Plant uptake of sulphur as related to changes in the HI-reducible and total sulphur fractions in soil

Although considerable progress has been made in relating extractable soil S to plant S availability, most of these studies determined the extractable soil S at the beginning of the experiment to use as an index of soil S status. This bears little or no relationship to the S taken up by plants during the entire growing season. The present study investigates the changes in extractable soil S with time and relates these to changes in plant S uptake. Six soils with different long-term fertiliser histories (0, 21, 42 kg of S as superphosphate ha^–1 applied since 1952) and animal camping treatments (camp and non-camp) were used in two pot systems (with and without plants). Carrier-free ³⁵SO₄–S was added to the soils, to provide the information on the transformations of recently added S between the different extractable S forms in soils and whether these transformations could predict plant-available S. The soils were pre-conditioned and then transferred to the glasshouse, where one set of pots were planted with perennial ryegrass (Lolium perenne L.) while the other set was left uncropped. Periodic plant harvests and soil samplings at four weekly intervals were conducted over a period of 20 weeks to determine plant S uptake and amounts of extractable soil S and ³⁵S forms using five extractants. Same extractions of soil S and ³⁵S were conducted for the initial soils. Results showed that HI-reducible and total soil S extracted by CaCl₂, KH₂PO₄ and by KCl at 40°C were utilised significantly by plants but not those extracted by NaHCO₃ and NaOH extractants. However, after the 8th week, plants continued to take up S even though levels of S extracted from the soil by CaCl₂, KH₂PO₄ and by KCl at 40°C remained low and unchanged. These results suggest that soil S taken up by plants after the 8th week period originated directly from the mineralisation of soil organic S from S pools other than those present in the extractable soil S forms. Similar results were shown by ³⁵S data, thereby confirming the complexity of determining plant S availability based on soil S extraction methods.     

Inorganic phosphate as a signaling molecule in osteoblast differentiation

The spatial and temporal coordination of the many events required for osteogenic cells to create a mineralized matrix are only partially understood. The complexity of this process, and the nature of the final product, demand that these cells have mechanisms to carefully monitor events in the extracellular environment and have the ability to respond through cellular and molecular changes. The generation of inorganic phosphate during the process of differentiation may be one such signal. In addition to the requirement of inorganic phosphate as a component of hydroxyapatite mineral, Ca(10)(PO(4))(6)(OH)(2), a number of studies have also suggested it is required in the events preceding mineralization. However, contrasting results, physiological relevance, and the lack of a clear mechanism(s) have created some debate as to the significance of elevated phosphate in the differentiation process. More recently, a number of studies have begun to shed light on possible cellular and molecular consequences of elevated intracellular inorganic phosphate. These results suggest a model in which the generation of inorganic phosphate during osteoblast differentiation may in and of itself represent a signal capable of facilitating the temporal coordination of expression and regulation of multiple factors necessary for mineralization. The regulation of protein function and gene expression by elevated inorganic phosphate during osteoblast differentiation may represent a mechanism by which mineralizing cells monitor and respond to the changing extracellular environment.     

The determination of labile soil phosphate as influenced by the time of application of labelled phosphate

Summary An experiment was conducted to determine the effect on the L-values of pre-equilibrating P³² with three soil types for 2, 1, and 0 months before sowing ryegrass. Resin and phosphoric acid were used as carriers. Equilibrium was established some twelve weeks after sowing and this time was virtually unaffected by the pre-equilibration treatments. The phosphate source was found to affect both P uptake and L-value; higher uptake and lower L-values were recorded from the resin.     

Inorganic phosphate regulates the binding of cofilin to actin filaments

Inorganic phosphate (Pi) and cofilin/actin depolymerizing factor proteins have opposite effects on actin filament structure and dynamics. Pi stabilizes the subdomain 2 in F-actin and decreases the critical concentration for actin polymerization. Conversely, cofilin enhances disorder in subdomain 2, increases the critical concentration, and accelerates actin treadmilling. Here, we report that Pi inhibits the rate, but not the extent of cofilin binding to actin filaments. This inhibition is also significant at physiological concentrations of Pi, and more pronounced at low pH. Cofilin prevents conformational changes in F-actin induced by Pi, even at high Pi concentrations, probably because allosteric changes in the nucleotide cleft decrease the affinity of Pi to F-actin. Cofilin induced allosteric changes in the nucleotide cleft of F-actin are also indicated by an increase in fluorescence emission and a decrease in the accessibility of etheno-ADP to collisional quenchers. These changes transform the nucleotide cleft of F-actin to G-actin-like. Pi regulation of cofilin binding and the cofilin regulation of Pi binding to F-actin can be important aspects of actin based cell motility.     

Agronomic effectiveness of partially acidulated phosphate rock as influenced by soil phosphorus-fixing capacity

A greenhouse study compared the effect of soil P-fixing capacity on the relative argonomic effectiveness (RAE) of partially acidulated phosphate rock (PAPR) and water-soluble P. Such information is lacking in the literature. Six soils varying widely in P-fixing capacity (5.6%–56.1%) were used. A phosphate rock (Huila PR) from Colombia was acidulated with H₂SO₄ at 50% of the level necessary to achieve full conversion to single superphosphate (SSP). Rates of P applied from PAPR or SSP were 0,05, 100, and 300 mg P kg⁻¹. The P fertilizers were mixed with the soils, and maize was grown for 6 weeks before harvest. The results show that the effectiveness of PAPR in increasing dry-matter yield and P uptake over yield and uptake obtained with SSP linearly increased as the soil P-fixing capacity increased. PAPR and SSP were equally effective in increasing dry-matter yield or P uptake at P-fixing capacities of 28% or 36%, respectively. PAPR was found to be more effective than SSP in soils (treated with Fe-gel) with P-fixing capacity higher than these values. The internal efficiency, which is defined as the ratio between dry-matter yield and P uptake, was the same for both PAPR and SSP in all the soils.     

Analysis of qPCR data by converting exponentially related Ct values into linearly related X0 values

A common method for calculating results from qPCR experiments is the comparative Ct method, also called the 2(-ΔΔCt) method. However, several assumptions are included in the 2(-ΔΔCt) method and standard statistical analyses are not directly applicable. Here, we describe a different method, the X(0) method, for result calculations and statistical analysis from qPCR experiments. The X(0) method differs from the 2(-ΔΔCt) method by introducing a conversion of the exponentially related Ct values into linearly related X(0) values, which represent the amount of starting material in a qPCR experiment. Results calculated by the X(0) method are illustrated for qPCR experiments with technical and biological replicates, including procedures to calculate standard deviations. Incorporation of primer efficiencies in calculations by the X(0) method is also described. Altogether, the X(0) method constitutes a very simple and accurate alternative to the 2(-ΔΔCt) method for result calculations from qPCR data.     

Mild phosphorus stress in barley and a related low-phosphorus-adapted barleygrass: Phosphorus fractions and phosphate absorption in relation to growth

Barleygrass ( Hordeum leporinum ) from Australian low-P (phosphorus) soils and commercial barley ( H. vulgare ) with high fertilizer requirements were grown in solution culture at 3 levels of P supply. The high-P-adapted barley produced more biomass at all levels of P supply and was more responsive to added P in terms of rate of tillering, rate of leaf production, final leaf size, and therefore total shoot weight compared to barleygrass. In both species root: shoot ratio decreased in response to improved tissue P status, even at P levels where total biomass did not respond to P supply. Removal of endosperm reserves of barley reduced total biomass to a greater extent than it altered phosphate absorption rate, thus increasing tissue P status and making plants less responsive to added P. Similarly, barleygrass had a slower growth rate but a comparable P absorption rate to that of barley. Thus barleygrass also accumulated tissue P and was unresponsive to added P. All phosphorus chemical fractions increased in response to improved tissue P status, but to differing extents (inorganic-P > nucleic acid-P > lipid-P > ester-P), suggesting that all P fractions (particularly inorganic P) serve, in part, a storage function. Both barleygrass and barley without endosperm had higher concentrations of all P fractions (particularly inorganic P) than did unaltered barley, but this was due entirely to their higher P status (due to slow growth) rather than to any major difference in P metabolism between species. We conclude that slow growth is more important than interspecific differences in P metabolism, P absorption, or efficiency of P utilization in explaining the success of barleygrass and other low-P-adapted species on infertile soils.     

Available soil nitrogen in relation to fractions of soil nitrogen and other soil properties

The available N of 27 soils from England and Wales was assessed from the amounts of N taken up over a 6-month period by perennial ryegrass grown in pots under uniform environmental conditions. Relationships between availability and the distribution of soil N amongst various fractions were then examined using multiple regression. The relationship: available soil N (mg kg^–1 dry soil)=(N_min×0.672)+(N_inc×0.840)+(N_mom×0.227)–5.12 was found to account for 91% of the variance in available soil N, where N_min=mineral N, N_inc=N mineralized on incubation and N_mom=N in macro-organic matter. The N mineralized on incubation appeared to be derived largely from sources other than the macro-organic matter because these two fractions were poorly correlated. When availability was expressed in terms of available organic N as % of soil organic N (N_ao) the closest relationship with other soil characteristics was: N_ao=[N_inc×(1.395–0.0347×CN_mom]+[N_mom×0.1416], where CN_mom=CN ratio of the macro-organic matter. This relationship accounted for 81% of the variance in the availability of the soil organic N.The conclusion that the macro-organic matter may contribute substantially to the available N was confirmed by a subsidiary experiment in which the macro-organic fraction was separated from about 20 kg of a grassland soil. The uptake of N by ryegrass was then assessed on two subsamples of this soil, one without the macro-organic matter and the other with this fraction returned: uptake was appreciably increased by the macro-organic matter.     

Phosphorus and micronutrient metal uptake by some tree species as affected by phosphate and lime applied to an acid sandy soil

Summary The effects of added P and lime on Douglas fir and Scots pine seedlings, and poplar and willow cuttings growing in a podzolic soil (pH 3.8, 90 ppm total P) were studied in pot experiments. Conifer dry weights responded best to P applied in the absence of lime, whereas liming to pH 4.3 promoted the P response of the broadleaved species. Normal rates of P, and of lime (broad-leaved species), by promoting growth, also raised total contents of P and metals (Zn, Mn, Cu, Fe) in the various plant parts (stems, foliage, roots), but generally lowered the metal concentrations. The results strongly suggest that P interfered with the root to shoot translocation of Cu, Fe and Al (Al only estimated in Scots pine), but not with that of Zn and Mn. It is postulated that internal plant tolerance (promoted by P) plays a more important part in neutralizing toxic metal concentrations (Zn, and possibly also Fe) in the soil than do exclusion mechanisms. High applications of P without Cu may depress growth, as demonstrated for willow. Water-soluble soil P data may be misinterpreted if other limiting soil factors (pH, Cu status) have not been eliminated.     

Contribution of bacterial cell nitrogen to soil humic fractions

Summary Living cells ofSerratia marcescens, uniformly labelled with¹⁵N, were added to samples of maple (Acer saccharum) and black spruce (Picea mariana) forest soils. After different periods of incubation from zero time to 100 days, the soils were subjected to alkali-acid and phenol extraction to provide humic acid, fulvic acid, humin and humoprotein fractions. Significant amounts of the cell nitrogen were recovered in the humic and fulvic acids immediately after addition. After incubation, less cell, nitrogen appeared in the humic acid and more in the fulvic acid. The amount of cell nitrogen recovered in the humin fraction increased with incubation. Roughly 5 to 10 per cent of the added cell nitrogen was found as amino acid nitrogen from humoprotein in a phenol extract of the humic acid. The data are consistent with the occurrence of co-precipitation of biologically labile biomass nitrogen compounds with humic polymers during the alkaline extraction procedure involved in the humic-fulvic fractionation.