Root Trait Importance for Phosphorus Acquisition Efficiency in Trifolium Subterraneum

Abstract

<p><i>Trifolium subterraneum</i> L. (subterranean clover) is the most widely grown annual pasture legume in the phosphorus (P) deficient soils of southern Australia. However, this legume has a high critical external P requirement (i.e. the amount of P required to achieve nearmaximum yield) and requires frequent applications of P fertiliser to achieve high productivity. In contrast, the critical external P requirements of constituent pasture grasses are much lower which means that the legume component of the pasture determines the level of P fertiliser application. Recent research has suggested that the difference in P requirement between <i>T. subterraneum</i> and constituent pasture grasses is associated with differences in root morphology. However, our understanding of the root traits that confer improved Pacquisition efficiency for <i>T. subterraneum</i> remains limited. Similarly, limited information is available regarding intraspecific variation in P-acquisition efficiency among <i>T. subterraneum</i> genotypes.</p> <p>Controlled-environment pot trials were used to investigate root morphological traits associated with P-acquisition efficiency among genotypes of <i>T. subterraneum</i>. The experiments were designed to determine: i) the intraspecific variation in P-acquisition efficiency among 26 genotypes and the root morphological traits important for P acquisition; ii) the effect of planting density on the proliferation of root length; iii) the effect of mycorrhizal colonisation on root acclimation responses to P deficiency; iv) the proliferation of root length in response to P stratification and the relative contribution of topsoil and subsoil roots to P acquisition; and v) the proliferation of root length in response to different P placement strategies and the expression of lateral root branching.</p> <p>There was significant intraspecific variation among the 26 genotypes of <i>T. subterraneum</i> to acquire P and yield when grown in the low-P soil treatment. In contrast, all genotypes were equally highly productive when grown in the high-P soil treatment. This meant that under P constraint, the relative shoot yields of the genotypes ranged between 38–71%. Variation in P acquisition was primarily associated with differences in root length, particularly in the nutrient-enriched topsoil layer (R² = 0.72, P < 0.001). Planting density differentially modified the root length proliferation of two contrasting genotypes which were selected based on differences in P-acquisition efficiency. Both genotypes proliferated root length in response to P stress (i.e. low-P soil) (by ~2-fold c.f. ~2.5-fold when grown at 6–12 plants pot^-1). However, the P-efficient genotype proliferated substantially more root length in response to space (i.e. low planting densities) than the P-inefficient genotype (by ~5-fold c.f.~2.5-fold when grown with 80 mg P kg soil^-1). Root responses to P deficiency were also modified by associations with mycorrhizal fungi. In particular, root acclimation to P deficiency was reduced by up to ~1.3-fold when plants were colonised by mycorrhizal fungi. Under these conditions, the difference in P-acquisition efficiency between the two contrasting genotypes was reduced when compared to the differences observed in the steampasteurised soil. In particular, both genotypes produced comparable root lengths and acquired similar amounts of P when grown in the different soil P distribution treatments. However, there remained marked differences in the relative contribution of subsoil roots to P acquisition from low-P soil (60% c.f. 74%) and the frequency of lateral root branching (77 branches m^-1 c.f. 45 branches m^-1) between the genotypes.</p> <p>Morphological traits such as root length, specific root length and root hair length varied among the genotypes of <i>T. subterraneum</i> in the controlled-environment pot trials. However, P acquisition was primarily associated with the development of root length in response to P deficiency. The development of root length was largely influenced by the concentration and volume of P enrichment, which demonstrated the potential for nutrient foraging by <i>T. subterraneum</i> plants. When genotypes were grown at equivalent planting densities and in the presence of mycorrhizal fungi, there was relatively limited variation in P-acquisition efficiency. However, there remained marked differences in the expression of root length proliferation which may determine the potential for P acquisition under certain environmental conditions. In particular, root elongation leading to increased subsoil exploration may improve P acquisition under dry surface soil conditions. Future breeding for P-acquisition efficiency within the <i>T. subterraneum</i> genome should focus on the nature of root length proliferation, while continuing to address agronomic traits such as biomass production and hardseededness.</p>