Critical phosphorus requirements of Trifolium species: The importance of root morphology and root acclimation in response to phosphorus stress.

Differences in root morphology and acclimation to low-phosphorus (P) soil were examined among eight legume species from the Trifolium Section Tricocephalum to understand how these root attributes determine P acquisition. Ornithopus sativus was included as a highly P-efficient benchmark species. Plants were grown as microswards in pots with five rates of P supplied in a topsoil layer to mimic uneven P distribution within a field soil profile. Topsoil and subsoil roots were harvested separately to enable measurement of the nutrient-foraging responses. Critical P requirement (lowest P supply for maximum yield) varied over a threefold range, reflecting differences in root morphology and acclimation of nutrient-foraging roots to P stress. Among the species, there was a 3.2-fold range in root length density, a 1.7-fold range in specific root length, and a 2.1-fold range in root hair length. O. sativus had the lowest critical P requirement, displayed a high root length density, the highest specific root length, and the longest root hairs. Acquisition of P from P-deficient soil was facilitated by development of a large root hair cylinder (i.e. a large root-soil interface). This, in turn, was determined by the intrinsic root morphology attributes of each genotype, and the plasticity of its root morphology response to internal P stress. Root acclimation in low-P soil by all species was mostly associated with preferential allocation of mass to nutrient-foraging roots. Only O. sativus and four of the Trifolium species adjusted specific root length beneficially, and only O. sativus increased its root hair length in low-P soil.

Plants in constrained canopy micro-swards compensate for decreased root biomass and soil exploration with increased amounts of rhizosphere carboxylates.

Root traits related to phosphorus (P) acquisition are used to make inferences about a species' P-foraging ability under glasshouse conditions. However, the effect on such root traits of constrained canopy spread, as occurs in dense pasture swards, is unknown. We grew micro-swards of Trifolium subterraneum L. and Ornithopus compressus L. at 15 and 60mg kg-1 soil P in a glasshouse. Shoots either spread beyond the pot perimeter or were constrained by a cylindrical sleeve adjusted to canopy height. After 8 weeks, shoot and root dry mass (DM), shoot tissue P concentration, rhizosphere carboxylates, arbuscular mycorrhizal (AM) fungal colonisation, total and specific root length (TRL and SRL respectively), average root diameter (ARD) and average root hair length (ARHL) were measured. In all species and treatments, constrained canopy spread decreased root DM (39-59%), TRL (27-45%) and shoot DM (10-28%), and increased SRL (20-33%), but did not affect ARD, ARHL and AM fungal colonisation. However, shoot P concentration and content increased, and rhizosphere carboxylates increased 3.5 to 12-fold per unit RL and 2.0- to 6.5-fold per micro-sward. Greater amounts of rhizosphere carboxylates when canopy spread was constrained appeared to compensate for reduced root growth enabling shoot P content to be maintained.

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.