rates of P supply below critical P. Species that maximised soil exploration by root morphology acclimation were able to prolong access to P in moderately P-deficient soil. However, among the species studied, it was those with an intrinsic capacity for a high root-hair-cylinder surface area (i.e. long roots and long root hairs) that achieved the lowest critical P requirement.

Differences in nutrient foraging among Trifolium subterraneum cultivars deliver improved P-acquisition efficiency

Background and aimsThe 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).MethodsThe 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.ResultsThe 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.ConclusionsThe results suggest variation within T. subterraneum for root proliferation and specific root length could be targeted in breeding for improved P-acquisition efficiency.