Nitric oxide protects sour pummelo (Citrus grandis) seedlings against aluminum-induced inhibition of growth and photosynthesis

Limited data are available on the amelioration of nitric oxide (NO) on aluminum (Al)-toxicity. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O × 0 and 10 μM sodium nitroprusside (SNP, an NO donor). Under Al stress, SNP increased root phosphorus (P) and Al, but decreased shoot Al. Al decreased photosynthesis, maximum quantum yield of primary photochemistry (F_v/F_m) and total performance index (PI_tot,sbs), but increased inactivation of oxygen-evolving complex (OEC), K-band and relative variable fluorescence at I-steps (V_I). SNP alleviated Al-induced changes for all these parameters. SNP stimulated Al-induced secretion of malate and citrate by excised roots from Al-treated seedlings, while Al did not increase their contents in roots. Antioxidant system in leaves and roots was up- and down-regulated by Al, respectively. SNP prevented Al-induced accumulation of malondialdehyde (MDA) in roots and leaves. In conclusion, SNP alleviates Al-induced inhibition of growth and impairment of the whole photosynthetic electron transport chain. This occurs through increasing Al-immobilization and P level in roots and Al-induced secretion of malate and citrate from roots, and decreasing Al accumulation in shoots. Thus, the decrease of photosynthesis is prevented. Increased P level and Al-immobilization in roots through SNP may be effected through enhanced secretion of malate and citrate.     

Iron Enhances Aluminum-induced Leaf Necrosis and Plant Growth Inhibition in Eucalyptus camaldulensis

The combined effects of excess Fe and Al on Eucalyptus camaldulensis Dehnh. were studied by investigating time course and visible symptoms of leaf necrosis, plant biomass, the status of some antioxidants and pigments and nutrient concentrations. Seedlings were grown hydroponically in nutrient solutions containing 0 or 500 μM AlCl₃, each with a FeSO₄ range of 1, 12 and 120 μM at pH 4.2. Leaf necrosis and plant growth inhibition were induced by Al and enhanced by the increase in Fe concentration. The process from the first appearance of necrotic spots to leaf death (shedding) of a leaf proceeded from a few days to about 20 days after the leaf had fully expanded. Either 120 μM Fe without Al or Al reduced plant growth to a similar extent but 120 μM Fe without Al did not cause leaf necrosis. In leaves, excess Fe (12 and 120 μM) without Al reduced concentration of ASC and GSH, while concentration of Fe, DHA and GSSG and DHA:ASC and GSSG:GSH ratios tended to increase with the increase in Fe concentration in treatment solution with or without Al. At 1 μM Fe, Al increased concentration of DHA and DHA:ASC and GSSG:GSH ratios. Catalase activity in leaves reduced with the increase in leaf Al concentration. At 1 μM Fe, Al greatly reduced concentrations of Fe and chlorophylls in leaves but increase two times Fe concentration in stems. These suggest that the enhancement effects of Fe on Al-induced leaf necrosis and plant growth inhibition can be discussed in context of the excess Fe itself weakens antioxidant capability of ASC–GSH cycle in leaves and greatly reduces plant growth; and the increase in Fe accumulation in stems is involved in Al-induced leaf chlorosis.     

Drought-induced increase in xylem malate and mannitol concentrations and closure of Fraxinus excelsior L. stomata

Changes in the malate and mannitol composition of ash leaf (Fraxinus excelsior L.) xylem sap were studied in response to water deficit. Xylem sap was collected by the pressure method from the petiole of leaves sampled on irrigated and non-irrigated ash seedlings. As the leaf water potential decreased from -0.3 to -3.0 MPa, there was a significant increase in malate and mannitol xylem concentrations, and a concomitant decrease in maximal stomatal conductance. The functional significance of the increased malate and mannitol concentrations was investigated by using a transpiratory bioassay with mature detached leaves which exhibited, for stomatal conductance, the typical pattern showed by expanded leaves during dark/light transitions. Supplying detached leaves with mannitol in a range of concentrations found in the xylem sap had no effect on stomatal movements, but malate, for concentrations between 0.5 and 3 mM, was effective in preventing stomatal opening. The ability of malate to inhibit stomatal opening appeared to be rather non-specific. Two structural malate analogues, citrate and aspartate or an unrelated anion, shikimate, also inhibited this process. Given the drought-induced increase in xylem malate concentrations, and the fact that the range of malate levels required to close stomata was very similar to that of the concentrations found in the xylem sap, it has been hypothesized that malate is involved in the stomatal closure of ash leaves under drying conditions.Key words: Fraxinus excelsior: L., malate, mannitol, xylem sap, stomata, water deficit.      

Effects of aluminium on growth and cation uptake in seedlings of Eucalyptus mannifera and Pinus radiata

This experiment was designed to examine the effects of aluminium (Al) on the growth of Pinus radiata (D. Don) and Eucalyptus mannifera subsp. mannifera (Mudie) seedlings in culture solutions in a glasshouse to help explain the failure of radiata pine trees on some acid, low fertility soils in Australia on which the native eucalypts flourish. Aluminium (Al) in culture solution increased the growth of roots and shoots of seedlings of both species but while growth of the eucalypt continued to increase with increases in Al to 2.222 μM, growth of the pine was largest at 370 μM Al. In addition to total root length, specific root length (length per unit dry weight), a measure of fineness of the root, increased in the eucalypt seedlings as the substrate Al increased. Growth of the shoots and roots of the pine in the absence of any added Al was extremely poor suggesting that Al, in low concentrations, may be an essential element or ameliorate some other factors in solution culture at low pH. Root and shoot concentrations of K increased with increasing Al, whilst Ca and Mg Concentrations decreased and Mn concentrations were unaffected in both species. Tissue Ca and Mg concentrations were 2 to 3 times higher in the eucalypt seedlings than the pine at all levels of added Al due to greater uptake of these elements by the eucalypt. In contrast, at the highest concentration of Al in the medium, shoot Al concentrations were lower in the cucalypt than in the pine due to a greater proportion of Al being retained in the eucalypt roots. These differences between the seedlings in terms of root growth and tissue cation concentrations may help explain the ability of eucalypt species to maintain vigorous growth on acid soils high in Al and low in Ca and P, where growth of the pines failed.     

施用磷对王桉 (Eucalyptus regnans F. Muell.)幼苗生长及其氨基酸组

王桉 (Eucalyptus regnans F. Muell.) 是澳大利亚桉树中最重要的商业用材和人工造林树种之一.研究王桉的施肥与其体内氨基酸的积累和转化及与食叶虫害之间的相关性具有重要的经济和生态意义.在温室内利用2种不同来源的土壤对王桉幼苗进行了不同磷施用量(100 kg hm~(-2)和 200 kg hm~(-2))处理.结果显示,不同土壤和不同磷施用量对苗木生长影响显著,但均未显著影响苗木各部分的氮和磷含量水平.苗木木质部渗出液中的氨基酸含量以谷氨酰胺为主,并与苗木生长和磷施用量呈反相关.不同土壤和磷施用量对苗木组织中游离氨基酸组分和含量的影响不显著,但游离氨基酸的组分和相对水平随叶龄变化明显,尤其是精氨酸在嫩叶氨基酸总量中只占2%～3%,但在老叶中占到20%多;精氨酸在老叶中的积累极有可能是某些蛋白质降解而精氨酸即时合成所致,因为精氨酸一般不在韧皮部转运.谷氨酰胺在树液中含量最高并与苗木生长呈反相关或许可以作为预测桉树发生食叶昆虫危害的一个有用指标.     

Calcicole behaviour of Callisthene fasciculata Mart., an Al‐accumulating species from the Brazilian Cerrado

• Most aluminium (Al)‐accumulating species are found on soils with high Al saturation and low Ca availability (Ca poor). Callisthene fasciculata Mart. (Vochysiaceae), however, is an Al‐accumulating tree restricted to Ca‐rich soils with low Al saturation in the Brazilian Cerrado savanna. Here we tested its calcicole behaviour, and the possible role of organic acids in detoxification of Al during the early stages of plant development.
• We assessed growth, dry mass, nutrients, Al and organic acids in seedlings grown for 50 days on two contrasting Cerrado soils; one with high Ca concentrations and low Al saturation and the other with low Ca availability and high Al saturation.
• Relative to plants on Ca‐rich soil, plants on Ca‐poor soil had necrotic spots and bronzing of leaves. Roots and shoots contained reduced concentrations of P and Cu, but higher concentrations of Fe, Al and citrate. Despite lower concentrations in the soil, Ca and Mg increased in shoots. Shoot concentrations of oxalate were also higher.
• We confirmed C. fasciculata as an Al‐accumulating species with calcicole behaviour. The increased concentrations of organic acids in plants with higher Al accumulation suggest that high availability of soluble Al does not prevent occurrence of this species on soils with high Al saturation. Instead, the absence of C. fasciculata from Ca‐poor soils is probably due to imbalances in tissue Fe, Cu and Zn imposed by this soil type.

     

Aluminium alleviates manganese toxicity to rice by decreasing root symplastic Mn uptake and reducing availability to shoots of Mn stored in roots

Background and Aims Manganese (Mn) and aluminium (Al) phytotoxicities occur mainly in acid soils. In some plant species, Al alleviates Mn toxicity, but the mechanisms underlying this effect are obscure.Methods Rice (Oryza sativa) seedlings (11 d old) were grown in nutrient solution containing different concentrations of Mn²⁺ and Al³⁺ in short-term (24 h) and long-term (3 weeks) treatments. Measurements were taken of root symplastic sap, root Mn plaques, cell membrane electrical surface potential and Mn activity, root morphology and plant growth.Key Results In the 3-week treatment, addition of Al resulted in increased root and shoot dry weight for plants under toxic levels of Mn. This was associated with decreased Mn concentration in the shoots and increased Mn concentration in the roots. In the 24-h treatment, addition of Al resulted in decreased Mn accumulation in the root symplasts and in the shoots. This was attributed to higher cell membrane surface electrical potential and lower Mn²⁺ activity at the cell membrane surface. The increased Mn accumulation in roots from the 3-week treatment was attributed to the formation of Mn plaques, which were probably related to the Al-induced increase in root aerenchyma.Conclusions The results show that Al alleviated Mn toxicity in rice, and this could be attributed to decreased shoot Mn accumulation resulting from an Al-induced decrease in root symplastic Mn uptake. The decrease in root symplastic Mn uptake resulted from an Al-induced change in cell membrane potential. In addition, Al increased Mn plaques in the roots and changed the binding properties of the cell wall, resulting in accumulation of non-available Mn in roots.     

Estimating the chemical compositions of soil solutions by obtaining saturation extracts or specific 1:2 by volume extracts

The aluminium tolerance of several tree species was studied in a cloud forest in Northern Venezuela, growing on a very acid soil and rich in soluble Al. The Al-accumulator species (>1000 ppm in leaves) were compared to non-accumulator ones in relation to total Al concentration in xylem sap, pH and Al concentration in vacuoles, and rhizosphere alkalinization capacity. The Al³⁺ concentration in the soil solution and the xylem sap were also measured. The results show that in the Al-accumulator plant Richeria grandis, xylem sap is relatively rich in Al and about 35% of it is present in ionic form. In the non-accumulator plant studied (Guapira olfersiana) there is no Al detectable in xylem sap. The pH of vacuolar sap of several Al-accumulator species studied was very acidic and ranged between 2.6–4.8, but the presence of Al in vacuoles was not correlated with the acidity of the vacuolar sap. Both Al-accumulator and non accumulator plants had the capacity to reduce acidity of the rhizosphere and increased the pH of the nutrient solution by one unit within the first 24 hours. Trees growing in natural, high acidity-high Al³⁺ environment show a series of tolerance mechanisms, such as deposition of Al in vacuoles, Al chelation and rhizosphere alkalinization. These partially ameliorate the toxic effects of this element, but they probably impose a high ecological cost in terms of photosynthate allocation and growth rate.     

Chemical forms of aluminum in xylem sap of tea plants (Camellia sinensis L.)

To identify the chemical forms of aluminum (Al) transported from roots to shoots of tea plants (C. sinensis L.), ²⁷Al-nuclear magnetic resonance and ¹⁹F NMR spectroscopy were used to analyze xylem sap.The concentration of Al in collected xylem sap was 0.29 mM, twice as high as that of F. Catechins were not detected in xylem sap. The concentration of malic acid in xylem sap was higher than that of citric acid, whereas the concentration of oxalic acid was negligible.There were two signals in the ²⁷Al NMR spectra of xylem sap, a larger signal at 11 ppm and a smaller one at −1.5 ppm. The former signal was consistent with the peak for an Al-citrate model solution, suggesting that an Al-citrate complex was present in xylem sap. Although the latter signal at −1.5 ppm was thought to indicate the presence of an Al-F complex (at 1.7 ppm) in xylem sap, there was only one signal at −122 ppm in the ¹⁹F NMR spectrum of xylem sap, indicating that the main F complex in xylem sap was F⁻.These results indicate that Al might be translocated as a complex with citrate, while Al-malate, Al-oxalate and Al-F complexes are not major Al complexes in xylem sap of tea plants.     

Form of aluminium for uptake and translocation in buckwheat (Fagopyrum esculentum Moench)

 The forms of Al for uptake by the roots and translocation from the root to the shoot were investigated in a buckwheat (Fagopyrum esculentum Moench, cv. Jianxi) that accumulates Al in its leaves. The Al concentration in the xylem sap was 15-fold higher in the plants exposed to AlCl₃ than in those exposed to an Al-oxalate (1:3) complex, suggesting that the roots take up Al in the ionic form. The Al concentration in the xylem sap was 4-fold higher than that in the external solution after a 1-h exposure to AlCl₃ solution and 10-fold higher after a 2-h exposure. The Al concentration in the xylem sap increased with increasing Al concentration in the external solution. The Al uptake was not affected by a respiratory inhibitor, hydroxylamine, but significantly inhibited by the addition of La. These results suggest that Al uptake by the root is a passive process, and La³⁺ competes for the binding sites for Al³⁺ on the plasma membrane. The form of Al in the xylem sap was identified by ²⁷Al-nuclear magnetic resonance analysis. The chemical shift of ²⁷Al in the xylem sap was around 10.9 ppm, which is consistent with that of the Al-citrate complex. Furthermore, the dominant organic acid in the xylem sap was citric acid, indicating that Al was translocated in the form of Al-citrate complex. Because Al is present as Al-oxalate (1:3) in the root, the present data show that ligand exchange from oxalate to citrate occurs before Al is released to xylem. Received: 10 December 1999 / Accepted: 3 February 2000     

Salicylic acid and salicylic acid glucoside in xylem sap of <Emphasis Type="Italic">Brassica napus</Emphasis> infected with <Emphasis Type="Italic">Verticillium longisporum</Emphasis>

Salicylic acid (SA) and its glucoside (SAG) were detected in xylem sap of Brassica napus by HPLC–MS. Concentrations of SA and SAG in xylem sap from the root and hypocotyl of the plant, and in extracts of shoots above the hypocotyl, increased after infection with the vascular pathogen Verticillium longisporum. Both concentrations were correlated with disease severity assessed as the reduction in shoot length. Furthermore, SAG levels in shoot extracts were correlated with the amount of V. longisporum DNA in the hypocotyls. Although the concentration of SAG (but not SA) in xylem sap of infected plants gradually declined from 14 to 35 days post infection, SAG levels remained significantly higher than in uninfected plants during the whole experiment. Jasmonic acid (JA) and abscisic acid (ABA) levels in xylem sap were not affected by infection with V. longisporum. SA and SAG extend the list of phytohormones potentially transported from root to shoot with the transpiration stream. The physiological relevance of this transport and its contribution to the distribution of SA in plants remain to be elucidated.     

Transpiration inhibition by stored xylem sap from well-watered maize plants

There is increasing evidence that a chemical signal exists in xylem sap of plants subjected to water deficits which influences physiological responses in plant shoots. An important method of studying this signal is the transpiration response of excised leaves exposed to xylem sap collected from plants. However, Munns et al [Plant, Cell & Environment 16, 867–877] cautioned that transpiration inhibition is observed when xylem sap collected from wheat and barley is stored before determining physiological activity. The objective of the study reported here was to determine if transpiration inhibition develops in maize sap collected from well-watered plants when the sap is stored under various conditions. It was found that storage of maize sap collected from well-watered plants for only 1 d at -20°C resulted in the development of substantial transpiration inhibition in bioassay leaves. Storage of sap at 4°C resulted in the development of the effect after 2 weeks, while storage at ?86°C showed only small transpiration inhibition after 3 weeks. The major source of the transpiration inhibition was the development of a substance in the stored sap that resulted in physical blockage of the transpiration stream in bioassay leaves. However, a small signal component may also have developed in the stored sap. Because of the possibility of ionic activity under freezing conditions at ?20°C, calcium was studied for its potential involvement in the transpiration inhibition. However, the calcium concentrations found to inhibit transpiration were nearly an order of magnitude larger than the calcium concentrations observed in xylem sap.     

Identification of three wheat globulin genes by screening a Triticum aestivum BAC genomic library with cDNA from a diabetes-associated globulin

Background

Plant systemic signaling characterized by the long distance transport of molecules across plant organs involves the xylem and phloem conduits. Root-microbe interactions generate systemic signals that are transported to aerial organs via the xylem sap. We analyzed the xylem sap proteome of soybean seedlings in response to pathogenic and symbiotic interactions to identify systemic signaling proteins and other differentially expressed proteins.

Results

We observed the increase of a serine protease and peroxidase in the xylem sap in response to Phytophthora sojae elicitor treatment. The high molecular weight fraction of soybean xylem sap was found to promote the growth of Neurospora crassa in vitro at lower concentrations and inhibit growth at higher concentrations. Sap from soybean plants treated with a P. sojae elicitor had a significantly higher inhibitory effect than sap from control soybean plants. When soybean seedlings were inoculated with the symbiont Bradyrhizobium japonicum, the abundance of a xyloglucan transendoglycosyl transferase protein increased in the xylem sap. However, RNAi-mediated silencing of the corresponding gene did not significantly affect nodulation in soybean hairy root composite plants.

Conclusion

Our study identified a number of sap proteins from soybean that are differentially induced in response to B. japonicum and P. sojae elicitor treatments and a majority of them were secreted proteins.     

Hybrid Weakness in Phaseolus vulgaris L. II. Disruption of Root-Shoot Integration

We have been examining the importance of the root system on shoot growth and development using a developmentally disabled hybrid of the common bean Phaseolus vulgaris L. Parental cultivars (P. Vulgaris cv. Redkloud of Mesoamerican origin, and P. vulgaris cv. Batt of Andean origin) grow normally, but crosses produce F1 hybrids exhibiting hybrid weakness associated with reduced root and shoot growth. In this study, applications of benzylaminopurine (BAP) to roots of F1 hybrids increased the number of root tips and leaves. Reciprocal grafting was used to study the effects of the root system on shoots. Grafting of roots of the Mesoamerican cultivar onto shoots of F1 hybrids increased the cytokinin concentrations in leaves of F1 hybrids and removed the characteristics associated with hybrid weakness. To determine whether factors in the xylem sap enhanced leaf growth, leaf discs were incubated on sap collected from Mesoamerican and Andean cultivars. Sap from Mesoamerican plants enhanced the growth of leaf discs excised from F1 hybrids more than sap collected from Andean cultivars. Estimates of the transport of zeatin riboside (ZR)–type cytokinins from roots of F1 hybrids indicated that transport out of hybrid roots was reduced compared with those transported out of Mesoamerican or Andean roots. Results suggest that ZR-type cytokinins are involved in hormonal integration between roots and shoots of P. vulgaris and that one of the barriers to hybridization between Andean and Mesoamerican landraces is related to hormone transport. Received October 15, 1998; accepted May 12, 1999     

Hydrogen sulfide alleviates aluminum toxicity in barley seedlings

Aims

Aluminum (Al) toxicity is one of the major factors that limit plant growth. Low concentration of hydrogen sulfide (H₂S) has been proven to function in physiological responses to various stresses. The objective of this study is to investigate the possible role of H₂S in Al toxicity in barley (Hordeum vulgare L) seedlings.

Methods

Barley seedlings pre-treated with sodium hydrosulfide (NaHS), a H₂S donor, and subsequently exposed to Al treatment were studied for their effects on root elongation, Al accumulation in seedlings, Al-induced citrate secretion and oxidative stress, and plasma membrane (PM) H⁺-ATPase expression.

Results

Our results showed that H₂S had significant rescue effects on Al-induced inhibition of root elongation which was correlated well with the decrease of Al accumulation in seedlings. Meanwhile, Al-induced citrate secretion was also significantly enhanced by NaHS pretreatment. Al-induced oxidative stress as indicated by lipid peroxidation and reactive oxygen species burst was alleviated by H₂S through the activation of the antioxidant system. Moreover, Al-induced reduction in PM H⁺-ATPase expression was reversed by exogenous NaHS.

Conclusions

Altogether, our results suggest H₂S plays an ameliorative role in protecting plants against Al toxicity by inducing the activities of antioxidant enzymes, increasing citrate secretion and citrate transporter gene expression, and enhancing the expression of PM H⁺-ATPase.     

Effect of cytokinins supplied via the xylem at multiples of endogenous concentrations on transpiration and senescence in derooted seedlings of oat and wheat

This study was conducted lo determine whether naturally occurring xylem cytokinins, when supplied to leaves via the xylem at approximately endogenous concentrations, increase transpiration and delay senescence in selected monocot species (oat and wheat). The concentrations of some of the major cytokinins (zeatin, dihydrozeatin, ciszeatin and their ribosides, the O-glucosides and nucleotides) were determined in the xylem exudate of oat and wheat seedlings by radioimmunoassay. Evidence is presented that the small volume of exudate (4–5 mm³) collected per plant was xylem sap in transit at the time of shoot excision. Using the data on cytokinin levels, the individual bases and ribosides (and a base/riboside mixture), at multiples of concentrations determined in xylem sap, were tested in transpiration and senescence bioassays. The individual O-glucosides (and mixtures of the O-glucosides) were similarly tested at (i) multiples of the molar concentrations of the corresponding bases and ribosides, and/or at (ii) multiples of the endogenous concentrations. Similarly, zeatin and dihydrozeatin nucleotides were tested at multiples of the molar concentration of zeatin riboside and, in some instances, at multiples of endogenous concentrations. Our results suggest that, at least in oat and possibly in wheat, zeatin-type bases, ribosides and O-glucosides supplied to the leaf in xylem sap are likely to play a role in regulating transpiration in vivo. O-glucosides in oat xylem sap may be important regulators of leaf senescence in the intact plant. The nucleotides were present in xylem sap at lower concentrations than most of the bases, ribosides and O-glucosides. The nucleotides appear likely to play a lesser role than the bases, riboside and O-glucosidcs in controlling transpiration and senescence in the intact plant.     

Long-distance translocation of polyamines in phloem and xylem of Ricinus communis L. plants

Polyamine content and enzyme activities in the biosynthetic and degradative pathways of polyamine metabolism were investigated in sieve-tube sap, xylem sap and tissues of seedlings and adult plants of Ricinus communis L. Polyamines were present in tissues and translocation fluids of both seedlings and adult plants in relatively high amounts. Only free polyamines were translocated through the plant, as indicated by the finding that only the free form was detected in the phloem and the xylem sap. Removal of the endosperm increased the polyamine content in the sieve-tube exudate of seedlings. The level and pattern of polyamines in tissue of adult leaves changed during leaf age, but not, however, in the sieve-tube sap. Xylem sap was relatively poor in polyamines. Polyamine loading in the phloem was demonstrated by incubating cotyledons with [¹⁴O]putrescine and several unlabelled polyamines. Feeding cotyledons with cadaverine and spermidine led to a decrease in the level of putrescine in sieve-tube sap, indicating a competitive effect. Comparison of polyamine content in the tissue and export rate showed that the export would deplete the leaves of polyamines within 1–3 d, if they were not replenished by biosynthesis. Polyamine biosynthesis in Ricinus proceeds mostly via arginine decarboxylase, which in vitro is 100-fold more active than ornithine decarboxylase. The highest arginine decarboxylase, ornithine decarboxylase and diamine oxidase activities were detected in cotyledons, while in sieve-tube sap only a slight arginine decarboxylase activity was found. Received: 18 March 1997 / Accepted: 20 August 1997     

Al Inhibits Both Shoot Development and Root Growth in als3, an Al-Sensitive Arabidopsis Mutant

In als3, an Al-sensitive Arabidopsis mutant, shoot development and root growth are sensitive to Al. Mutant als3 seedlings grown in an Al-containing medium exhibit severely inhibited leaf expansion and root growth. In the presence of Al, unexpanded leaves accumulate callose, an indicator of Al damage in roots. The possibility that the inhibition of shoot development in als3 is due to the hyperaccumulation of Al in this tissue was examined. However, it was found that the levels of Al that accumulated in shoots of als3 are not different from the wild type. The inhibition of shoot development in als3 is not a consequence of nonspecific damage to roots, because other metals (e.g. LaCl3 or CuSO4) that strongly inhibit root growth did not block shoot development in als3 seedlings. Al did not block leaf development in excised als3 shoots grown in an Al-containing medium, demonstrating that the Al-induced damage in als3 shoots was dependent on the presence of roots. This suggests that Al inhibition of als3 shoot development may be a delocalized response to Al-induced stresses in roots following Al exposure.     

Effects of phosphorus supply on growth and nitrogen fractions in xylem sap and foliage of Eucalyptus regnans (F.Muell.), E. nitens (Maiden) and E. globulus (Labill.) seedlings: implications for herbivory

Rates of growth of seedlings of E. globulus, E. regnans and E. nitens were related to phosphorus supply in two soils but concentrations of total nitrogen and total phosphorus in most plant tissues did not vary significantly among soil or phosphorus treatments. Differences in concentrations of nitrogen and phosphorus and in the composition of the pool of free amino-acids among leaves at different stages of development were far greater than differences between treatments. The most significant of these differences were several-fold greater concentrations of arginine in the oldest leaves and these are most likely due to protein degradation and/or in situ synthesis since arginine is not generally phloem mobile. The concentration of reduced nitrogen in xylem sap was inversely related to growth and glutamine was by far the dominant nitrogenous solute. We suggest that specific nitrogenous solutes may be useful indices of the nitrogen status of eucalypt tissues for insect herbivores.     

Aluminum-mycorrhizal interactions in the physiology of pitch pine seedlings

Aluminum (Al) in the rhizosphere adversely affects plant nutrition and growth. Although many conifer species, and pitch pine (Pinus rigida) in particular, have evolved on acidic soils where soluble Al is often high, controlled environment studies often indicate that Al interferes with seedling growth and nutrient relations. Under normal field conditions, conifer roots grow in a symbiotic relationship with ectomycorrhizal fungi, and this association may modulate the effects of Al on root physiology. To investigate the influence of mycorrhizal infection on Al toxicity, pitch pine seedlings were grown with or without the ectomycorrhizal symbiont Pisolithus tinctorius and were exposed to low levels of Al in sand culture. Aluminum at 50 μM reduced nonmycorrhizal seedling growth and increased foliar Al concentrations, but did not alter photosynthetic gas exchange or other aspects of seedling nutrition. Nonmycorrhizal seedlings exposed to 200 μM Al exhibited decreased growth, increased transpiration rates, decreased water use efficiency, increased foliar Al and Na levels, and reduced foliar P concentrations. Seedlings inoculated with P. tinctorius exhibited unaltered growth, physiological function, and ionic relations when exposed to Al. The fungal symbiont evidently modulated ionic relations in the rhizosphere, reducing Al-P precipitation reactions, Al uptake, and subsequent root and shoot tissue Al exposure.