Tight control of nitrate acquisition in a plant species that evolved in an extremely phosphorus‐impoverished environment

Hakea prostrata (Proteaceae) has evolved in an extremely phosphorus (P)‐limited environment. This species exhibits an exceptionally low ribosomal RNA (rRNA) and low protein and nitrogen (N) concentration in its leaves. Little is known about the N requirement of this species and its link to P metabolism, despite this being the key to understanding how it functions with a minimal P budget. H. prostrata plants were grown with various N supplies. Metabolite and elemental analyses were performed to determine its N requirement. H. prostrata maintained its organ N content and concentration at a set point, independent of a 25‐fold difference nitrate supplies. This is in sharp contrast to plants that are typically studied, which take up and store excess nitrate. Plants grown without nitrate had lower leaf chlorophyll and carotenoid concentrations, indicating N deficiency. However, H. prostrata plants at low or high nitrate availability had the same photosynthetic pigment levels and hence were not physiologically compromised by the treatments. The tight control of nitrate acquisition in H. prostrata retains protein at a very low level, which results in a low demand for rRNA and P. We surmise that the constrained nitrate acquisition is an adaptation to severely P‐impoverished soils.     

Exudation of carboxylates in Australian Proteaceae: chemical composition

Roots of a wide range of plant species exude carboxylates, such as citrate, into the rhizosphere. In the present study, seedlings of a range of Australian Banksia, Hakea and Dryandra species (Proteaceae) were assayed for their exudation of carboxylates. All of these species (Hakea prostrata, Hakea undulata, Hakea petiolaris, Hakea baxteri, Banksia grandis, Banksia prionotes, Banksia occidentalis and Dryandra sessilis) form cluster roots when grown in nutrient solution with a low phosphate concentration. Exudation of carboxylates was studied for cluster roots and non‐cluster roots separately, and for the entire root system. Cluster roots of these Proteaceae exuded malate, malonate, lactate, acetate, maleate, citrate, fumarate, cis‐ and trans‐aconitate. The relative contributions of each of these carboxylates differed between species. Malate, malonate, lactate, citrate and trans‐aconitate, however, were invariably present in large proportions of total carboxylate exudation. Non‐cluster roots of H. prostrata exuded a spectrum of carboxylates (mainly malonate, lactate and citrate), which differed somewhat from the exudation pattern of cluster roots (mainly malate, malonate, lactate and citrate). The rate of exudation for cluster roots of the seven species was approximately 1·6 nmol g^?1 FM s^?1, which is considerably higher than that reported for a variety of crop and native species that do or do not form cluster roots. Contrary to what occurs in the cluster roots of Lupinus albus, which release carboxylates accompanied by protons so that the rhizosphere is acidified, the present Proteaceae exude the carboxylates as anions without concomitant proton release. The role of carboxylates in the mobilization of phosphate and other nutrients from soil is discussed.     

Cluster Roots: A Curiosity in Context

Cluster roots are an adaptation for nutrient acquisition from nutrient-poor soils. They develop on root systems of a range of species belonging to a number of different families (e.g., Proteaceae, Casuarinaceae, Fabaceae and Myricaceae) and are also found on root systems of some crop species (e.g., albus, Macadamia integrifoliaandCucurbita pepo). Their morphology is variable but typically, large numbers of determinate branch roots develop over very short distances of main root axes. Root clusters are ephemeral, and continually replaced by extension of the main root axes. Carboxylates are released from cluster roots at very fast rates for only a few days during a brief developmental window termed an ‘exudative burst’. Most of the studies of cluster-root metabolism have been carried out using the crop plant L. albus, but results on native plants have provided important additional information on carbon metabolism and exudate composition. Cluster-root forming species are generally non-mycorrhizal, and rely upon their specialised roots for the acquisition of phosphorus and other scarcely available nutrients. Phosphorus is a key plant nutrient for altering cluster-root formation, but their formation is also influenced by N and Fe. The initiation and growth of cluster roots is enhanced when plants are grown at a very low phosphate supply (viz. ≤1 μM P), and cluster-root suppression occurs at relatively higher P supplies. An important feature of some Proteaceae is storage of phosphorus in stem tissues which is associated with the seasonality of cluster-root development and P uptake (winter) and shoot growth (summer), and also maintains low leaf [P]. Some species of Proteaceae develop symptoms of P toxicity at relatively low external P supply. Our findings with Hakea prostrata (Proteaceae) indicate that P-toxicity symptoms result after the capacity of tissues to store P is exceeded. P accumulation in H. prostrata is due to its strongly decreased capacity to down-regulate P uptake when the external P supply is supra-optimal. The present review investigates cluster-root functioning in (1) L.albus (white lupin), the model crop plant for cluster-root studies, and (2) native Proteaceae that have evolved in phosphate-impoverished environments.     

Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system

The present study was carried out to investigate whether the P concentration in the roots or the shoots controls the growth and citrate exudation of cluster roots in white lupin (Lupinus albus L). Foliar P application indicated that low P concentration in the shoots enhanced cluster‐root growth and citrate‐exudation rate more so than low P concentration in the roots. In the split‐root study, the P concentration in the shoots increased with increased P supply (1, 25 or 75 mmol m^?3 P), to the ‘privileged’ root halves. Roots ‘deprived’ of P invariably had the same low P concentrations, whereas those in the ‘privileged’ roots increased with increasing P supply (1, 25 or 75 mmol m^?3 P). Nevertheless, the proportion of the total root mass allocated to cluster roots, and the citrate‐exudation rates from the root halves were always similar on both root halves, irrespective of P supply, and decreased with increasing shoot P concentrations. Peak citrate exudation rates from developing cluster roots were significantly faster from cluster roots on the ‘deprived’ root halves when the ‘privileged’ half was exposed to 1 mmol m^?3 P as compared with 25 or 75 mmol m^?3 P. The possibility that changes in the concentrations of P fractions in the root halves influenced cluster‐root growth and citrate exudation was discounted, because there were no significant differences in insoluble organic P, ester‐P and inorganic P among all ‘deprived’ root halves. The results indicate that cluster‐root proportions and citrate exudation rates were regulated systemically by the P status of the shoot, and that P concentrations in the roots had little influence on growth and citrate exudation of cluster roots in L. albus.     

Changes in NO3− and K+ uptake by four species in flowing solution culture in response to increased irradiance

Net rates of NO₃^? and K⁺ uptake were compared for oilseed rape (Brassica napus L. cv. Jet neuf), perennial ryegrass (Lolium perenne L. cv. S23), Italian ryegrass (Lolium multiflorum Lam. cv. Augusta) and wheat (Triticum aestivum L. cv. Fen-man) in flowing solution culture during a 4-day sequence of low-low-high-high natural irradiance. Concentrations of NO₃^? (10 μM) and K⁺ (2.5 μM) in solutions were maintained automatically and hourly variation in net uptake of these ions was measured. During the 2 days of low irradiance (<1 MJ m^?2 day^?1) the uptake rates of both ions by all species were low at <1 mmol NO₃^?, m^?2 h^?1 and <0.4 mmol K⁺ m^?2 h^?1. Uptake increased in each species during the first day of high irradiance (7.90 MJ m^?2 day^?1) to >4 mmol NO₃^? m^?2 h^?1 and >1.4 mmol K⁺ m^?1 h^?1. These higher rates were maintained throughout the following night. The lag-time between maximum irradiance and the onset of the highest acceleration in uptake was greater for NO₃^? (5–8 h) than for K⁺ (≤1 h) in rape, wheat and Italian ryegrass. Uptake of NO₃^?, by perennial ryegrass showed an almost constant acceleration for 18 h following maximum irradiance. In all species the measured maximum inflows (uptake rate per unit root length) of both ions were greater than theoretical maximum potential inflows to a non-competing infinite-sink root in soil, by factors of 7 and 36, respectively, for NO₃^? and K⁺, averaged over all species.     

Positive effects of continuous low nitrate levels on growth and photosynthesis of Alexandrium tamarense (Gonyaulacales, Dinophyceae)

The growth and photosynthesis of Alexandrium tamarense (Lebour) Balech in different nutrient conditions were investigated. Low nitrate level (0.0882 mmol/L) resulted in the highest average growth rate from day 0 to day 10 (4.58 × 10² cells mL^?1 d^?1), but the lowest cell yield (5420 cells mL^?1) in three nitrate level cultures. High nitrate‐grown cells showed lower levels of chlorophyll a‐specific and cell‐specific light‐saturated photosynthetic rate (P_m^{chl a} and P_m^cell), dark respiration rate (R_d^chla and R_d^cell) and chlorophyll a‐specific apparent photosynthetic efficiency (α^chla) than was seen for low nitrate‐grown cells; whereas the cells became light saturated at higher irradiance at low nitrate condition. When cultures at low nitrate were supplemented with nitrate at 0.7938 mmol/L in late exponential growth phase, or with nitrate at 0.7938 mmol/L and phosphate at 0.072 mmol/L in stationary growth phase, the cell yield was drastically enhanced, a 7–9 times increase compared with non‐supplemented control culture, achieving 43 540 cells mL^?1 and 52 300 cells mL^?1, respectively; however, supplementation with nitrate in the stationary growth phase or with nitrate and phosphate in the late exponential growth phase increased the cell yield by no more than 2 times. The results suggested that continuous low level of nitrate with sufficient supply of phosphate may facilitate the growth of A. tamarense.     

Physiological and morphological adaptations of herbaceous perennial legumes allow differential access to sources of varyingly soluble phosphate

The aim of this study was to investigate the capacity of three perennial legume species to access sources of varyingly soluble phosphorus (P) and their associated morphological and physiological adaptations. Two Australian native legumes with pasture potential (Cullen australasicum and Kennedia prostrata) and Medicago sativa cv. SARDI 10 were grown in sand under two P levels (6 and 40 µg P g^?1) supplied as Ca(H₂PO₄)₂·H₂O (Ca‐P, highly soluble, used in many fertilizers) or as one of three sparingly soluble forms: Ca₁₀(OH)₂(PO₄)₆ (apatite‐P, found in relatively young soils; major constituent of rock phosphate), C₆H₆O₂₄P₆Na₁₂ (inositol‐P, the most common form of organic P in soil) and FePO₄ (Fe‐P, a poorly‐available inorganic source of P). All species grew well with soluble P. When 6 µg P g^?1 was supplied as sparingly soluble P, plant dry weight (DW) and P uptake were very low for C. australasicum and M. sativa (0.1–0.4 g DW) with the exception of M. sativa supplied with apatite‐P (1.5 g). In contrast, K. prostrata grew well with inositol‐P (1.0 g) and Fe‐P (0.7 g), and even better with apatite‐P (1.7 g), similar to that with Ca‐P (1.9 g). Phosphorus uptake at 6 µg P g^?1 was highly correlated with total root length, total rhizosphere carboxylate content and total rhizosphere acid phosphatase (EC 3.1.3.2) activity. These findings provide strong indications that there are opportunities to utilize local Australian legumes in low P pasture systems to access sparingly soluble soil P and increase perennial legume productivity, diversity and sustainability.     

Discrimination of nitrogen isotopes during absorption of ammonium and nitrate at different nitrogen concentrations by rice (Oryza sativa L.) plants

Studies of uptake of ionic sources of N by two hydroponically grown rice (Oryza sativa L.) cultivars (paddy‐field‐adapted Koshihikari and dryland‐adapted Kanto 168) showed that the magnitude of the nitrogen isotope fractionation (?) for uptake of NH₄⁺ depended on the concentrations of NH₄⁺ and cultivar (averaging –6·1‰ for Koshihikari and –12·0‰ for Kanto 168 at concentrations from 40 to 200 mmol m^?3 and, respectively, –13·4 and –28·9‰ for the two cultivars at concentrations from 0·5 to 4 mol m^?3). In contrast, the ? for uptake of NO₃^? in similar experiments was almost insensitive to the N concentration, falling within a much narrower range (+3·2‰ to –0·9‰ for Koshihikari and –0·9‰ to –5·1‰ for Kanto 168 over NO₃^? concentrations from 0·04 to 2 mol m^?3). From longer term experiments in which Norin 8 and its nitrate‐reductase deficient mutant M819 were grown with 2 or 8 mol m^?3 NO₃^? for 30 d, it was concluded that the small concentration‐independent isotopic fractionation during absorption of this ion was not related to nitrate reductase activity.     

Variation in seedling growth of 11 perennial legumes in response to phosphorus supply

Phosphorus (P) deficiency is a major problem for Australian agriculture. Development of new perennial pasture legumes that acquire or use P more efficiently than the current major perennial pasture legume, lucerne (Medicago sativa L.), is urgent. A glasshouse experiment compared the response of ten perennial herbaceous legume species to a series of P supplies ranging from 0 to 384 µg g^?1 soil, with lucerne as the control. Under low-P conditions, several legumes produced more biomass than lucerne. Four species (Lotononis bainesii Baker, Kennedia prorepens F.Muell, K. prostrata R.Br, Bituminaria bituminosa (L.) C.H.Stirt) achieved maximum growth at 12 µg P g^?1 soil, while other species required 24 µg P g^?1. In most tested legumes, biomass production was reduced when P supply was ≥192 µg g^?1, due to P toxicity, while L. bainesii and K. prorepens showed reduced biomass when P was ≥24 µg g^?1 and K. prostrata at ≥48 µg P g^?1 soil. B. bituminosa and Glycine canescens F.J.Herm required less soil P to achieve 0.5 g dry mass than the other species did. Lucerne performed poorly with low P supply and our results suggest that some novel perennial legumes may perform better on low-P soils.     

Sensitivity of different Lupinus species to calcium under a low phosphorus supply

To study mechanism underpinning the calcifuge habit of some Lupinus species, especially under low‐phosphorus (P) conditions, Lupinus species that were likely to respond differently to calcium (Ca) availability were assembled, and the sensitivity to Ca under a low‐P supply was assessed. Seven Lupinus species (9 genotypes, L. albus L. cv Kiev, L. albus L. P26766, L. angustifolius L. cv Mandelup, L. angustifolius L. P26723, L. luteus L. cv Pootalong, L. hispanicus ssp. bicolor Boiss. and Reut. P22999, L. pilosus Murr. P27440, L. cosentinii Guss. P27225, and L. atlanticus Gladst. P27219) were grown hydroponically at 10 or 6000 μM Ca. Leaf symptoms, gas exchange and biomass were recorded; leaf and root nutrient concentrations were analysed, and the leaf cell types in which Ca and P accumulated were determined using elemental X‐ray microanalyses. Calcium toxicity was demonstrated for L. angustifolius P26723, L. hispanicus ssp. bicolor. P22999, and L. cosentinii P27225, whereas the other species were tolerant of a high Ca supply under low‐P conditions. In addition, genotypic differences in Ca toxicity were found within L. angustifolius. Most Ca accumulated in the mesophyll cells in all species, whereas most P was located in epidermal cells.     

Mechanisms for tolerance of very high tissue phosphorus concentrations in Ptilotus polystachyus

Study of plants with unusual phosphorus (P) physiology may assist development of more P‐efficient crops. Ptilotus polystachyus grows well at high P supply, when shoot P concentrations ( [P] ) may exceed 40 mg P g^?1 dry matter (DM). We explored the P physiology of P. polystachyus seedlings grown in nutrient solution with 0–5 mM P. In addition, young leaves and roots of soil‐grown plants were used for cryo‐scanning electron microscopy and X‐ray microanalysis. No P‐toxicity symptoms were observed, even at 5 mM P in solution. Shoot DM was similar at 0.1 and 1.0 mM P in solution, but was ～14% lower at 2 and 5 mM P. At 1 mM P, [P] was 36, 18, 14 and 11 mg P g^?1 DM in mature leaves, young leaves, stems and roots, respectively. Leaf potassium, calcium and magnesium concentrations increased with increasing P supply. Leaf epidermal and palisade mesophyll cells had similar [P]. The root epidermis and most cortical cells had senesced, even in young roots. We conclude that preferential accumulation of P in mature leaves, accumulation of balancing cations and uniform distribution of P across leaf cell types allow P. polystachyus to tolerate very high leaf [P].     

Moderating mycorrhizas: arbuscular mycorrhizas modify rhizosphere chemistry and maintain plant phosphorus status within narrow boundaries

Pastures often experience a pulse of phosphorus (P) when fertilized. We examined the role of arbuscular mycorrhizal fungi (AMF) in the uptake of P from a pulse. Five legumes (Kennedia prostrata, Cullen australasicum, Bituminaria bituminosa, Medicago sativa and Trifolium subterraneum) were grown in a moderate P, sterilized field soil, either with (+AMF) or without (?AMF) addition of unsterilized field soil. After 9–10 weeks, half the pots received 15 mg P kg^?1 of soil. One week later, we measured: shoot and root dry weights; percentage of root length colonized by AMF; plant P, nitrogen and manganese (Mn) concentrations; and rhizosphere carboxylates, pH and plant‐available P. The P pulse raised root P concentration by a similar amount in uncolonized and colonized plants, but shoot P concentration increased by 143% in uncolonized plants and 53% in colonized plants. Inoculation with AMF decreased the amount of rhizosphere carboxylates by 52%, raised rhizosphere pH by ～0.2–0.7 pH units and lowered shoot Mn concentration by 38%. We conclude that AMF are not simply a means for plants to enhance P uptake when P is limiting, but also act to maintain shoot P within narrow boundaries and can affect nutrient uptake through their influence on rhizosphere chemistry.     

Tissue and cellular phosphorus storage during development of phosphorus toxicity in Hakea prostrata (Proteaceae)

Storage of phosphorus (P) in stem tissue is important in Mediterranean Proteaceae, because proteoid root growth and P uptake is greatest during winter, whereas shoot growth occurs mostly in summer. This has prompted the present investigation of the P distribution amongst roots, stems, and leaves of Hakea prostrata R.Br. (Proteaceae) when grown in nutrient solutions at ten P-supply rates. Glasshouse experiments were carried out during both winter and summer months. For plants grown in the low-P range (0, 0.3, 1.2, 3.0, or 6.0 micromol d(-1)) the root [P] was > stem and leaf [P]. In contrast, leaf [P] > stem and root [P] for plants grown in the high-P range (6.0, 30, 60, 150, or 300 micromol P d(-1)). At the highest P-supply rates, the capacity for P storage in stems and roots appears to have been exceeded, and leaf [P] thereafter increased dramatically to approximately 10 mg P g(-1) dry mass. This high leaf [P] was coincident with foliar symptoms of P toxicity which were similar to those described for many other species, including non-Proteaceae. The published values (tissue [P]) at which P toxicity occurs in a range of species are summarized. X-ray microanalysis of frozen, full-hydrated leaves revealed that the [P] in vacuoles of epidermal, palisade and bundle-sheath cells were in the mM range when plants were grown at low P-supply, even though very low leaf [P] was measured in bulk leaf samples. At higher P-supply rates, P accumulated in vacuoles of palisade cells which were associated with decreased photosynthetic rates.     

UPTAKE AND TRANSLOCATION OF TI FROM NANOPARTICLES IN CROPS AND WETLAND PLANTS

Bioavailability of engineered metal nanoparticles affects uptake in plants, impacts on ecosystems, and phytoremediation. We studied uptake and translocation of Ti in plants when the main source of this metal was TiO₂ nanoparticles. Two crops (Phaseolus vulgaris (bean) and Triticum aestivum (wheat)), a wetland species (Rumex crispus, curly dock), and the floating aquatic plant (Elodea canadensis, Canadian waterweed), were grown in nutrient solutions with TiO₂ nanoparticles (0, 6, 18 mmol Ti L^?1 for P. vulgaris, T. aestivum, and R. crispus; and 0 and 12 mmol Ti L^?1 for E. canadensis). Also examined in E. canadensis was the influence of TiO₂ nanoparticles upon the uptake of Fe, Mn, and Mg, and the influence of P on Ti uptake. For the rooted plants, exposure to TiO₂ nanoparticles did not affect biomass production, but significantly increased root Ti sorption and uptake. R. crispus showed translocation of Ti into the shoots. E. canadensis also showed significant uptake of Ti, P in the nutrient solution significantly decreased Ti uptake, and the uptake patterns of Mn and Mg were altered. Ti from nano-Ti was bioavailable to plants, thus showing the potential for cycling in ecosystems and for phytoremediation, particularly where water is the main carrier.     

Phosphorus uptake and translocation in salt-stressed melon plants

Melon seedlings (Cucumis melo L. cv.Galia) were grown hydroponically to study the effect of salinity (80 mmol/LNaCl) on phosphate (Pi) uptake and translocation at two levels of Pi (25 μmol/L and 1 mmol/L). Net uptake rates of Pi were determined by depletionof the medium and by plant content. Salinity decreased Pi uptake at low Pi (high affinity uptake mechanism), 25 μmol/L, although no specific competitive inhibition of Pi uptake by Cl⁻ was observed. When plants were grown with high Pi (1 mmol/L), the uptake of Pi through the low affinity system was increased by 80 mmol/L NaCl. Salinity also reduced the phosphorus flux, as Pi, through the xylem. It is hypothesised that high levels of NaCl decrease the mobility of Pi stored in vacuoles, and as a result, inhibit export from this storage compartment to other parts of the plant.     

A comparison of the effects of auxin on cluster root initiation and development in Grevillea robusta Cunn. ex R. Br. (Proteaceae) and in the genus Lupinus (Leguminosae)

The effect of NAA (naphthaleneacetic acid) on the development of cluster roots in members of the Proteaceae and Leguminosae was investigated. The exogenous addition of NAA led to initiation of cluster roots in phosphate conditions normally inhibitory for their development, but initiation took place within the limits of the cluster pattern under –P conditions. There was no change in spacing within the cluster root nor between cluster roots in Grevillea robusta Cunn. ex R. Br. or in rootlet length or cluster root length. In Lupinus albus L., change in rootlet length and cluster root length was noted at 10^-10 and 10¹² M NAA. In L. albus, the length of time that roots were exposed to NAA does not appear to be important, with similar levels of cluster root initiation after 48 h and 7 days. Cluster root production in G. robusta differed from that in L. albus in terms of the concentration of NAA needed to induce initiation, and in the effects of extremely low levels of NAA on rootlet numbers and lengths. L. arboreus L. does not produce cluster roots under –P conditions. Furthermore, neither L. arboreus L., L. angustifolius L., L. luteus L. nor L. mutabilis L. were induced to produce cluster roots under –P conditions, nor under +P conditions in the presence of exogenous NAA. Thus, exogenous NAA only leads to the induction of cluster roots, at levels of P normally inhibitive of their development, in species of Lupinus that produce them under –P conditions. Auxin-induced cluster roots develop within the same constraints as those developing under –P conditions. NAA does not induce cluster roots in species of Lupinus that do not produce them under –P conditions.     

White lupin utilizes soil phosphorus that is unavailable to soybean

White lupin (Lupinus albus L. var. Ultra) and soybean (Glycine max L. var. Elgin) were grown in an acidic soil low an available phosphorus (P) to investigate their different capacities to acquire soil phosphorus. Experiments done in the controlled environment of a biotron were supplemented with four separate greenhouse experiments. Lupin and soybean were grown in monoculture and intercropped on a soil with low available P that was labeled with carrier-free ³²P as phosphate. Lupin had significantly lower values of specific activity of ³²P and higher values of isotopically exchangeable P than soybean in all cases. The results show that lupin utilizes soil P from a normally non-labile pool of soil P that is not utilized by soybean.     

Nickel toxicity and peroxidase activity in seedlings of Triticum aestivum L.

Ni²⁺ toxicity was evaluated in Triticum aestivum L. by its effects on root and shoot length, dry matter production and water content. Over a threshold value of 20 mmol m^?3 Ni²⁺ the degree of toxicity increases as a function of the Ni²⁺ concentration in the medium. Ni²⁺-treated roots show enhanced lipid peroxidation; the higher Ni²⁺ treatment (40mmol m^?3) also increases leakage of K⁺. In roots and shoots, Ni²⁺ enhances both guaiacol and syringaldazine extracellular peroxidase activity. The increase in extracellular peroxidase activity is also associated with an increase in the phenolic contents of roots and shoots. The observed growth inhibition might be partly the result of the effect of Ni²⁺ on cell turgor and cell-wall extensibility. Intracellular soluble peroxidases are also stimulated by Ni²⁺; such effects, independently of the substrate, were detected in extracts of Ni²⁺-treated shoots at a lower Ni²⁺ concentration than in the roots. Intracellular peroxidases might act as scavengers of peroxide radicals produced as a result of nickel toxicity.     

Phosphorus benefits of different legume crops to subsequent wheat grown in different soils of Western Australia

Certain legume crops, including white lupin (Lupinus albus L.), mobilise soil-bound phosphorus (P) through root exudates. The changes in the rhizosphere enhance P availability to these crops, and possibly to subsequent crops growing in the same soil. We conducted a pot experiment to compare phosphorus acquisition of three legume species with that of wheat, and to determine whether the legume crops influence growth and P uptake of a subsequent wheat crop. Field pea (Pisum sativum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.) and wheat (Triticum aestivum L.) were grown in three different soils to which we added no or 20 mg P kg^–1 soil (P0, P20). Growth, P content and rhizosphere carboxylates varied significantly amongst crops, soils and P levels. Total P content of the plants was increased with applied phosphorus. Phosphorus content of faba bean was 3.9 and 8.8 mg/pot, at P0 and P20, respectively, which was about double that of all other species at the respective P levels. Field pea and white lupin had large amounts of rhizosphere carboxylates, whereas wheat and faba bean had negligible amounts in all three soils at both P levels. Wheat grew better after legumes than after wheat in all three soils. The effect of the previous plant species was greater when these previous species had received P fertiliser. All the legumes increased plant biomass of subsequent wheat significantly over the unplanted pots in all the soils. Faba bean was unparalleled in promoting subsequent wheat growth on all fertilised soils. This experiment clearly demonstrated a residual benefit of the legume crops on the growth of the subsequent wheat crop due to enhanced P uptake. Faba bean appeared to be a suitable P-mobilising legume crop plant for use in rotations with wheat.     

Regulation of the High-Affinity Nitrate Transport System in Wheat Roots by Exogenous Abscisic Acid and Glutamine

Nitrate is a major nitrogen （N） source for most crops. Nitrate uptake by root cells is a key step of nitrogen metabolism and has been widely studied at the physiological and molecular levels. Understanding how nitrate uptake is regulated will help us engineer crops with improved nitrate uptake efficiency. The present study investigated the regulation of the high-affinity nitrate transport system （HATS） by exogenous abscisic acid （ABA） and glutamine （Gin） in wheat （Triticum aestivum L.） roots. Wheat seedlings grown in nutrient solution containing 2 mmol/L nitrate as the only nitrogen source for 2weeks were deprived of N for 4d and were then transferred to nutrient solution containing 50 μmol/L ABA, and 1 mmol/L Gin in the presence or absence of 2 mmol/L nitrate for 0, 0.5, 1, 2, 4, and 8 h. Treated wheat plants were then divided into two groups. One group of plants was used to investigate the mRNA levels of the HATS components NRT2 and NAR2 genes in roots through semi-quantitative RT-PCR approach, and the other set of plants were used to measure high-affinity nitrate influx rates in a nutrient solution containing 0.2 mmol/L ^15N-labeled nitrate. The results showed that exogenous ABA induced the expression of the TaNRT2.1, TaNRT2.2, TaNRT2.3, TaNAR2.1, and TaNAR2.2 genes in roots when nitrate was not present in the nutrient solution, but did not further enhance the induction of these genes by nitrate. Glutamine, which has been shown to inhibit the expression of NRT2 genes when nitrate is present in the growth media, did not inhibit this induction. When Gin was supplied to a nitrate-free nutrient solution, the expression of these five genes in roots was induced. These results imply that the inhibition by Gin of NRT2 expression occurs only when nitrate is present in the growth media. Although exogenous ABA and Gin induced HATS genes in the roots of wheat, they did not induce nitrate influx.