Selecting plants with improved root hair growth is a key strategy for improving phosphorus-uptake efficiency in agriculture. While significant inter- and intraspecific variation is reported for root hair length, it is not known whether these phenotypic differences are exhibited under conditions that are known to affect root hair elongation. This work investigates the effect of soil strength, soil water content (SWC) and soil particle size (SPS) on the root hair length of different root hair genotypes of barley. The root hair and rhizosheath development of five root hair genotypes of barley ('Hordeum vulgare' L.) was compared in soils with penetrometer resistances ranging from 0.03 to 4.45 MPa (dry bulk densities 1.2-1.7 g cm⁻³). A "short" (SRH) and "long" root hair (LRH) genotype was selected to further investigate whether differentiation of these genotypes was related to SWC or SPS when grown in washed graded sand. In low-strength soil (<1.43 MPa), root hairs of the LRH genotype were on average 25 % longer than that of the SRH genotype. In high-strength soil, root hair length of the LRH genotype was shorter than that in low-strength soil and did not differ from that of the SRH genotype. Root hairs were shorter in wetter soils or soils with smaller particles, and again SRH and LRH did not differ in hair length. Longer root hairs were generally, but not always, associated with larger rhizosheaths, suggesting that mucilage adhesion was also important. The root hair growth of barley was found to be highly responsive to soil properties and this impacted on the expression of phenotypic differences in root hair length. While root hairs are an important trait for phosphorus acquisition in dense soils, the results highlight the importance of selecting multiple and potentially robust root traits to improve resource acquisition in agricultural systems.

Tropical pasture systems are typically dominated by C₄ grasses growing on nitrogen (N) deficient soils. Under these conditions, N₂-fixing legumes should have a competitive advantage, yet low legume contents are often reported in these systems. This work investigates whether below-ground competition for phosphorus (P) is limiting the ability of legumes to compete in swards of C₄ grasses when grown in a sand matrix. The external P requirement of a subtropical legume (butterfly pea, 'Clitoria ternatea' L.) and a C₄ grass (buffel grass, 'Cenchrus ciliaris' L.) were initially determined in a P-response experiment. Four rates of P (4.6-78.2 mg P kg-¹ of Colwell P) were subsequently selected to investigate the growth response of the butterfly pea when grown with and without competition from a sward of N-deficient buffel grass. Shoot dry matter was determined over successive cuts and P uptake determined at the final harvest at 72 days. Buffel grass dominated the mixed swards and reduced the shoot dry matter production of the butterfly pea by >50% relative to the pure swards. A significant difference in the soil P response curve and shoot P uptake of butterfly pea was not detected between pure swards and those with competition from buffel grass. The ability of C₄ grasses to acquire and convert resources (i.e. light, water and nutrients) more efficiently into shoot dry matter is likely to be a major factor resulting in grass-dominated pastures in tropical systems.

Root hairs are a key trait for improving the acquisition of phosphorus (P) by plants. However, it is not known whether root hairs provide significant advantage for plant growth under combined soil stresses, particularly under conditions that are known to restrict root hair initiation or elongation (e.g. compacted or high-strength soils). To investigate this, the root growth and P uptake of root hair genotypes of barley, 'Hordeum vulgare' L. (i.e. genotypes with and without root hairs), were assessed under combinations of P deficiency and high soil strength. Genotypes with root hairs were found to have an advantage for root penetration into high-strength layers relative to root hairless genotypes. In P-deficient soils, despite a 20% reduction in root hair length under high-strength conditions, genotypes with root hairs were also found to have an advantage for P uptake. However, in fertilized soils, root hairs conferred an advantage for P uptake in low-strength soil but not in high-strength soil. Improved root-soil contact, coupled with an increased supply of P to the root, may decrease the value of root hairs for P acquisition in high-strength, high-P soils. Nevertheless, this work demonstrates that root hairs are a valuable trait for plant growth and nutrient acquisition under combined soil stresses. Selecting plants with superior root hair traits is important for improving P uptake efficiency and hence the sustainability of agricultural systems.

Mycorrhizal symbioses are considered the most important mutualism on Earth. This symbiosis occurs between plant roots and mycorrhizal fungi in the rhizosphere. Mycorrhizal hyphae facilitates the exploration of a greater volume of soil by plant roots and increased water and nutrient uptake, especially phosphorus (P) which is immobile in the soil. This consequently improves plant growth, particularly at early growth stages. Cotton (Gossypium hirsutum L.) is a mycorrhizal dependant crop and mycorrhization clearly improves early growth and nutrient uptake. Although mycorrhizal fungi also alleviate abiotic stresses, adverse abiotic soil condition might restrict mycorrhizal colonisation and associated nutrient uptake. One possible adverse environmental condition is elevated soil sodicity. In Australia the majority of cotton growing regions are sodic. Sodicity creates adverse physical and chemical conditions, including waterlogging, hard-setting, high bulk density, high pH, and high soil solution sodium (Na), which may affect mycorrhizal colonisation of cotton plants.

This thesis aimed to investigate the percentage of root length colonised by mycorrhizae and nutrient uptake of cotton in a range of naturally sodic soils. This thesis also attempted to estimate the relative hyphal contribution to early P uptake of cotton in sodic soil conditions.

A series of glasshouse experiments was conducted in order to assess mycorrhizal colonisation and a number of different colonisation techniques in moderately (ESP 10– 15) and highly (ESP>15) sodic soils. Standard techniques to inoculate cotton plants in sodic soils were unsuccessful until live hyphal cultures were introduced.

In a separate glasshouse experiment, changes in mycorrhizal colonisation and nutrient uptake of cotton in a range of naturally non-sodic and low-sodic soils from cotton production areas in southern Queensland and northern New South Wales, with different exchangeable Na percentages (ESP) (ranged between 1.4 and 9.8) was investigated. Linear mixed model analysis showed minimal effects of sodicity, when ESP was less than 10, on mycorrhizal colonisation, associated plant growth and nutrient uptake. Principle component and regression analysis showed that other sources of variation including soil pH and soil extractable P, rather than sodicity, might drive cotton colonisation in Vertosols with low to moderate ESP. The colonisation percentage was positively linearly correlated with P, Mg, and Zn uptake of cotton plants.

An isotope experiment was established to assess the mycorrhizal colonisation and nutrient uptake of cotton plants under highly (ESP 21) and less (ESP 7) sodic soil conditions with two rates of applied P. The relative hyphal contribution to P uptake was quantified using dual isotope labelling techniques (³²P and ³³P). Root colonisation and P uptake of mycorrhizal cotton plants reduced by 16% and 20%, respectively, in highly sodic soil as compared to plants in low sodic soil, however, the relative proportion of P delivered via hyphal pathways (³²P from root-free hyphal compartment) was similar. Under high P conditions, the relative increase in the proportion of ³³P (root + hyphae compartment) taken up by inoculated plants was greater in the low sodic soil relative to the high sodic soil. Mycorrhization improved early seedling vigour, and nutrient uptake.

Overall, these results confirmed that early growth and nutrient uptake of cotton, especially P and Zn, benefits from mycorrhizal association in both non-sodic and sodic soils. However, mycorrhizal colonisation establishment in sodic soils is not as straight forward as in non-sodic soils. In the absence of fresh hyphal material, reliance on spore germination to colonise cotton roots might be unsuccessful in moderately- and highlysodic soils. Therefore, the presence of a fresh mycorrhizal hyphal network within the inoculum source may play an important role in initiating colonisation of cotton roots in sodic soils with ESP greater than 15.

These results indicate that higher levels of sodicity restrict mycorrhizal colonisation of cotton. Reduced colonisation and hyphal exploration of the soil, possibly due to the physical and chemical constraints imposed by highly-sodic soil, rather than poorer mycorrhizal function, might be one of the responsible factors for limited early P uptake of cotton in highly-sodic soil. However, mycorrhizal colonisation and associated nutrient uptake (P, Zn, Ca, Mg, K, Mn) of cotton was not dominated by sodicity and associated physical/chemical conditions (waterlogging, high bulk density, high pH, high soil solution Na) occurring in soils when ESP is less than 10. Further investigation into mycorrhizal spore density, species diversity and mycorrhizal proliferation under sodic soil conditions is warranted.