Phosphorus fractions in leaves.

Leaf phosphorus (P) comprises four major fractions: inorganic phosphate (Pi ), nucleic acids, phospholipids, P-containing metabolites and a residual fraction. In this review paper, we investigated whether allocation of P fractions varies among groups of terrestrial vascular plants, and is indicative of a species' strategy to use P efficiently. We found that as leaf total P concentration increases, the Pi fraction increases the most, without a plateau, while other fractions plateau. Variability of the concentrations of leaf P fractions is greatest among families > species(family) > regions > plant life forms. The percentage of total P allocated to nucleic acid-P (20-35%) and lipid-P (14-34%) varies less among families/species. High photosynthetic P-use efficiency is associated with low concentrations of all P fractions, and preferential allocation of P to metabolite-P and mesophyll cells. Sequential resorption of P from senescing leaves starts with Pi , followed by metabolite-P, and then other organic P fractions. Allocation of P to leaf P fractions varies with season. Leaf phytate concentrations vary considerably among species, associated with variation in photosynthesis and defence. Plasticity of P allocation to its fractions is important for acclimation to low soil P availability, and species-specific P allocation is needed for co-occurrence with other species.

Adaptive shoot and root responses collectively enhance growth at optimum temperature and limited phosphorus supply of three herbaceous legume species.

BACKGROUND AND AIMS: Studies on the effects of sub- and/or supraoptimal temperatures on growth and phosphorus (P) nutrition of perennial herbaceous species at growth-limiting P availability are few, and the impacts of temperature on rhizosphere carboxylate dynamics are not known for any species. METHODS: The effect of three day/night temperature regimes (low, 20/13 °C; medium, 27/20 °C; and high, 32/25 °C) on growth and P nutrition of Cullen cinereum, Kennedia nigricans and Lotus australis was determined. KEY RESULTS: The highest temperature was optimal for growth of C. cinereum, while the lowest temperature was optimal for K. nigricans and L. australis. At optimum temperatures, the relative growth rate (RGR), root length, root length per leaf area, total P content, P productivity and water-use efficiency were higher for all species, and rhizosphere carboxylate content was higher for K. nigricans and L. australis. Cullen cinereum, with a slower RGR, had long (higher root length per leaf area) and thin roots to enhance P uptake by exploring a greater volume of soil at its optimum temperature, while K. nigricans and L. australis, with faster RGRs, had only long roots (higher root length per leaf area) as a morphological adaptation, but had a higher content of carboxylates in their rhizospheres at the optimum temperature. Irrespective of the species, the amount of P taken up by a plant was mainly determined by root length, rather than by P uptake rate per unit root surface area. Phosphorus productivity was correlated with RGR and plant biomass. CONCLUSIONS: All three species exhibited adaptive shoot and root traits to enhance growth at their optimum temperatures at growth-limiting P supply. The species with a slower RGR (i.e. C. cinereum) showed only morphological root adaptations, while K. nigricans and L. australis, with faster RGRs, had both morphological and physiological (i.e. root carboxylate dynamics) root adaptations.

Multiple adaptive responses of Australian native perennial legumes with pasture potential to grow in phosphorus- and moisture-limited environments.

BACKGROUND AND AIMS: Many Australian legumes have evolved in low-phosphorus (P) soils and low-rainfall areas. Therefore a study was made of the interaction of soil [P] and water availability on growth, photosynthesis, water-use efficiency (WUE) and P nutrition of two Australian native legumes with pasture potential, Cullen australasicum and C. pallidum, and the widely grown exotic pasture legume, lucerne (Medicago sativa). METHODS: Plants were grown in a glasshouse at 3, 10 and 30 mg P kg(-1) dry soil for 5 months. At week 10, two drought treatments were imposed, total pot dried (all-dry) and only top soil dried (top-dry), while control pots were maintained at field capacity. KEY RESULTS: Shoot dry weight produced by lucerne was never higher than that of C. australasicum. For C. pallidum only, shoot dry weight was reduced at 30 mg P kg(-1) dry soil. The small root system of the Cullen species was quite plastic, allowing plants to access P and moisture efficiently. Lucerne always had a higher proportion of its large root system in the top soil layer compared with Cullen species. All species showed decreased photosynthesis, leaf water potential and stomatal conductance when exposed to drought, but the reductions were less for Cullen species, due to tighter stomatal control, and consequently they achieved a higher WUE. All species showed highest rhizosphere carboxylate concentrations in the all-dry treatment. For lucerne only, carboxylates decreased as P supply increased. Citrate was the main carboxylate in the control and top-dry treatments, and malate in the all-dry treatment. CONCLUSIONS: Multiple adaptive responses of Cullen species and lucerne favoured exploitation of low-P soils under drought. The performance of undomesticated Cullen species, relative to that of lucerne, shows their promise as pasture species for environments such as in south-western Australia where water and P are limiting, especially in view of a predicted drying and warming climate.

Tightening the Phosphorus Cycle through Phosphorus-Efficient Crop Genotypes.

We are facing unprecedented phosphorus (P) challenges, namely P scarcity associated with increasing food demand, and an oversupply of P fertilisers, resulting in eutrophication. Although we need a multidisciplinary approach to systematically enhance P-use efficiency, monodisciplinary studies still prevail. Here, we propose to tighten the P cycle by identifying P-efficient crop genotypes, integrating four plant strategies: increasing P-acquisition efficiency, photosynthetic P-use efficiency and P-remobilisation efficiency, and decreasing seed phytate P concentrations. We recommend P-efficient genotypes together with diversified cropping systems involving complementary P-acquisition strategies as well as smart P-fertiliser management to enhance P-use efficiency in agriculture dependent on soil P status. These strategies will reduce P-fertiliser requirements and offsite environmental impacts, while enhancing seed quality for human and livestock nutrition.