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.     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjected to 15 d of nitrogen deprivation were resupplied for 10 d with 1.0 mol m-3 nitrogen provided as NO3-, NH4+, or NH4(+) + NO3- in flowing hydroponic culture. Plants in a fourth hydroponic system received 1.0 mol m-3 NO3- during both stress and resupply periods. Concentrations of soluble carbohydrates and organic acids in roots increased 210 and 370%, respectively, during stress. For the first day of resupply, however, specific uptake rates of nitrogen, determined by ion chromatography as depletion from solution, were lower for stressed than for non-stressed plants by 43% for NO3- resupply, by 32% for NH4(+) + NO3- resupply, and 86% for NH4+ resupply. When specific uptake of nitrogen for stressed plants recovered to rates for non-stressed plants at 6 to 8 d after nitrogen resupply, carbohydrates and organic acids in their roots had declined to concentrations lower than those of non-stressed plants. Recovery of nitrogen uptake capacity of roots thus does not appear to be regulated simply by the content of soluble carbon compounds within roots. Solution concentrations of NH4+ and NO3- were monitored at 62.5 min intervals during the first 3 d of resupply. Intermittent 'hourly' intervals of net influx and net efflux occurred. Rates of uptake during influx intervals were greater for the NH4(+)-resupplied than for the NO3(-)-resupplied plants. For NH4(+)-resupplied plants, however, the hourly intervals of efflux were more numerous than for NO3(-)-resupplied plants. It thus is possible that, instead of repressing NH4+ influx, increased accumulation of amino acids and NH4+ in NH4(+)-resupplied plants inhibited net uptake by stimulation of efflux on NH4+ absorbed in excess of availability of carbon skeletons for assimilation. Entry of NH4+ into root cytoplasm appeared to be less restricted than translocation of amino acids from the cytoplasm into the xylem.     

Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake

Bethlenfalvay, G. J., Brown, M. S., Ames, R. N. and Thomas, R. S. 1988. Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake. - Physiol. Plant. 72: 565–571.
Soybean [ Glycine max (L.) Merr.] plants were grown in pot cultures and inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or provided with P fertilizer (non-VAM plants). After an initial growth period (21 days), plants were exposed to cycles of severe, moderate or no drought stress over a subsequent 28-day period by rewatering at soil water potentials of -1.0, -0.3 or -0.05 MPa. Dry weights of VAM plants were greater at severe stress and smaller at no stress than those of non-VAM plants. Phosphorus fertilization was applied to produce VAM and non-VAM plants of the same size at moderate stress. Root and leaf P concentrations were higher in non-VAM plants at all stress levels. All plants were stressed to permanent wilting prior to harvest. VAM plants had lower soil moisture content at harvest than non-VAM plants. Colonization of roots by G. mosseae did not vary with stress, but the biomass and length of the extraradical mycelium was greater in severely stressed than in non-stressed plants. Growth enhancement of VAM plants relative to P-fertilized non-VAM plants under severe stress was attributed to increased uptake of water as well as to more efficient P uptake. The ability of VAM plants to deplete soil water to a greater extent than non-VAM plants suggests lower permanent wilting potentials for the former.     

Effects of salinity and phosphate on ion distribution in lupin leaflets

Lupin ( Lupinus luteus L. cv. Weiko III) were grown in nutrient solution over a range of inorganic phosphate (P_i) concentrations, with or without 50 m M NaCl. Plants with high P_i (2 m M ) and salt showed progressive leaf necrosis and had higher concentrations of total phosphate than plants grown with high P_i alone. Most of the extra total phosphate in salt treated plants was in the P_i form. P_i supply did not influence Na⁺, K⁺ or Cl⁻ concentrations in epidermal vacuoles or mesophyll cells. However, epidermal vacuoles accumulated more monovalent cations (Na⁺ and K⁺) than Cl⁻, and in vacuoles of plants grown with 0.1 m M P_i additional P_i was accumulated, possibly to maintain charge balance. Plants grown with 2 m M P_i did not accumulate additional P_i in epidermal vacuoles, but showed higher phosphorus levels in cell walls. It is suggested that at moderate phosphorus concentrations P_i plays a role in epidermal osmotic adjustment, possibly explaining the beneficial role of additional phosphorus on salt stressed plants. At high P_i supply with salt, P_i does not contribute to osmotic adjustment and instead accumulates in cell walls. However, the cause of leaf damage under conditions of high phosphorus supply and salinity is still not entirely clear.     

Regulation of phosphate influx in barley roots: Effects of phosphate deprivation and reduction of influx with provision of orthophosphate

Barley plants were grown in nutrient solutions, which were maintained at either 0 (-P) or 15 μ M orthophosphate (+P). After 11 days phosphate influx into the intact roots of the -P plants began to increase by comparison with +P plants. During this period differences became apparent between the treatments in absolute growth rates, as well as in the root:shoot ratios. Phosphate influx in the -P plants continued to increase as a function of time, to a maximum value of 2.4 μmol (g fresh wt)^-1h^-1 at 16 days after germination. This rate was 6 times higher than influx values for +P plants of the same age. During the period of enhanced uptake phosphate was strongly correlated (r²= 0.77) with root organic phosphate concentration. – The enhancement of inorganic phosphate influx into intact roots of -P plants was rapidly reduced by the provision of 15 μ M orthophosphate. Typically, within 4 h of exposure to this concentration of phosphate, influx values fell from 1.80 ± 0.20 to 0.75 ± 0.03 μmol (g fresh wt)^-1 h^-1, while inorganic phosphate concentrations of the roots increased from 0.12 to 1.15 μmol (g fresh wt)^-1 during the same period. Hill plots of the influx data obtained during this period, treating root inorganic phosphate as an inhibitor of influx, gave Hill coefficients close to 2. The rapidity of the reduction of influx associated with increased root inorganic phosphate together with the Hill plot data provide evidence for an allosteric inhibition of influx by internal inorganic phosphate.     

Phosphate Uptake, Proton Extrusion and Membrane Electropotentials of Phosphorus-Deficient Trifolium repens L

Trifolium repens plants were grown to produce a range of levelsof phosphate stress. The most severely stressed plants absorbedphosphate at twice the rate for plants grown in adequate phosphate.Characteristics of phosphate uptake, membrane electrical potentials(     

Influence of phosphate-stress on phosphate absorption and translocation by various parts of the root system of Hordeum vulgare L. (barley)

Plants of Hordeum vulgare (barley) were grown initially in a solution containing 150 M phosphate and then transferred on day 6 to solutions with (+P) and without (-P) phosphate supplied. After various times plants from these treatments were supplied with labelled phosphate. Analysis of plant growth and rates of labelled phosphate uptake showed that a general enhancement of uptake and translocation was found, in plants which had been in the-P solution, several days before the rate of dry matter accumulation was affected. Subsequently a detailed analysis of phosphate uptake by segments of intact root axes showed that the enhancement of phosphate uptake by P-stress occurred first in the old and mature parts of the seminal root axis and last in the young zones 1 cm from the root apex. During this transition period there were profound changes in the pattern of P absorption along the length of the root. Most of the additional P absorbed in response to P-stress was translocated to the shoot, particularly in older zones of the axis. Enhancement of phosphate uptake in young zones of nodal axes occurred at an earlier stage than in seminal axes. The results are related to the P-status of shoots and root zones and discussed in relation to the general control by the shoot of phosphate transport in the root.     

Chlorsulfuron reduces rates of zinc uptake by wheat seedlings from solution culture

Wheat plants differing in zinc efficiency (Excalibur; Zn-efficient, Gatcher and Durati; Zn-inefficient) were grown in HEDTA chelate-buffered nutrient solution in controlled conditions and supplied with 0 or 40 g chlorsulfuron L^-1 . Zinc uptake rates of 12-d-old plants were measured over 80 or 90 minutes using⁶⁵ Zn added to nutrient solutions. Increasing the zinc concentration of the solution increased the rate of zinc uptake, while the percentage of zinc transported to shoots was decreased. Addition of chlorsulfuron to uptake solutions for 90 minutes did not influence rate of zinc uptake or transport of zinc to shoots. Pretreating plants with chlorsulfuron for 5 days decreased zinc uptake rates, but transport to shoots was proportionally increased. Three-day pretreatment with chlorsulfuron was the minimum required for significant differences in uptake and transport of zinc to occur. Plants exposed to chlorsulfuron for 3 days required a further 5 days of growth in chlorsulfuron-free solutions before uptake rates recovered to control plant rates. It is concluded that chlorsulfuron deleteriously but reversibly affects uptake of zinc across the plasma membrane after prolonged exposure.     

Effect of Cytokinin 4-PU-30 on the Lipid Composition of Water Stressed Bean Plants

Fourteen days-old bean plants, grown on sand with Knop's nutrient solution were subjected to water stress (three days without irrigation). The stress led to a decrease in almost all lipid classes except phospholipids in the primary leaves. The content of palmitic acid increased, and that of the linolenic acid decreased. An increase of hexadecenoic acid in phospholipids was also observed. Rewatering for 24 h led to the recovery of the stressed plants including that of the photosynthetic apparatus, but the changes in the lipid composition were insignificant. The spraying of the plants before and after the water stress with 5 × 10-6 M solution of the phenylurea cytokinin 4-PU-30 alleviated negative effect of water stress on the lipid membrane composition permitting the plants to resist the harmful environment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Levels of phosphate esters in spirodela

The duckweed Spirodela oligorrhiza was grown in sterile nutrient solutions that contained 1 mm phosphate-(32)P at various specific activities. In solutions with activities higher than 2 muc per mumole per ml, plant growth was inhibited after a time, and the physical appearance of the plants was affected. The critical level of radiation, at which growth was first affected, corresponded to 5 kilorads.Plants were grown for 9 days (5 generations) in a culture solution containing phosphate at 0.5 muc per mumole per ml (radiation load approx 0.5 kilorads) so that all phosphorus-containing materials in the tissue became uniformly labeled. The various radioactive compounds were extracted, chromatographed, identified, and their radioactivity was measured. From this radioactivity plus the specific activity of the supplied phosphate, the amount of each compound was calculated. The data constitute a complete balance-sheet for phosphorus in a plant tissue. The identity of 98% of the phosphorus in the tissue was determined. Inorganic phosphate (32,700 mmumoles/g fr wt) was the predominant phosphorus-containing compound; RNA (5100 mmumoles P/g fr wt) was the main organic phosphate; phosphatidyl choline (1600 mmumoles/g fr wt) was the main phospholipid, and glucose-6-phosphate (500 mmumoles/g fr wt) the main acid-soluble phosphate ester. Amounts of other phosphorus compounds are given.     

Accumulation of free proline and glycine betaine in Aster tripolium subjected to a saline shock: A kinetic study related to light period

On transferring three-week-old plants of Aster tripolium L. growing in a half strength Hoagland's medium to the same medium containing 333 m M NaCl a very quick uptake of salt and, after a lag phase of 3 to 5 h, an increase in free proline level was observed. During the time course of imino acid storage, the accumulation rates were higher in the light than in the dark, thereby suggesting some kind of photocontrol on solute metabolism. At zero time, high levels of glycine betaine were present in young plants grown without salt. However, after the application of saline shock, the betaine level also increased significantly. The highest rate of betaine accumulation was detected during the third day of treatment when the rate of proline storage decreased. Glycine betaine storage could also be linked to light dependent processes; whatever its importance in response to salt shock was, the levels observed were lower than those of plants directly grown on 333 m M NaCl for three weeks. When saline stressed plants were transferred to a medium without NaCl, the proline level quickly decreaed while that of glycine betaine remained stable.     

Effects of sulphate and pH on the plant-availability of phosphate adsorbed on goethite

The adsorption of phosphate on metal (hydr)oxides may be influenced by the pH and by the adsorption of other ions. In this study, the influence of sulphate and pH on phosphate adsorption on goethite and the availability to plants of adsorbed phosphate was examined. Maize plants were grown on suspensions of goethite with adsorbed phosphate, containing the same total amount of phosphate and either 0.11 mM or 2.01 mM sulphate at pH 3.7, 4.6 or 5.5. The uptake of phosphorus by the plants increased with the larger sulphate concentration and decreasing pH. Mean P uptake in the treatment with 2.01 mM sulphate and pH 3.7 was 55 µmol plant^-1, whereas in the treatment with 0.11 mM sulphate and pH 5.5 it was 2 µmol plant^-1. Batch adsorption experiments using³² P and speciation modelling of ion adsorption showed that in the presence of sulphate, the phosphate concentration in solution strongly increased with decreasing pH, due to competitive adsorption between sulphate and phosphate on goethite. Modelled phosphate concentrations in solution in the uptake experiment were all below 0.6 µM and correlated well with the observed P uptake. This correlation indicates that the strong influence of the sulphate concentration and pH on the plant-availability of adsorbed phosphate results from the competition between sulphate and phosphate for adsorption on goethite.     

The bacterial wilt,uptake of phosphate,and phosphate ester levels in the resistant and susceptible alfalfa plants

7 days or 7 weeks old alfalfa plants (Medicago sativa L.), susceptible (S) and resistant (R) to bacterial wilt, were inoculated withCorynebacterium michiganense pv.insidiosum and on day 8 and 15 after inoculation the levels of acid-soluble phosphate esters (P-esters) were determinated by means of³²P labelling in the shoots or roots. The most significant changes were recorded in the roots of the older R plants grown in full Knop nutrient solutions on day 8 after inoculation. The marked reduction of inorganic phosphate (P₁) uptake by whole R plants is accompanied by a decrease in the levels of fructose-l, 6-bisphosphate (Fru-P₂), glucose-6-phosphate (Glc-6-P), fructose-6-phosphate (Fru-6-P), adenosine mono-, and diphosphate (AMP and ADP), phosphorylcholine (P-choline) and a proportional increase in the level of P₁. In the S plants, infection affected neither P₁ uptake nor P₁ proportions. In the plants grown after inoculation in diluted Knop’s solutions (0.147 mM KH₂PO₄), infection induced a reduction of the radial transport of P₁ to the segments of R roots whereas a reduction of the levels was only recorded in some P-esters [AMP, ADP, dihydroxyacetone phosphate (DHAP), and P-choline, but no decrease of Fru-P₂, Glc-6-P and Fru-6-P]. In the S plants, P₁ transport and the levels of P-esters were increased by the infection. P₁ transport exhibited considerable metabolic dependence (DNP, DCCD). Bacterial infection probably had no influence on the activity of the plasma membrane ATPases.     

Uptake and long-distance transport of phosphate,potassium and chloride in relation to internal ion concentrations in barley: evidence of non-allosteric regulation

The extent to which uptake and transport of either phosphate, potassium or chloride are controlled by the concentration of these ions within the root, perhaps through an allosteric mechanism, was investigated with young barley plants in nutrient solution culture. Plants were grown with their roots divided between two containers, such that a single seminal root was continuously supplied with all the required nutrient ions, while the remaining four or five seminal roots were either supplied with the same solution (controls) or, temporarily, a solution lacking a particular nutrient ion (nutrient-deficient treatment). Compared with controls, there was a marked stimulation of uptake and transport of labelled ions by the single root following 24 h or more of nutrient dificiency to the remainder of the root system. This stimulation, which comprised an increased transport to the shoot and, for all ions except Cl^-, increased transport to the remainder of the root system, took place without appreciable change in the concentration of particular ions within the single root. However, nutrient deficiency quickly caused a lower concentration of ions in the shoot and the remaining roots. The results are discussed in relation to various mechanisms, proposed in the literature, by which the coordination of ion uptake and transport may be maintained within the plant. We suggest that under our conditions any putative allosteric control of uptake and transport by root cortical cells was masked by an alternative mechanism, in which ion influx appears to be regulated by ion efflux to the xylem, perhaps controlled by the concentration of particular ions recycled in the phloem to the root from the shoot.     

Water Potential and Stomatal Resistance of Sunflower and Soybean Subjected to Water Stress during Various Growth Stages

Plants of two varieties of soybean (Glycine max (L.) Merr.) and two varieties of sunflower (Helianthus annuus L.) were grown in controlled environments and subjected to water stress at various stages of growth. Leaf resistances and leaf water potentials were measured as stress developed. In soybeans the upper leaf surface had a higher resistance than the lower surface at all leaf water potentials and growth stages. Resistance of the upper surface began to increase at a higher water potential and increased more than the resistance of the lower surface. Resistances returned to prestress values 4 days after rewatering. In sunflowers upper and lower leaf surfaces had similar resistances at all water potentials and growth stages. Leaf resistances were higher in sunflower plants stressed before flowering than in those stressed later. Sunflower plants stressed to −16 bars recovered their prestress leaf resistance and water potential a few days after rewatering, but leaves of sunflower plants stressed to −23 bars died. Leaves of soybean and sunflower plants stressed before flowering suffered less injury than those of older plants and sunflowers stressed after flowering suffered more injury than soybeans.     

Influence of temperature on root development and phosphate influx in winter wheat grown at different P levels

Root development was studied in winter wheat ( Triticum aestivum L. cv Starke II) grown at 5,10, 15 and 20°C in nutrient solutions with phosphate concentrations of 10, 100 or 1000 μM . The plants were grown for 38 days (5 and 10°C), 19 days (15°C) or 14 days (20°C). At the end of the cultivation period the phosphate influx in the roots was determined with ³²P-phosphate. Root development (lateral and seminal roof length and number) was monitored throughout the cultivation period on the same individuals by repeated (approximately every second day) photocopying of the roots for measurements with digitizer and appropriate software. The 5°C treatment yielded no laterals, and the seminals were only slightly affected by the different phosphate treatments. The 10 μM phosphate treatment gave high root:shoot dry weight ratio, high average lateral root length and high specific root length [m root (g root fresh weight)^-1]. The 1000 μM phosphate treatment yielded the highest number of laterals per m seminal root, and usually also the highest absolute numbers. Phosphate influx decreased with increased P status of the roots. It is argued that phosphate influx is dependent on factors such as P status, root geometry and relative root extension rate.     

Drought Induces Oxidative Stress and Enhances the Activities of Antioxidant Enzymes in Growing Rice Seedlings

When rice seedlings grown for 10 and 20 days were subjected to in vitro drought stress of −0.5 and −2.0 MPa for 24 h, an increase in the concentration of superoxide anion (O₂^.−), increased level of lipid peroxidation and a decrease in the concentration of total soluble protein and thiols was observed in stressed seedlings compared to controls. The concentration of H₂O₂ as well as ascorbic acid declined with imposition of drought stress, however glutathione (GSH) concentration declined only under severe drought stress. The activities of total superoxide dismutases (SODs) as well as ascorbate peroxidase (APX) showed consistent increases with increasing levels of drought stress, however catalase activity declined. Mild drought stressed plants had higher guaiacol peroxidase (GPX) and chloroplastic ascorbate peroxidase (c-APX) activity than control grown plants but the activity declined at the higher level of drought stress. The activities of enzymes involved in regeneration of ascorbate i.e. monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) were higher in drought stressed plants compared to controls. Results suggest that drought stress induces oxidative stress in rice plants and that besides SOD, the enzymes of ascorbate-glutathione cycle, which have not been studied in detail earlier under stressful conditions, appear to function as important component of antioxidative defense system under drought stress.     

Pre-harvest retting of flax: effects of water stress on uptake and efficacy of glyphosate

The effects of water stress on the uptake, translocation and efficacy of glyphosate in flax were investigated in relation to pre-harvest retting. Glyphosate (applied at a rate equivalent to 1.44 kg a.e. ha^-1 at 0, 10, 20 or 30 days after the start of flowering) caused little desiccation of flax grown in pots under restricted watering. Glyphosate application to well-watered plants caused the moisture content to decline from an initial value of 70 – 80% to approximately 40% at 3 wk after spraying. Glyphosate was applied 2 wk after the mid-point of flowering to flax grown in soil with moisture contents of 35, 31, 26, 22, 16 or 12%. Soil moisture levels (16% and 12%) which restricted evapotranspiration also reduced the efficacy of glyphosate but did not affect uptake of ¹⁴C-glyphosate. Translocation of ¹⁴C-glyphosate out of treated leaves was reduced only in the most severely stressed plants (12% soil moisture). Experiments with young plants (4 wk old) confirmed that water stress slightly reduced downward translocation of glyphosate. When the herbicide was applied to young plants under conditions which minimised differences in translocation, 10.8 μg glyphosate was sufficient to desiccate unstressed plants but 108 μg had little effect on stressed plants. This indicates that, in addition to any reduction in translocation which occurs during drought, water stress may reduce the susceptibility of flax to glyphosate. Thus only relief of plant water stress by irrigation is likely to improve response of the flax crop to glyphosate.     

The pattern of carboxylate exudation in Banksia grandis (Proteaceae) is affected by the form of phosphate added to the soil

Roots of a wide range of plant species exude carboxylates, e.g. citrate, into the rhizosphere, to mobilise sparingly available phosphate. We investigated the carboxylates in root exudates of Banksia grandisWilld. (Proteaceae), which occurs on severely phosphate-impoverished soils in Western Australia. Plants were grown in pots with a nutrient-poor quartz sand, with phosphate, at 25 g P g^–1, added as either K-phosphate, glycerol phosphate, Fe-phosphate or Al-phosphate.Plants grown on Fe-phosphate or Al-phosphate formed `proteoid' or `cluster' roots, and exuded significant amounts of carboxylates. Plants grown on K-phosphate did not form cluster roots; their leaves were chlorotic, and some of these plants died during the experiment. Plants grown on glycerol phosphate did have cluster roots, but their leaves also became chlorotic, albeit later in the experiment.Tri- and dicarboxylates (citrate, 60%; malate, 25%; trans-aconitate, 14%) were the major carboxylates in root exudates when P was supplied as Al-phosphate. The same tri- and dicarboxylates were also exuded when P was supplied as Fe-phosphate (31, 14 and 12%, respectively). In addition, these plants exuded monocarboxylates (lactate, 30%; acetate, 12%). We analysed the effect of the different carboxylates on the mobilisation of phosphate and Fe in two different types of soils. The ecological significance of the difference in exudate spectrum for the mobilisation of nutrients and for the detoxification of aluminium is discussed.Because the leaves of plants grown with K-phosphate or glycerol-phosphate appeared chlorotic, we analysed the concentrations of P, Fe, Zn, Mn and Cu in these leaves. Only the concentration of total P was considerably higher in leaves of plants grown with K- or glycerol-phosphate than that in leaves of plants grown with Fe- or Al-phosphate. Both the concentration of total Fe and that of reduced Fe was the same in chlorotic leaves as that in leaves of plants grown with Fe- or Al-phosphate, which had a healthy appearance. It is concluded that P-induced chlorosis was not due to a lack of total or reduced Fe; it may have been due to precipitation of Fe by phosphate.     

Effect of phosphate fertiliser and plant density on phosphate inflow into ryegrass roots in soil

Summary Ryegrass plants were grown in pots of Horotiu sandy loam, either singly or as a dense sward, with a range of P fertiliser rates and regular harvests. Plants were non-mycorrhizal. P inflows into roots increased with P fertiliser rate. Sward plants absorbed up to 14.3% of the P fertiliser added, and single plants up to 7.6%. Sward plants absorbed most of the fertiliser P available to them within two weeks of germinating. After that, root growth ceased except at the highest P fertiliser rate used, and P inflow into roots decreased from 10^–15 to 10^–16 mole/cm/sec. Single plants, however, had continuous root growth and P uptake throughout the two month experiment, except for those at the two lowest fertiliser rates. Singly grown plants absorbed much more P than each plant in the dense swards. In single plants, root length/root weight ratio increased with increasing P fertiliser. Plant growth was dependant on continued P uptake by the roots, which only occurred if new roots were continally produced and new volumes of soil tapped for P. Non-growing root systems absorbed very little P. re]19760201