Phosphorus (P) is an important nutrient required for plant growth and its management in soil is critical to ensure sustainable and profitable agriculture that has minimal impact on the environment. Although soils may contain a large amount of total P, only a small proportion is immediately available to plants. Australian soils often have low availability of P for plant growth and P-based fertilisers are, therefore, commonly used to correct P deficiency and to maintain productivity. For many soils, the sustained use of P fertiliser has resulted in an accumulation of total P, a proportion of which is in forms that are poorly available to most plants. The efficiency with which different P fertilisers are used in agricultural systems depends on their capacity to supply P in a soluble form that is available for plant uptake (i.e. as orthophosphate anions). In addition to fertiliser source, the availability of P in soil is influenced to a large extent by physico-chemical and biological properties of the soil. Plant access to soil P is further affected by root characteristics (e.g. rate of growth, specific root length, and density and length of root hairs) and biochemical processes that occur at the soil–root interface. The ability of roots to effectively explore soil, the release of exudates (e.g. organic anions and phosphatases) from roots that influence soil P availability, and the association of roots with soil microorganisms such as mycorrhizal fungi are particularly important. These processes occur as a natural response of plants to P deficiency and, through better understanding, may provide opportunities for improving plant access to soil and fertiliser P in conventional and organic agricultural systems.

We investigated (1) the effect of constant and altered inorganic phosphate (P i ) supply (1-100 mmol m -3 ) on proteoid root production by white lupin (Lupinus albus L.); and (2) the variation in citrate efflux, enzyme activity and phosphate uptake along the proteoid root axis in solution culture. Proteoid root formation was greatest at P i solution concentrations of 1-10 mmol m -3 and was suppressed at 25 mmol m -3 P i and higher. Except at 1 mmol m -3 P i , the formation of proteoid roots did not affect plant dry matter yields or shoot to root dry matter ratios, indicating that proteoid roots can form under conditions of adequate P supply and not at the expense of dry matter production. Plants with over 50% of the root system as proteoid roots had tissue P concentrations considered adequate for maximum growth, providing additional evidence that proteoid roots can form on P-sufficient plants. There was an inverse relationship between the P i concentration in the youngest mature leaf and proteoid root formation. Citrate efflux and the activities of enzymes associated with citric acid synthesis (phosphoenolpyruvate carboxylase and malate dehydrogenase) varied along the proteoid root axis, being greatest in young proteoid rootlets of the 1-3 cm region from the root tip. Citrate release from the 0-1 and 5-9 cm regions of the proteoid root was only 7% (per unit root length) of that from the 1-3 cm segment. Electrical potential and 32 P i uptake measurements showed that P i uptake was more uniform along the proteoid root than citrate efflux.

Atmospheric CO2 levels are increasing, but little is known about how this will affect tissue concentrations and the partitioning of agriculturally important nutrients such as nitrogen (N) within crop plants. To investigate this, a glasshouse experiment was conducted in which wheat, a C3 species, and maize, a C4 species, were grown for 8 weeks at high CO2 (1500 cm3 m-3) on N supplies ranging from deficient (0.5 mol m-3) to more than adequate for maximum growth (25 mol m-3). Wheat responded to both CO2 enrichment and N supply; maize responded only to N supply. CO2-enriched wheat produced about twice the dry matter of control plants at all levels of N supply. Tiller and ear numbers were increased by CO2 enrichment irrespective of N supply. Enriched wheat plants had a lower Leaf Area Ratio but higher Net Assimilation Rate and Relative Growth Rate than control plants. There was no effect of CO2 enrichment on specific leaf weight. The enriched plants had lower shoot to root dry matter ratios than the controls at 6 mol m-3 N and higher. Shoot to root dry matter ratios of both wheat and maize increased with increasing N supply. CO2-enriched wheat plants accumulated more N than the controls but the proportional increase in N content was not as great as that in dry matter, with the result that concentrations of total-N and nitrate-N were lower in all organs of enriched plants, including ears. Nitrate reductase activity was lower in enriched than in control wheat plants. N-use efficiency by wheat was increased by CO2 enrichment. From a practical point of view, the study indicates that critical total-N and NO3-N concentrations used to diagnose the N status of wheat will need to be reassessed as global CO2 levels increase. Elevated CO2 may also reduce the protein content of grain and thus the baking quality of hard wheats.

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SummaryBarley ( Hordeum vulgare L.), genetically modified with the Al 3+ resistance gene of wheat ( TaALMT1 ), was compared with a non-transformed sibling line when grown on an acidic and highly phosphate-fixing ferrosol supplied with a range of phosphorus concentrations.In short-term pot trials (26 days), transgenic barley expressing TaALMT1 (GP-ALMT1) was more efficient than a non-transformed sibling line (GP) at taking up phosphorus on acid soil, but the genotypes did not differ when the soil was limed. Differences in phosphorus uptake efficiency on acid soil could be attributed not only to the differential effects of aluminium toxicity on root growth between the genotypes, but also to differences in phosphorus uptake per unit root length. Although GP-ALMT1 out-performed GP on acid soil, it was still not as efficient at taking up phosphorus as plants grown on limed soil. GP-ALMT1 plants grown in acid soil possessed substantially smaller rhizosheaths than those grown in limed soil, suggesting that root hairs were shorter. This is a probable reason for the lower phosphorus uptake efficiency. When grown to maturity in large pots, GP-ALMT1 plants produced more than twice the grain as GP plants grown on acid soil and 80% of the grain produced by limed controls. Expression of TaALMT1 in barley was not associated with a penalty in either total shoot or grain production in the absence of Al 3+ , with both genotypes showing equivalent yields in limed soil. These findings demonstrate that an important crop species can be genetically engineered to successfully increase grain production on an acid soil.

Conroy, J. and Hocking, P. 1993. Nitrogen nutrition of C, plants at elevated atmospheric CO2 concentrations. -Physiol. Plant. 89: 570-576.The atmospheric CO2 concentration has risen from the preindustrial level of approximately 290 |xl L' to more than 350 jxl 1"' in 1993. The current rate of rise is such that concentrations of 420 ^,11"' are expected in the next 20 years. For C, plants, higher CO2 levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO2 due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production ef crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO2 levels were considerably lower. These standards will need to be re-evaluated as the CO2 concentration continues to rise. Lower levels of leaf nitrog;en will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.