Physiological Characterization of a Single-Gene Mutant of Pisum sativum Exhibiting Excess Iron Accumulation: I. Root Iron Reduction and Iron Uptake

Root systems of mutant (E107) and parental (cv `Sparkle') Pisum sativum genotypes were studied to determine the basis for excess Fe accumulation in E107. Plants were grown with (+Fe-treated) or without (−Fe-treated) added Fe(III)-N,N'-ethylenebis[2-(2-hydroxyphenyl)glycine] in aerated nutrient solutions. Daily measurements of Fe(III) reduction indicated a four-to seven-fold higher reduction rate in +Fe- or −Fe-treated E107, and −Fe-treated Sparkle, when compared with +Fe-treated Sparkle. An agarose-based staining technique used to localize Fe(III) reduction, revealed Fe(III) reduction over most of the length of the roots (but not at the root apices) in both E107 treatments and −Fe-treated Sparkle. In +Fe-treated Sparkle, Fe(III) reduction was either nonexistent or localized to central regions of the roots. Measurements of short-term Fe influx (with 0.1 millimolar ⁵⁹Fe(III)-ethylenediaminetetraacetic acid) was also enhanced (threefold) in +Fe- or −Fe-treated E107 and −Fe-treated Sparkle, relative to +Fe-treated Sparkle. The physiological characteristics of E107 root systems, which are similar to those seen in Fe-deficient Sparkle, have led us to conclude that the mutation causes E107 to act functionally as an Fe-deficient plant, and appears to explain the excess Fe accumulation in E107.     

Pleiotropic Effects of brz: A Mutation in Pisum sativum (L.) cv ;Sparkle' Conditioning Decreased Nodulation and Increased Iron Uptake and Leaf Necrosis

Treatment of Pisum sativum (L.) cv `Sparkle' with ethylmethane sulfonic acid produced a stable mutant, E107, which forms few nodules. The mutant allele exhibits other pleiotropic properties including bronze necrotic spots on the leaflets and high accumulation of iron in the shoot. The mutant phenotype is under monogenic recessive control. The gene, designated brz (bronze), is nonallelic with two other genes conditioning necrotic spots on leaves of other mutants of P. sativum. The brz allele was located on chromosome 4 by linkage with wax production controlled by alleles at the was locus.     

Excessive aluminium accumulation in the pea mutant E107 (brz)

E107 is a pleiotropic mutant of peaPisum sativum cv. ‘Sparkle’, characterized by forming few nodules and developing bronze necrotic spots on older leaves. The mutant accumulates Al and has symptoms typical of Al toxicity. The lateral roots of E107 are fewer (40%) and shorter (50%) than those of its parent. High concentrations of Al accumulate in E107 shoots (1000 mg kg^-1) and roots (3000 mg kg^-1), and three-week old E107 plants extrude 2.5 times more protons than ‘Sparkle’ plants of similar age. Al concentrations of the roots of the mutant and of its parent ‘Sparkle’ are similar for the first two weeks of growth. Thereafter they differ. In 2 week old plants Al continues to accumulate in excessive amounts in E107 primary and lateral roots whereas in ‘Sparkle’ roots, it reaches a plateau. In E107, Al is erratically distributed in the walls of root hairs and epidermal cells in both primary and lateral roots. Some of these cells have also Al in their nucleus.     

Ethylene involvement in the over-expression of Fe(III)-chelate reductase by roots of E107 pea [Pisum sativum L. (brz, brz)] and chloronerva tomato (Lycopersicon esculentum L.) mutant genotypes

Recently, ethylene was reported to be involved in the regulation of Fe(III)-chelate reducing capacity by cucumber (Cucuinis sativus L.) roots. Here, we studied the effect of two ethylene inhibitors, aminooxyacetic acid (AOA) and cobalt, on the Fe(III) reducing capacity in roots of mutant genotypes [E107 pea [Pisum sativum L. (brz, brz)] and chloronerva tomato (Lycopersicon esculentum L.) that exhibit high rates of Fe(III)-chelate reduction and excessive iron accumulation. The ethylene inhibitors, AOA and cobalt, markedly inhibited Fe(III)-chelate reducing capacity in roots of both genotypes. Over-expression of root Fe(III) reductase activity by both mutants appears to be related to ethylene. Possibly, both mutants are genetically defective in their ability to regulate root ethylene production. The large inhibitory effect of both ethylene inhibitors on Fe(III)-chelate reducing capacity in roots of the mutant tomato genotype, chloronerva, disputes the contention that the nicotianamine-Fe(II) complex is the repressior of the gene responsible for Fe(III)-chelate reductase activity, as previously suggested by others. However, since nicotianamine shares the same biosynthetic precursor as ethylene, i.e. S-adenosyl methionine, nicotianamine may affect Fe(III)-chelate reductase activity in dicot and non-grass monocot roots by influencing ethylene biosynthesis.     

Iron requirement for and effects of promoters and inhibitors of ethylene action on stimulation of Fe(III)-chelate reductase in roots of strategy I species

Stimulation of root Fe(III) reductase activity by iron additions to iron-deficient growth media may be the result of iron activation of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase required for ethylene biosynthesis. Two different ethylene inhibitors, aminooxyacetic acid (AOA) (20 m; ACC synthase inhibitor) and cobalt (3 m CoCl₂; ACC oxidase inhibitor), were used to study the effects of iron supply and cobalt inhibition on ethylene action in controlling the activity of Fe(III)-chelate reductase in pea (Pisum sativum L.) roots. Supplying 20 gm m Fe(III)-N,N-ethylenebis[2-(2-hydroxypheyl)-glycine [Fe(III)-EDDHA] to either cobalt-treated, iron-deficient Sparkle (normal parent) or E107 (brz mutant genotype) pea seedlings reversed the negative effects of cobalt on root Fe(III)-reductase activity. Re-supplying 20 m Fe(III)-EDDHA to iron-deficient, AOA-treated seedlings did not enhance root Fe(III)-reductase. Apparently, cobalt competes with iron for the active site in ACC oxidase during ethylene synthesis. Inhibition of root reductase activity by cobalt treatment lowered manganese, zinc, magnesium and potassium content of mutant E107 pea seedlings. In contrast, iron enhancement of root reductase activity in iron-deficient, cobalt-treated E107 seedlings resulted in higher seedling accumulations of manganese, zinc, magnesium and potassium. These results support the hypothesis that root cell plasma membrane reductase activity plays a role in cation uptake by root cells.     

Ethylene Inhibitors Partly Restore Nodulation to Pea Mutant E 107 (brz)

E107 (brz) is a pleiotropic mutant of pea (Pisum sativum L. cv Sparkle) characterized by low nodulation, leaf necrosis, excessive ion accumulation, and decreased plant size. The defective nodulation of E107 was studied by light microscopy of lateral roots. The number of infections per centimeter of lateral root was only a third that of Sparkle. Moreover, most of the infections were aborted early; i.e. in only 14% of the infections did the infection thread penetrate beyond the epidermis. Nodulation of E107 was partly restored by treating the plant with the ethylene inhibitors aminoethoxyvinylglycine (AVG) or Ag⁺. Treatment with Ag⁺ did not increase the number of infections, but half of the infections went to completion. Ag⁺ and AVG did not alter the size of the mutant, the accumulation of cations in its shoots, nor the leaf necrosis. Thus, in E107, nodule development can be uncoupled from other pleiotropic characteristics.     

Accumulation of apoplastic iron in plant roots : a factor in the resistance of soybeans to iron-deficiency induced chlorosis?

We hypothesized that the resistance of Hawkeye (HA) soybean (Glycine max L.) to iron-deficiency induced chlorosis (IDC) is correlated to an ability to accumulate a large pool of extracellular-root iron which can be mobilized to shoots as the plants become iron deficient. Iron in the root apoplast was assayed after efflux from the roots of intact plants in nutrient solution treated with sodium dithionite added under anaerobic conditions. Young seedlings of HA soybean accumulated a significantly larger amount of extracellular iron in their roots than did either IDC-susceptible PI-54619 (PI) soybean or IDC-resistant IS-8001 (IS) sunflower (Helianthus annus L.). Concurrently, HA soybean had much higher concentrations of iron in their shoots than either PI soybean or IS sunflower. The concentration of iron in the root apoplast and in shoots of HA soybean decreased sharply within days after the first measurements of extracellular root iron were made, in both +Fe and −Fe treatments. The accumulation of short-term iron reserves in the root apoplast and translocation of iron in large quantities to the shoot may be important characteristics of IDC resistance in soybeans.     

Contact with ballotini (glass spheres) stimulates exudation of iron reducing and iron chelating substances from barley roots

Contact between roots and Fe-containing solid substrate is known to facilitate acquisition of iron by plants, but the actual mechanism of this contact effect is not yet clear. This study was undertaken to evaluate the effect of root contact with ballotini (glass spheres) on exudation of substances capable of reducing or chelating insoluble Fe(III) compounds by the roots of barley (Hordeum vulgare L. cv. Europa) seedlings. Seedlings with roots encountering mechanical impedance (i.e., in contact with ballotini) produced more lateral roots than the seedlings with unimpeded (i.e., freely suspended) roots in the nutrient solution. Nutrient solution bathing roots in contact with ballotini showed higher concentrations of Fe(III)-chelating (83% on day 7) and Fe(III)-reducing (107% on day 12) substances than solutions bathing unimpeded roots. The pH of all solutions rose continuously during the course of the experiment but was always lower (by a nonsignificant degree) in the solutions with roots in contact with ballotini than in those with unimpeded roots. The data indicate that under natural soil conditions the amount of Fe-chelating and Fe-reducing root exudates may be higher than is usually measured from roots of terrestrial plants artificially suspended in nutrient solution.     

Does Iron Deficiency in Pisum sativum Enhance the Activity of the Root Plasmalemma Iron Transport Protein?

Roots of Fe-sufficient and Fe-Deficient pea (Pisum sativum L.) were studied to determine the effect of Fe-deficiency on the activity of the root-cell plasmalemma Fe²⁺ transport protein. Rates of Fe(III) reduction and short-term Fe²⁺ influx were sequentially determined in excised primary lateral roots using Fe(III)-ethylene-diaminetetraacetic acid (Fe[III]-EDTA). Since the extracellular Fe²⁺ for membrane transport was generated by root Fe(III) reduction, rates of Fe²⁺ influx for each root system were normalized on the basis of Fe(III) reducing activity. Ratios of Fe²⁺ influx to Fe(III) reduction (micromole Fe²⁺ absorbed/micromole Fe[III] reduced) revealed no enhanced Fe²⁺ transport capacity in roots of Fe-deficient peas (from the parental genotype, Sparkle) or the functional Fe-deficiency pea mutant, E107 (derived from Sparkle), relative to roots of Fe-sufficient Sparkle plants. Data from studies using 30 to 100 micromolar Fe(III)-EDTA indicated a linear relationship between Fe²⁺ influx and Fe(III) reduction (Fe²⁺ generation), while Fe²⁺ influx saturated at higher concentrations of Fe(III)-EDTA. Estimations based on current data suggest the Fe²⁺ transport protein may saturate in the range of 10^−4.8 to 10⁻⁴ molar Fe²⁺. These results imply that for peas, the physiological rate limitation to Fe acquisition in most well-aerated soils would be the root system's ability to reduce soluble Fe(III)-compounds.     

豌豆Sparkle及其单基因突变体E107的铁锰吸收及转移效率

用豌豆Ｓｐａｒｋｌｅ及其单基因突变体Ｅ１０７进行的水培的试验表明,－Ｆｅ和＋Ｆｅ处理的Ｅ１０７幼苗以及－Ｆｅ处理的Ｓｐａｒｋｌｅ幼苗均表现出根系Ｈ＋分泌量大、根系Ｆｅ（Ⅲ）还原力强等特点,其中尤以＋Ｆｅ处理的Ｅ１０７最为突出;而十Ｆｅ处理的Ｓｐａｒｋｌｅ则无以上特点。与Ｓｐａｒｋｌｅ相比,Ｅ１０７各处理的地上部Ｆｅ、Ｍｎ合量均很高,但根部含量则相反。与Ｓｐｅｋｌｅ相比,Ｅ１０７—Ｆｅ处理表现为Ｆｅ高效,即使在＋Ｆｅ处理下,Ｅ１０７仍表现出－Ｆｅ条件下的根系生理特性,活化并还原了根际大量Ｆｅ（Ⅲ）和Ｍｎ,因而它对Ｆｅ、Ｍｎ具有较高的吸收效率,但是这些元素并不在根系中贮存,而是源源不断地运输到地上部,并在叶片中累积乃至使叶片中毒坏死,充分表现了Ｅ１０７单基因突变体对Ｆｅ、Ｍｎ也具有较高的转移效率。     

Differential response of salt-marsh species to variation of iron and manganese

Salt-marsh plants of the lower, middle and upper marsh were compared in their response to iron and manganese. The species studied showed differential sensitivity to high concentrations of Fe (1 000 μM) and Mn (10 000 μM) in hydroculture experiments, species of the lower marsh being more resistant than species of the upper marsh. Fe and Mn concentrations in the root were higher than in the shoot, which was also found in plants inundated with seawater. High Fe and Mn concentrations in the root are probably the result of the oxidizing power of plant roots with a subsequent low translocation of Fe (II) and Mn (II) to the shoot. At high (toxic) Fe and Mn levels in the nutrient solution, Fe and Mn concentrations were much higher in the shoots of sensitive species than in resistant species. The P content of roots and shoots was not influenced by increased Fe and Mn concentrations. Fe and Mn resistance in Spartina anglica and Juncus gerardii, may be in part due to a high root porosity. Other species, however, that are similarly resistant to Fe and Mn lack a well-developed aerenchym. Root porosity, radial oxygen loss and Fe (II) and Mn (II) exclusion by oxidation to Fe (III) (hydr)oxides deposited on the roots form part of the resistance mechanism of hygrohalophytes to Fe and Mn; the differences in this respect between the species may also be due to other metabolic aspects.     

The effect of different iron concentrations on lead accumulation in hydroponically grown <Emphasis Type="Italic">Matthiola flavida</Emphasis> Boiss

The general relationship between heavy metals and mineral nutrition of plants grown in polluted environments is one of the most important factors for modifying the toxic properties of these metals. To study the effect of iron and lead pollution on the growth of Matthiola flavida a factorial research was undertaken in the form of a completely randomized design with four replications in hydroponic culture. After germination, seedlings were transferred to a hydroponic culture. During the pre-treatment step, a series of plants contained enough iron (+Fe), whereas the second series was without iron (?Fe). After the pre-treatment step, both series of plants were treated with three levels of iron as FeEDDHA and two levels of lead as Pb(NO₃)₂. The results showed that in both series of plants, lead reduced the root growth, shoot height, shoot and root dry weight. For 5 µM lead concentration, with increasing concentration of iron in the nutrient solution, concentration of lead in the roots and shoots decreased. For 1 µM concentration of lead, increasing the iron concentration in the nutrient solution reduced the concentration of lead in the roots, but had no significant effect on the amount of lead in shoots. Lead accumulation in shoots and roots of plants with Fe was more than plants without Fe. Also, in low concentrations of iron, the amount of iron in shoot increased with the increase of lead concentration in the nutrient solution. The results showed that in lead pollution, iron has a positive effect on investigated traits.     

Ethylene as a Possible Mediator of Light- and Nitrate-Induced Inhibition of Nodulation of Pisum sativum L. cv Sparkle

R82 (sym-17), a stable mutant of Pisum sativum L. cv Sparkle, is described. The shoot growth of the mutant was less than that of its parent under light or dark growth conditions. Gibberellic acid treatment did not normalize the shoot growth of R82. The mutant had thick and short roots. It formed few nodules, but the specific nitrogenase activity was not affected. R82 produced and contained more ethylene than Sparkle. It also contained more free 1-amino-cyclopropane-1-carboxylic acid than did its parent in both the shoot and the root. The root tip of R82 had a lower activity of ethylene-forming enzyme than that of Sparkle, whereas the whole shoot of R82 had a similar activity. The sensitivity of R82 to exogenous ethylene was not more than that of Sparkle. Exogenous ethylene treatments did not make Sparkle mimic R82, and inhibitors of ethylene biosynthesis or action did not normalize the phenotype of R82. The data suggest that the primary effect of sym-17 is not the enhanced ethylene production.     

Morphogenetic responses obtained from a variety of somatic explant tissues of data palm

Shoot tip, bud, leaf, stem and root explants from bearing trees, offshoots, seedlings, and asexual plantlets ofPhoenix dactylifera L. were cultured on modified Murashige and Skoog nutrient media containing 3 g/l activated charcoal, 100 mg/l 2,4-dichlorophenoxyacetic acid, 3 mg/l N ⁶-(Δ²-isopentyl)adenine to obtain callus. Differential morphogenetic responses were obtained from calli dependent on the explant type and parent source. Subcultured shoot tips and leafy lateral buds callus on nutrient media devoid of charcoal and supplemented with 0.1 mg/l α-naphthaleneacetic acid (NAA) produced adventitious plantlets. Subcultured leaf calli produced roots only. Root callus failed to exhibit any morphogenetic response upon subculturing. Undifferentiated non-leafy buds and stem tissues did not give rise to callus, regardless of the parent source. Generally, the best callus and embryogenetic responses from explants were obtained from seedling and plantlet parent sources. Similarly, organogenetic responses such as root formation and shoot development from shoot tips cultured on media containing 10 mg/l NAA were also related to the parent explant source. Mention of a trademark or proprietary product in the paper does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be available.     

The crucial role of roots in increased cadmium-tolerance and Cd-accumulation in the pea mutant <Emphasis Type="Italic">SGECd</Emphasis><Superscript><Emphasis Type="Italic">t</Emphasis></Superscript>

Elucidation of mechanisms underlying plant tolerance to cadmium, a widespread toxic soil pollutant, and accumulation of Cd in plants are urgent tasks. For this purposes, the pea (Pisum sativum L.) mutant SGECd^t (obtained by treatment of the laboratory pea line SGE with ethylmethane sulfonate) was reciprocally grafted with the parental line SGE, and four scion/rootstock combinations were obtained: SGE/SGE, SGECd^t/SGECd^t, SGE/SGECd^t, and SGECd^t/SGE. They were grown in hydroponics in the presence of 1 μM CdCl₂ for 30 d. The SGE and SGECd^t scions on the SGECd^t rootstock had a higher root and shoot biomass and an elevated root and shoot Cd content compared with the grafts having SGE rootstock. Only the grafts with the SGE rootstock showed chlorosis and roots demonstrating symptoms of Cd toxicity. The content of nutrient elements in roots (Fe, K, Mg, Mn, Na, P, and Zn) was higher in the grafts having the SGECd^t rootstock, and three elements, namely Ca, Fe, and Mn, were efficiently transported by the SGECd^t root to the shoot of these grafts. The content of other measured elements (K, Mg, Na, P, and Zn) was similar in the root and shoot in all the grafts. Then, the non-grafted plants were grown in the presence of Cd and subjected to deficit or excess concentrations of Ca, Fe, or Mn. Exclusion of these elements from the nutrient solution retained or increased differences between SGE and SGECd^t in growth response to Cd toxicity, whereas excess of Ca, Fe, or Mn decreased or eliminated such differences. The obtained results assign a principal role of roots to realizing the increased Cd-tolerance and Cdaccumulation in the SGECd^t mutant. Efficient translocation of Ca, Fe, and Mn from roots to shoots appeared to counteract Cd toxicity, although Cd was actively taken up by roots and accumulated in shoots.     

Influence of external zinc and phosphorus supply on Cd uptake by rice (Oryza sativa L.) seedlings with root surface iron plaque

Rice seedlings were grown in hydroponic culture to determine the effects of external Zn and P supply on plant uptake of Cd in the presence or absence of iron plaque on the root surfaces. Iron plaque was induced by supplying 50 mg l⁻¹ Fe²⁺ in the nutrient solution for 2 day. Then 43-day-old seedlings were exposed to 10 μmol l⁻¹ Cd together with 10 μmol l⁻¹ Zn or without Zn (Zn–Cd experiment), or to 10 μmol l⁻¹ Cd with 1.0 mmol l⁻¹ P or without P (P–Cd experiment) for another 2 day. The seedlings were then harvested and the concentrations of Fe, Zn, P and Cd in dithionite–citrate–bicarbonate (DCB) extracts and in roots and shoots were determined. The dry weights of roots and shoots of seedlings treated with 50 mg l⁻¹ Fe were significantly lower than when no Fe was supplied. Adsorption of Cd, Zn and P on the iron plaque increased when Fe was supplied but Cd concentrations in DCB extracts were unaffected by external Zn or P supply levels. Cd concentrations in shoots and roots were lower when Fe was supplied. Zn additions decreased Cd concentrations in roots but increased Cd concentrations in shoots, whereas P additions significantly increased shoot and root Cd concentrations and this effect diminished when Fe was supplied. The percentage of Cd in DCB extracts was significantly lower than in roots or shoots, accounting for up to 1.8–3.8% of the plant total Cd, while root and shoot Cd were within the ranges 57–76% and 21–40% respectively in the two experiments. Thus, the main barrier to Cd uptake seemed to be the root tissue and the contribution of iron plaque on root surfaces to plant Cd uptake was minor. The changes in plant Cd uptake were not due to Zn or P additions altering Cd adsorption on iron plaque, but more likely because Zn or P interfered with Cd uptake by the roots and translocation to the shoots.     

Photosynthetic pigments content, <Emphasis Type="Italic">δ</Emphasis>-aminolevulinic acid dehydratase and acid phosphatase activities and mineral nutrients concentration in cadmium-exposed <Emphasis Type="Italic">Cucumis sativus</Emphasis> L.

In this study, the effects of cadmium chloride (CdCl₂) on plant growth, histology of roots, photosynthetic pigments content, δ-aminolevulinic acid dehydratase (ALA-D; E.C. 4.2.1.24) and acid phosphatase activities (AP; E.C. 3.1.3.2), soluble phosphorus (Pi) measurement and mineral nutrients content in cucumber seedlings (Cucumis sativus L.) were investigated. Cucumber seedlings were grown in vitro in an agar-solidified substrate containing four CdCl₂ treatments (0, 100, 400, and 1000 μM) for ten days. Cd was readily absorbed by seedlings and its content was greater in the roots than in the shoot. Cd reduced shoot and root length, and fresh and dry biomass of seedlings. Inhibition of root cell elongation in Cd-treated seedlings was observed by the increase of the mean radial size of cells belonging to three zones of the root tip. The highest level of Cd reduced in a similar manner chlorophyll a, chlorophyll b and total chlorophyll contents. Increasing concentrations of Cd resulted in a linear decrease in carotenoids levels of cotyledons. Interestingly, the ALA-D activity in cotyledons was inhibited only at the highest level of Cd. Root and shoot AP activities were, respectively, activated and inhibited at all CdCl₂ concentrations. Root Pi concentration was increased in all Cd treatments and it was not altered in the shoot tissues. Moreover, in general, the nutrient contents were increased in the root and decreased in the shoot. Therefore, we suggest that Cd affects negatively growth, photosynthetic pigments, ALA-D and AP activities and partition of mineral nutrients in cucumber seedlings.     

Carbon Cost of the Fungal Symbiont Relative to Net Leaf P Accumulation in a Split-Root VA Mycorrhizal Symbiosis

Translocation of ¹⁴C-photosynthates to mycorrhizal (+ +), half mycorrhizal (0+), and nonmycorrhizal (00) split-root systems was compared to P accumulation in leaves of the host plant. Carrizo citrange seedlings (Poncirus trifoliata [L.] Raf. × Citrus sinensis [L.] Osbeck) were inoculated with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck and Smith. Plants were exposed to ¹⁴ CO₂ for 10 minutes and ambient air for 2 hours. Three to 4% of recently labeled photosynthate was allocated to metabolism of the mycorrhiza in each inoculated root half independent of shoot P concentration, growth response, and whether one or both root halves were colonized. Nonmycorrhizal roots respired more of the label translocated to them than did mycorrhizal roots. Label recovered in the potting medium due to exudation or transport into extraradical hyphae was 5 to 6 times greater for (+ +) versus (00) plants. In low nutrient media, roots of (0+) and (+ +) plants transported more P to leaves per root weight than roots of (00) plants. However, when C translocated to roots utilized for respiration, exudation, etc., as well as growth is considered, (00) plant roots were at least as efficient at P uptake (benefit) per C utilized (cost) as (0+) and (+ +) plants. Root systems of (+ +) plants did not supply more P to leaves than (0+) plants in higher nutrient media, yet they still allocated twice the ¹⁴C-photosynthate to the mycorrhiza as did (0+) root systems. This indicates there is an optimal level of mycorrhizal colonization above which the plant receives no enhanced P uptake yet continues to partition photosynthates to metabolism of the mycorrhiza.     

Ribulose bisphosphate carboxylase, protein and nitrogen in Scots pine seedlings cultivated at different nutrient levels

Seedlings of Scots pine (Pinus sylvestris L.) of a northern provenance were cultivated in nutrient solution for 10 weeks in a climate chamber. The nutrient solution (renewed by solution exchange) contained 2.5, 10 or 50 mg N I^?1. All other essential elements were added in optimal proportion to the nitrogen. Seedlings cultivated at 10 and 50 mg N I^?1 were similar with respect to all characteristics studied. Seedlings cultivated at 2.5 mg N I^?1 showed a lower growth rate, especially for the shoot, and an altered morphology, with high root:shoot ratios and long, slender roots. The nitrogen concentrations in shoot and needles as well as in whole seedlings were not significantly affected by the nitrogen supply, while the nitrogen concentrations in the roots were somewhat lower at 2.5 mg N I^?1. Ribulose bisphosphate carboxylase (EC 4.1.1.39) activity and the concentrations of carboxylase, total and soluble protein and of chlorophyll in the needles were consistently much lower for seedlings cultivated at 2.5 mg N I^?1, than for seedlings grown at higher nutrient levels. A close correlation was observed between activity and concentration of the carboxylase (r=0.95). Carboxylase activity and protein were more sensitive to a low nutrient supply than was chlorophyll. The data show how activity and concentration of ribulose bisphosphate carboxylase and the concentrations of soluble and total protein and of chlorophyll in needles of pine seedlings can be negatively affected by the nutrient supply, also when the nitrogen concentrations in the needles are close to those observed at optimal nutrient supply. It is suggested that pine seedlings store assimilated non-protein nitrogen in the needles when protein synthesis is under restraint. The nitrogen concentration in needles and seedlings could not be used as a measure of the physiological state of the seedlings.     

The effects of water and multi-nutrient stress on xylem sap chemistry,photosynthesis and transpiration of seedlings of two Eucalypts

Summary Ten seedlings each of Eucalyptus kitsoniana Maiden and Eucalyptus globulus Labill. were subjected to two levels of water stress and two levels of nutrient stress (macro and micro-nutrients) in a greenhouse for 3 weeks. The objectives were to determine the degree to which seedlings show differences in sap chemistry, photosynthesis and transpiration that relate to the environments in which these two species live. Whole plants were then extracted for xylem sap using a pressure chamber and the sap was analyzed for 14 elements using an inductively coupled plasma spectrometer and a nitrometer. For E. kitsoniana water and nutrient stress, applied separately or in combination, significantly reduced leaf conductance, transpiration, photosynthesis and midday water potential. Nutrient stress alone had less effect than water stress on most functions measured. Water stress alone reduced the root/shoot ratio; the combination of water and nutrient stress increased the root/shoot ratio, primarily because of reduced shoot weight. In E. kitsoniana, water stress alone or in combination with nutrient stress increased the xylem sap concentrations of B and Si. Multi-nutrient stress alone, or in combination with water stress, significantly decreased sap Zn and K. For this species, sap N was decreased by nutrient stress, but increased by water stress. E. globulus had significantly lower transpiration rates and less root mass than E. kitsoniana. Slightly lower leaf conductance and photosynthesis were not significant in E. globulus compared to E. kitsoniana. Water and nutrient stress reduced conductance, transpiration (except for nutrient stress) and photosynthesis, and the effects of water stress on E. globulus were greater than the effects of nutrient stress. Midday water potential was reduced by water stress. Water or nutrient stress alone did not alter seedling root/shoot ratio, but the combination of water and nutrient stress significantly increased the root/shoot ratio for both species. For E. globulus, sap concentrations of Mn, Na, Si and K were increased by water stress (alone or in combination with nutrient stress). Sap N increased with water stress or combined stresses, but decreased under nutrient stress alone. When the two species were compared, E. globulus generally had lower or similar nutrient concentrations in the sap, with Ca, Mg, Mn and P significantly lower than in E. kitsoniana. Seedlings of these two species show strong site adaptations to water and nutrient availability.