Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.     

The use of P32 in studies on the uptake of phosphorus by plants

Summary A fertilizer experiment with calcium phosphates labelled with P³² was carried out in pots with barley as a test plant. By determining the ratio of P³² in fertilizer and plant phosphorus after more than 7 weeks the content of exchangeable phosphate in the soil could be calculated. The results of the calculation were independent of the amount of phosphate added providing it was soluble.     

Regulation of phosphate uptake reveals cyanobacterial bloom resilience to shifting N:P ratios

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Physiological changes in, and phosphate uptake by potato plants during development of, and recovery from phosphate deficiency

The development of phosphate deficiency (P-stress) was observed in rooted sprouts of Solanum tuberosum L. cv. Desiree growing in solutions without phosphate. Shoot growth was inhibited by P-stress within 3 to 5 days of terminating the phosphate supply, while significant effects on root growth were not recorded until 7 to 9 days. Thus, the shoot:root dry weight ratio decreased from 4.3 to 2.6 over a 10-day period. Growth in the absence of an exogenous phosphate supply progressively diluted the phosphorus in the plant. The proportional decrease in concentration was similar in roots and shoots over a 7-day period, even though the former were growing more quickly. The potential for phosphate uptake per unit weight of root increased rapidly during the first 3 days of P-stress. When the plants were provided subsequently with a labelled, 1 mol m^?3 phosphate solution, the absorption rate was 3 to 4-fold greater than that of control plants which had received a continuous phosphate supply. The increased rate of uptake by P-stressed plants was accounted for by an increase (3-fold) in the V_max of system 1 for phosphate transport and by a marked increase in the affinity of the system for phosphate (decrease in K_m). In the early stages of P-stress, before marked changes in growth were measured, the proportion of labelled phosphate translocated to the shoots increased slightly relative to the controls when a phosphate supply was restored. In the later stages of stress a greater proportion was retained in the root system of P-stressed plants than in that of controls. In plants with roots divided between solutions containing or lacking a phosphate supply, the increased absorption rate was determined by the general demand for phosphate in the plant and not by the P-status of the particular root where uptake was measured. By contrast, the poportion translocated was strongly dependent on the P-status of the root. The restoration of a phosphate supply to P-stressed plants was marked by a rapid increase in the P concentration in snoots and roots which returned to levels similar to unstressed controls within 24 h. The enhanced uptake rate persisted for at least 5 days, resulting in supra-normal concentrations of P in both shoots and roots, and in the formation of extensive necrotic areas between the veins of mature leaves. Autoradiographs showed accumulations of ³²P in these lesions and at the points where guttation droplets formed on leaves.     

On the effects of phosphorus on zinc uptake by barley

Summary The response of barley to phosphate application and the effect of applied phosphorus on the uptake of soil zinc by the crop were tested in pot, Neubauer and incubaticn experiments on four soils differing in native phosphate status.There was a response of the yield to phosphorus application in alluvial soils from Clementina (Ecuador) and Bangla Desh. Barley grown on red soil from Bangla Desh and glacial clay from Uppsala did not show any response to P application.Applied P³² was fixed to a great extent in all the soils studied. But in phosphate-deficient soils, a much higher degree of sorption of P³² was recorded than in phosphate-rich soils. Compared to phosphate-rich soils, the utilization of fertilizer phosphorus is higher than of native soil P in the case of phosphate-deficient soils. It was observed that uptake of Zn⁶⁵ by the crop is counteracted by phosphorus application at the three stages of crop growth studied.This work was financially supported by the International Atomic Energy Agency, Vienna, Austria, and by the National Council for Forestry and Agricultural Research, Sweden.     

Direct evidence on participation of root hairs in phosphorus (32P) uptake from soil

Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer ³²P (185 kBq g^-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the ³²P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total ³²P uptake). The uptake of³² P from soil via the root hairs only was quantified by measuring activity of ³²P in the plant shoot (³²P uptake only via root hairs).The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the ³²P labelled soil, the quantity of ³²P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of ³²P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.     

Growth and phosphate uptake kinetics of the toxic dinoflagellate Alexandrium tamarense from Hiroshima Bay in the Seto Inland Sea, Japan

Shellfish poisoning by the toxic dinoflagellate Alexandrium tamarense (Lebour) Balech occurred for the first time in Hiroshima Bay, Japan, in 1992. Oyster culture in the bay produces as much as 60% of the total production in Japan, and it suffered severe damage. In the present study, we experimentally investigated the growth rate and phosphate uptake kinetics of A. tamarense, Hiroshima Bay strain. A short-term phosphate uptake experiment revealed that the maximum uptake rate was 1.4 pmol P cell^-1 per h and the half-saturation constant was 2.6 umol L^-1. In semicontin-uous culture, the maximum specific growth rate and the minimum phosphorus cell quota were 0.54 day^-1 and 0.56 pmol P cell^-1, respectively. These uptake rates suggest that A. tamarense is a poor phosphorus competitor compared with other species. However, the large phosphorus storage capacity (Q_pmax/q_o= 36), the surge phosphorus uptake ability (V_s/V_i= 4.1) and the low growth rate would be advantageous for surviving brief periods of phosphorus limitation which frequently occur in Hiroshima Bay.     

Native soil and fresh fertilizer phosphorus uptake as affected by rate of application and P fertilizers

³²P labelled fertilizers were used to measure native soil and fresh fertilizer phosphorus uptake byLolium perenne L. in greenhouse experiments. The P source evaluation was carried out for multiple rates of application for a standard P fertilizer (DAP) on low and medium soil P levels and for North Carolina rock phosphate (RP) at the medium soil P level only. At the low soil P level, the native P uptake increase was independent of P-DAP applied, and represented 19% of the nil P uptake. At the medium soil P level, the variability of the native soil as a nutrient P source depended on the phosphate fertilizer applied, and the rate of application. Consequently the amount of total P uptake could conceal differences in P fertilizer evaluations as the nutrient P source. Fresh P uptake increased linearly with the rates of P applied as standard or tested P fertilizer. The comparison of various P sources by means of fresh P uptake ratio (i.e. fresh P uptake from tested phosphate divided by fresh P uptake from standard phosphate) was independent of the rate of application. It was therefore suggested that various phosphorus sources be evaluated by measuring the fresh P uptake for a single rate of application.     

In vitro studies of p32 uptake in mouse liver mitochondria

Isolated mouse liver mitochondria were incubated in two types of P³²-labelled sucrose-phosphate buffers. The first contained no added ATP or oxidizable substrate. The second contained added ATP. Samples were taken at specified times, up to 60 minutes, and analyses were made of the mitochondrial TCA-soluble inorganic P³² and the total mitochondrial residue P³¹ and P³². The results of the analyses showed that when the phosphorus inhibition index (the ratio of the amount of incubation inorganic phosphorus to the square of the amount of tyrosine in the mitochondria) was high, inorganic P³² uptake was low and vice versa. In accordance with established data, increased P³² uptake was obtained when ATP was added. ATP was found to stabilize the turnover of TCA-insoluble residue phosphorus as well as to maintain the TCA-soluble orthophosphate pool. These results support findings regarding the inhibitory and controlling effects of incubation medium phosphate in the regulation of inorganic phosphorus uptake.     

Uptake of phosphate from soils and fertilizers as affected by soil P availability and solubility of phosphorus fertilizers

Fertilizers labelled with ³²P were used to measure amounts of phosphorus, P_s and P_F, taken up by Lolium perenne from available soil P and from P fertilizer respectively, when applied at a rate of 66 mg P·(kg soil^–1) in greenhouse experiments. The quantity P_s of phosphorus taken up from soil in the presence of P fertilizer was compared to the quantity P_o taken up from soil without P fertilizer. The quantity (P_s–P_o) is positive for low P_o values, i.e. in soils poor in available phosphorus, but is negative for high P_o values indicating that an input of P fertilizer can induce a decrease in the utilization of available soil phosphorus. Moreover, for a given soil, the quantity (P_s–P_o) depends on the chemical form of the fertilizer. The standard method of evaluation of P fertilizer efficiency is based on the assumption that P_s=P_o, but P_s can differ from P_o. This result can explain the contradictory data published from field experiments about the efficiency of the various P fertilizers.     

The fate of18O labelled phosphate in soil/plant systems

KH₂PO₄ labelled with¹⁸O and³²P was mixed with soil that was placed in pots in which grass seed was sown. Grass samples were taken after 5, 7, and 12 weeks. The dilution factor (DF) for¹⁸O in the first cut was much greater than the DF for³²P, indicating that the bulk of the¹⁸O in the absorbed phosphate was lost. The DFs for¹⁸O and³²P determined in phosphate extracted from the soil at the end of the pot experiment indicated that half the¹⁸O excess in the applied phosphate was lost.A succeeding experiment showed no loss of¹⁸O when the treated soil was shaken for 3 months with water to which a germicide was added. Thus, the loss of¹⁸O was presumably caused by biochemical processes which brought about the replacement of¹⁸O by¹⁶O. We suggest that the loss of¹⁸O from applied labelled phosphate may be used as a measure of biological activity in soil.     

Use of partially acidulated rock phosphate as a possible means of minimising phosphate fixation in acid soils

Summary To examine the possibility of minimising phosphate fixation the lateritic soil at various levels of liming was incubated with phosphate rock from U.A.R. acidulated to different degree viz. 0, 10, 20, 50, and 100 per cent both with phosphoric and nitric acid. The soil was incubated for 90 days on addition of different phosphate carriers at the rate of 100 ppm total P₂O₅ containing different proportion of water-soluble, citrate-soluble and insoluble phosphorus. Samples were drawn at an interval of 30 days. Bray's p₁ and pH of the soil samples were measured. The dry-matter yield and uptake of phosphorus by two successive crops of maize grown in pots, the treatments being same as in incubation study, were well correlated with the Bray's p₁. Ground rock phosphate and 10 per cent acidulated material were effective in minimising the fixation in soil of pH 4.0 whereas 50 per cent acidulation was suitable for soils of higher pHi.e. 5.6 and 6.5. H₃PO₄ acidulated material was proved superior to HNO₃ acidulated product. The use of partially acidulated rock phosphate for acid soils may be recommended to receive economic return. Associate Professor Senior Research Assistant.     

A study of the mechanism of soil-phosphate uptake in relation to plant species

Summary The concept of plant available and unavailable soil phosphate has been examined by growing a range of plant species in soils well mixed with carrier free P³².It was found that although activities of 5–100 µC per kg soil caused changes in dry matter and P uptake, they had no influence on the specific activity per unit dose.Only small and agriculturally insignificant differences have been found in the proportion of soil P to added P³² taken up by the different species in the acid and neutral soils employed. It is considered that such differences as there are may be due to exchange of phosphate between seed and soil. Marked differences, however, occurred in the total phosphate absorbed by the crops.These results support the view that a fraction of the soil phosphate exists in a labile pool which will exchange with added P³² and will maintain a definite equilibrium potential in the soil solution. Plants do not appear to have a means of increasing significantly the size of the labile pool under the experimental conditions described. The disparity, however, between the total phosphate uptake especially in very deficient soils does suggest that certain species are more efficient than others in the absorption of soil phosphate at low potential.     

Incorporation of P32 in phosphate esters of the sugar cane plant and the effect of Si and Al on the distribution of these esters

Summary Good separation of phosphate esters labeled with P³² was obtained on paper-chromatography using the ascending technique in two dimensions at right angles.The pattern of phosphate esters in chromatographic resolution was similar for both the roots and leaves of sugar cane but there were differences in the relative proportions of the individual esters. Significant amounts of some esters, of which G-6-P constituted the main bulk, were labeled and identified after short incubation periods of excised roots in the radioactive phosphate solution.Although Al pre-treatment at the level of 0.5 mM Al₂(SO₄)₃ stimulated labeled phosphate uptake, there was no effect on the extent of phosphorylation. With Si pre-treatment, however, a significant increase in the degree of phosphorylation was noted although no effect was obtained on labeled phosphate uptake     

Root surface phosphatase activities and uptake of 32P-labelled inositol phosphate in field-collected gray birch and red maple roots

 This study examined select, naturally-occurring tree mycorrhizae for differences related to efficiency of organic phosphorus hydrolysis in forest soils. We investigated the activity of several phosphatases and root respiration in field-collected ectomycorrhizae of American beech and gray birch and VAM of red maple. Root materials were collected in the early and late growing season from a common soil type. American beech occurred in a late-successional stand, whereas gray birch and red maple grew in a mid-successional stand. All of the root types examined had phosphatase activities with p-nitrophenyl phosphate, bis-p-nitrophenyl phosphate and phytic acid and thus the potential to mineralize monoester and diester forms of organic phosphorus. Rates of hydrolysis at pH 5.0 were greatest with p-nitrophenyl phosphate. Although enzyme activity varied with season and ectomycorrhizal morphotype, VAM roots of red maple consistently had the lowest enzyme activities on a length and dry weight basis. Comparison of ³²P uptake from inositol phosphate by gray birch and red maple roots suggested that phosphomonoesterase activity was linked to P uptake from this source. Differences between species in oxygen consumption rates were less pronounced than those observed for enzymatic activities, suggesting similar short-term energy demands by the root types examined. The quantitative differences observed between plants growing on a common soil potentially relate to differences in host demand or reflect differences in basic morphology and/or physiology of associated mycobionts. Further study is necessary to understand the importance of these enzymes in the functional ecology of mycorrhizal fungi. Accepted: 20 December 1996     

The influence of soil volume on the absorption of soil phosphorus by plants and on the determination of labile soil phosphorus

Summary In a pot experiment the soil volume available for ryegrass growth was varied. There was relatively greater uptake of phosphorus from the smaller amounts of soil and an increase in the pool of labile phosphorus (the L-value). It appears that an equilibrium exists in the soil between non-labile and labile phosphorus, and this equilibrium may be disturbed by the removal of phosphorus by plants.The variations of L-value with time followed a pattern indicating the initial influence of seed-borne phosphorus and slow isotopic exchange of the added carrier-free P³² with soil phosphorus.     

Uptake of 3HHO and 32P by roots of wheat and rape

Summary Direct measurements were made of ³HHO and ³²P taken up from labelled soil by roots of wheat (Triticum aestivum L.) and rape (Brassica campestris L.). Single roots were encased in labelled soil for 3 days, and the amount of ³HHO and ³²P retained in the shoots was determined. Plants were grown to five stages of maturity in growth boxes under controlled conditions. Roots were labelled at up to four depths (to 90 cm) depending on the rooting depth at each stage of maturity. Uptake of ³HHO per unit length of root increased as the plant age increased, while uptake of ³²P decreased to below detection levels by 45 days after germination. Larger amounts of both nutrients were translocated to and retained in the shoots from surface roots than from roots located deeper in the soil although the soil was uniform in temperature, bulk density, and composition throughout the growth boxes. Wheat roots were more efficient than rape roots in absorbing ³HHO; however, rape roots took up larger amounts of ³²P per unit length of root. Neither native nor added P located more than 30 cm deep is of much importance to these annual crops, since uptake is minimal and the main demand for this nutrient occurs at early growth stages when the root system is restricted to the surface layers. re]19750812     

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.     

Comparative study of metabolism and forms of transport of phosphate between Ascophyllum nodosum and Polysiphonia lanosa

Polysiphonia lanosa (L.) Tandy is a marine red alga that usually grows epiphytically on the fucale Ascophyllum nodosum (L.) Le Jolis. The present work was conducted in order to obtain more information on the relationships between these two algae, especially as regards the metabolism and long-distance transport of phosphorus. Three types of experiments were carried out using labelled phosphorus. (1) Comparative study of the metabolism of ³²P₁ absorbed by the tissues of each species. By means of two-dimensional chromatography and autoradiography, it was shown that ³²P₁ was rapidly incorporated into organic soluble compounds (adenosine triphosphate, hexose monophosphate, uridine diphosphoglucose, phosphoenolpyruvate + phosphoglyceric acid). Although the two algae belong to different phylae the phosphorylated compounds were not very different. The energy charges (0. 72 for both species) were in the usual range for aerobic plant tissues. On the other hand the incorporation of ³²P₁ into the insoluble P₀ fraction was doubled in P. lanosa compared to in A. nodosum (ca 80 and 40%, respectively). At the source level, the air bladder of A. nodosum. the same soluble compounds (inorganic phosphate, P₁ adenosine triphosphate, hexose monophosphate. etc.) represented the likely forms transported. A part of the soluble P₀ fraction may return to the P₁ fraction. (2) In translocation experiments conducted in situ, ³²P₁ locally injected into an air bladder moved over long distances not only through the thallus of A. nodosum but also into P. lanosa. The reciprocal transfer remained unsuccessful. (3) The ³²P₁ represented the predominant compound identified in the two species: this argues in favour of P₁ as the translocated form of phosphorus. Our results support the hypothesis of a parasitic rather than a simple epiphytic relationship between the two algae.     

The influence of phosphate concentration on the kinetics of uptake by maize roots

In an experiment with native maize roots depending on different phosphorus concentration in the external solution (0.001 … 50 mM P), the multiphasic character of the kinetics of phosphate uptake has been stated. The single phases are characterized by the different values of K_m and V_max. In the wide range of concentrations the isotherm of the phosphate uptake has five evident phases. The character of kinetics for the uptake of phosphate is analogical to the kinetics of the enzymatic reactions described by the Michaelis-Menten equation. On the other hand the linear dependence for the inactivated root was determined,i.e. the uptake of phosphate versus different phosphorus concentration in the external solution. The graphic representation of the logarithmic values for the phosphorus taken up versus the different phosphorus concentration in the external solution gives the biphasic course including concentration less than 1.0 mM P and more than 1.0 mM P. Within the framework of the concentration range the following values of V_max, K_m and ϕ_in were calculated under the conditions if the concentration of phosphorus is less than 1.0mMP: V_max = 1.705 μmol P × g^-1h^-1, K_m = 0.057 mM P and ϕ_in = 0.83,i.e. if the concentration of phosphorus is more than 1.0mM P: V_max = 40 μmol P × g^-1 h^-1, K_m = 16.66 mM and ϕ_in = 20. According to these results, the phosphate concentration in the external solution influences the activity of the transport mechanisms concerning their conformative changes which discretely change their working regime of membrane transport. This is also demonstrated in the change of values V_max, K_m and ϕ_in.