Root excretion of carboxylic acids and protons in phosphorus-deficient plants

Phosphorus deficiency-induced metabolic changes related to exudation of carboxylic acids and protons were compared in roots of wheat (Triticum aestivum L. cv Haro), tomato (Lycopersicon esculentum L., cv. Moneymaker), chickpea (Cicer arietinum) and white lupin (Lupinus albus L. cv. Amiga), grown in a hydroponic culture system. P deficiency strongly increased the net release of protons from roots of tomato, chickpea and white lupin, but only small effects were observed in wheat. Release of protons coincided with increased exudation of carboxylic acids in roots of chickpea and white lupin, but not in those of tomato and wheat. P deficiency-induced exudation of carboxylic acids in chickpea and white lupin was associated with a larger increase of carboxylic acid concentrations in the roots and lower accumulation of carboxylates in the shoot tissue compared to that in wheat and tomato. - Citric acid was one of the major organic acids accumulated in the roots of all investigated species in response to P deficiency, and this was associated with increased activity and enzyme protein levels of PEP carboxylase, which is required for biosynthesis of citrate. Accumulation of citric acid was most pronounced in the roots of P-deficient white lupin, chickpea and tomato. Increased PEP carboxylase activity in the roots of these plants coincided with decreased activity of aconitase, which is involved in the breakdown of citric acid in the TCA cycle. In the roots of P-deficient wheat plants, however, the activities of both PEP carboxylase and aconitase were enhanced, which was associated with little accumulation of citric acid. The results suggest that P deficiency-induced exudation of carboxylic acids depends on the ability to accumulate carboxylic acids in the root tissue, which in turn is determined by biosynthesis, degradation and partitioning of carboxylic acids or related precursors between roots and shoot. In some plant species such as white lupin, there are indications for a specific transport mechanism (anion channel), involved in root exudation of extraordinary high amounts of citric acid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Responses of ethylene biosynthesis to saline stress in seedlings of eight plant species

The effect of salinity (100 mM NaCl) on ethylene metabolism in the early phase of vegetative development of several plant species has been investigated. The effects of saline treatment on shoot and root growth, ranged in sensitivity with respect to species: pepper (Capsicum annum L. cv Pairal) > tomato (Lycopersicon esculentum Mill. cv Malpica) > broccoli (Brassica oleraceae L. var. Italica Plenk. cv Marathon F1) ≅ lettuce (Lactuca sativa var. longifolia Lam. cv Inverna) ≅ melon (Cucumis melo L. cv Ruano F1, Roche type) > bean (Phaseolus vulgaris L. cv. Gator Green 15) ≅ spinach (Spinacia oleracea L. cv Boeing) > beetroot (Beta vulgaris L. var. crassa (Alef.) J. Helm. cv Detroit). After saline treatment, ethylene production increased 4.2-fold in pepper shoots. Significant increases were also found in shoots of tomato, broccoli and bean. In contrast, salinity decreased shoot ethylene production rate in melon, spinach, and beetroot. In roots, the general effect of salinity was a decrease in ethylene production, especially in broccoli and bean, except in tomato root, in which a sharp increase in ethylene production occurred. In general, saline treatment increased total ACC concentration in both shoot and root in most of the plant species examined, which was related to plant sensitivity to salinity. For example, pepper shoot was the most sensitive to saline treatment, showing the highest fresh weight inhibition and the highest increase in total ACC concentration (8.5-fold), while, beetroot was less affected by salinity and showed no effect on total ACC concentration in response to saline treatment.     

Changes in Root Resistance as a Function of Applied Suction, Time of Day, and Root Temperature

Water uptake rate of decapitated root systems of cotton (Gossypium hirsutum L.), tomato (Lycopersicon esculentum L. cv. Rutgers), and kidney bean (Phaseolus vulgaris L.) plants shows an exponential increase with applied suction up to about —1 bar. The water uptake rate was higher on the descending path of applied suction than on the ascending path, indicating a hysteresis effect in the roots. The root resistance in a cotton plant increased between 3-to 5-fold during the photoperiod of 12 hours. The water uptake rate increased with increasing temperature of the root medium up to 30°C in cotton and 25°C in tomato and bean plants.     

Functional xylem anatomy in root-shoot junctions of six cereal species

In cereals, the formation of safety zones in the root-shoot junction could protect the vessels of roots from embolism originating in the shoot. The root-shoot junction was examined both anatomically, with a light microscope, and experimentally, using a pressurized-air method, in the base of seminal and adventitious roots of maize (Zea mays L. cv. Seneca 60-II), a corngrass mutation of maize (Cg mutant), sorghum (Sorghum bicolor L. cv. Ho-Pak), winter oats (Avena sativa L. cv. Ogle), spring wheat (Triticum aestivum L. cv. Glenlea), winter wheat (T. aestivum cv. Monopol), winter barley (Hordeum vulgare L. cv. Wysor), spring rye (Secale cereale L. cv. JO-02 Finland), and winter rye (S. cereale cv. Musketeer). Two types of hydraulic architecture were found in the cereal roots: (i) a very safe root vessel system, as in winter rye, in which the vessels of the roots are separated from those of the shoots by tracheids, versus (ii) a completely unsafe system, as in corngrass, where the vessels in the root are continuous with the vessels in the shoot. The xylem anatomy of the seminal roots is generally correlated with the species-specific overall root morphology. Rye, wheat and barley, which develop four to six seminal roots, show a high degree of vascular segmentation resulting in, the formation of safe root vessels; maize, sorghum and oats, which typically develop a primary seminal root, contain unsafe vessels that are continuous through the mesocotyl and through the first node. In adventitious roots, vascular segmentation is not related to overall root morphology. Differences in the proportion of safe adventitious roots in which all the vessels end in the root-shoot junction range from 9 to 98% in the cereals studied. In the unsafe roots of these cereals, the number of vessels per root that are continuous through the junction range from 1 to 14. As significant differences in vascular segmentation of the root-shoot junction occur not only between species, but also between cultivars, we suggest that selection based on the occurrence of safety zones might be used in breeding programs designed to improve adaptation of cereals to drought and cold temperatures.This research project was supported by the Natural Sciences and Engineering Research Council of Canada through an International Scientific Exchange Award to R.A. and Dr. C.A. Peterson, and through an Operating Grant to M.G. We thank Dr. G. McLeod (Agriculture Canada, Swift Current, Sask., Canada), Dr. N.P.A. Huner (University of Western Ontario, London, Ont., Canada) and Dr. W.F. Tracy (University of Wisconsin, Madison, USA) for providing seeds; Dr. C.A. Peterson and Dr. W.B. McKendrick (University of Waterloo) for use of the Zeiss photomicroscopes; Dr. M.A. Dixon (University of Guelph, Guelph, Ont.) for use of the Moore pressure gauge; and Dr. R.J. O'Hara-Hines (University of Waterloo) for statistical advice.     

The Decreased Cold-Resistance of Chilling-Sensitive Plants Is Related to Suppressed CO2 Assimilation in Leaves and Sugar Accumulation in Roots

Tomato (Lycopersicon esculentum L., cv. Sibirskii skorospelyi) and cucumber (Cucumis sativus L., cv. Konkurent) plants were grown in a soil culture in a greenhouse at an average daily temperature of 20°C and ambient illumination until the development of five and eight true leaves, respectively. During the subsequent three days, some plants were kept in a climatic chamber at 6°C in the light, whereas other plants remained in a greenhouse (control). The cold-resistance of cucumber leaves and roots, as assayed from the electrolyte leakage, was reduced after cold exposure stronger than cold-resistance of tomato organs. The ratio photosynthesis/dark respiration was lower in cucumber than in tomato leaves at all measurement temperatures. The concentrations of sugars (sucrose + glucose + fructose) increased in chilled tomato roots but decreased in cucumber roots. Cold exposure changed the activities of various invertase forms (soluble and insoluble acidic and alkaline invertases). The total invertase activity and the ratio of mono- to disaccharides increased. The lower cucumber cold-resistance is related to the higher sensitivity of its photosynthetic apparatus to chilling and, as a consequence, insufficient root supply with sugars.     

Effect of Pseudomonas fluorescens on the root exudates of two tomato mutants differently sensitive to Fe chlorosis

Plants with different Fe-mobilization properties are known to differ in the amount and kind of Fe-reducing and Fe-chelating compounds exuded by their roots. Although rhizosphere bacteria are known to affect the exudation of organic compounds by the plant roots, their effect on the root exudates of plants differing in Fe-mobilization properties is not known. We studied the effect of Pseudomonas fluorescens, on the exudation of sugars and organic and amino acids by roots of an iron chlorosis-resistant (T3238FER) and a chlorosis-susceptible (T3238fer) tomato mutant. Under sterile conditions two tomato mutants grew equally well and did not differ in the total amount of sugars and organic acid exuded by their roots. More amino acids, however, were exuded by the roots of T3238FER than T323fer. Mutants differed in the amount of oxalic acid and the amino acids Ala, Asp, Gaba, Gln, Gly, His, Hyl, Ile, Leu, Lys, Phe, Pro, and Val exuded by their roots into sterile rooting media. Addition of P. fluorescens to the rooting medium did not affect the growth of T3238FER but stimulated the root growth of chlorosis-susceptible T3238fer, reduced the amounts of glucose, arabinose and fructose but increased the amount of sucrose, reduced the amounts of fumaric, malic and oxalic acid but increased the amounts of citric and succinic acid in the rooting media of both mutants. P. fluorescens resulted in the following changes in the amino acids in the rooting media: reduced the amounts of Gly, Leu, and Lys in T3238FER, and of Asp, Gln, Hyp, and Ile in T3238fer, and increased the amounts of Cys, Glu, His, Hyp, Ile, Phe and Tyr in T3238FER and of Ala, Glu, His, Phe, and Ser in T323fer—in cases more than 40-fold. These differential effects of P. fluorescens in altering the pattern of organic and amino acids compounds with some Fe-chelating properties detected in the rooting medium of these two mutants may indicate that the differences in Fe-chlorosis susceptibility of these tomato mutants may be the result of, or modified by, the interactions between plant roots and rhizosphere microorganisms. We postulate that the Fe-chlorosis susceptibility in plants may be the product of the interactions between soil microorganisms and plant roots, and may not be solely related to the plant per se.     

Secretion of malate and citrate from roots is related to high Al-resistance in Lespedeza bicolor

Lespedeza bicolor (Lespedeza bicolor Turcz. cv. Jiangxi) is a leguminous shrub that is well adapted to acid infertile soils. However, the mechanisms of aluminum resistance in this species have not been established. This study aimed to assess the possible resistance mechanisms of this plant to Al. An Al-sensitive species of Lespedeza, sericea lespedeza [Lespedeza cuneata (Dum.-Cours.) G. Don cv. Zhejiang], was used as a reference. The roots of L. bicolor secreted both malate and citrate after exposure to Al, but roots of L. cuneata did not. The secretion of organic acids from L. bicolor was specific to Al; neither 15-day P starvation nor 50 μM lanthanum induced the secretion of these organic acid anions. Secretion of organic acid anions in L. bicolor was detected after 3–6 h exposure to Al, and the amount increased significantly after 6 h exposure, suggesting that this plant shows a pattern II-type organic acid secretion. This is supported by the finding that the secretion was significantly inhibited by a protein-synthesis inhibitor, cycloheximide. Two kinds of anion-channel inhibitors had different effects on Al-induced secretion of organic acids: 9-anthracene carboxylic acid completely inhibited secretion, phenylglyoxal had no effect. Root elongation in L. bicolor was more severely inhibited by Al in the presence of 9-anthracene carboxylic acid. All these results indicated that the secretion of malate and citrate is a specialized response to Al stress in L. bicolor roots, which might be one of the Al-resistance mechanisms in this species.     

Lipids from Bean, Barley and Sugar Beet in Relation to Salt Resistance

A comparison was made between the lipid and fatty acid composition of the salt-sensitive bean (Phaseolus vulgaris L. cv. Saxa), the less salt-sensitive barley (Hordeum vulgaris L. cv. Wisa) and the salt-tolerant sugar beet (Beta vulgaris L. cv. Kawemono). Sugar beet roots showed a higher content of sterol components and sulfolipid as compared with bean and barley roots. The lipids of sugar beet roots contained more linoleic acid and less linolenic acid than those of bean and barley roots. For barley and sugar beet roots a higher amount of extra-long chain fatty acids was observed than for bean roots. It was concluded that differences in membrane structure are correlated with differences in membrane permeability to sodium and chloride and in salt-resistance of the studied species.     

Sequence and tissue-specific expression of a putative peroxidase gene from wheat (Triticum aestivum L.)

We have used a cDNA clone encoding a pathogen-induced putative wheat peroxidase to screen a genomic libary of wheat (Triticum aestivum L. cv. Cheyenne) and isolated one positive clone, lambda POX1. Sequence analysis revealed that this clone contains a gene encoding a putative peroxidase with a calculated pI of 8.1 which exhibits 58% and 83% sequence identity to the amino acid sequence of the turnip (Brassica rapa) peroxidase and a pathogen-induced putative wheat peroxidase, respectively. The two introns in the wheat gene are at the same positions as introns in the peroxidase genes of tomato and horseradish. Results of S1-mapping experiments suggest that this gene is neither pathogen-nor wound-induced in leaves but is constitutively expressed in roots.     

Pattern of aluminum-induced secretion of organic acids differs between rye and wheat

Al-Induced secretion of organic acids from the roots has been considered as a mechanism of Al tolerance, but the processes leading to the secretion of organic acids are still unknown. In this study, the secretion pattern and alteration in the metabolism of organic acids under Al stress were examined in rye (Secale cereale L. cv King) and wheat (Triticum aestivum L. cv Atlas 66). Al induced rapid secretion of malate in the wheat, but a lag (6 and 10 h for malic and citric acids, respectively) between the exposure to Al and the secretion of organic acids was observed in the rye. The activities of isocitrate dehydrogenase, phosphoenolpyruvate carboxylase, and malate dehydrogenase were not affected by Al in either plant. The activity of citrate synthase was increased by the exposure to Al in the rye, but not in the wheat. The secretion of malate was not suppressed at low temperature in the wheat, but that of citrate was stopped in the rye. The Al-induced secretion of citrate from roots of the rye was inhibited by the inhibitors of a citrate carrier, which transports citrate from the mitochondria to the cytoplasm. All of these results suggest that alteration in the metabolism of organic acids is involved in the Al-induced secretion of organic acids in rye, but only activation of an anion channel seems to be responsible for the rapid secretion of malate in the wheat.     

Translocation and Complex Formation of Root-applied 2,4-D and Picloram in Susceptible and Tolerant Species

Translocation and complex formation of ¹⁴C-labelled 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-amino-3,5,6-trichloropicolinic acid (picloram) in seedlings of sunflower (Helianthus annuus L. var. uniflorus), rape (Brassica napus L. cv. Nilla), wheat (Triticum aestivum L. cv. Starke), and Norway spruce (Picea abies (L.) H. Karst.) were studied. The herbicides were absorbed through the roots from the nutrient solution, Picloram was well translocated to the shoots of the four species; while the acropetal translocation of 2,4-D was small except in rape. In 2,4-D-susceptible sunflower and rape and in picloramsusceptible sunflower and spruce the herbicides were recovered mainly in the uncomplexed form. In 2,4-D tolerant wheat and spruce most of the absorbed 2,4-D was converted into water-soluble or TCA-insoluble complexes. In picloram-tolerant wheat and in relatively picloram-tolerant rape, the absorbed picloram was also converted into complexes recovered predominantly in the water-soluble fraction. Most of the complexes released free herbicides by hydrolyzing in NaOH or HCI. The results further support the hypothesis that complex formation counteracts herbicide toxicity.     

Root herbivory in vitro: Interactions between roots and aphids grown in aseptic coculture

Summary An in vitro coculture system has been established to study interactions between roots and aphids. Eight aphid species (Aphis spiraecola P., Trama rara M., Macrosiphum euphorbiae S., Rhopalosiphum padi L., Sitobion avenae F., Rhopalosiphum maidis F., Metopolophium dirhodum W., and Pemphigus populivenae F.) were reared on six species of hairy root cultures, Carthamus tinctorius L. cv N10, Tagetes patula L., Trichosanthes cucumerina L. var anguina, Hyoscyamus muticus L., Nicotiana tabacum L., and Beta vulgaris L. subsp. vulgaris. All species of aphids survived on root cultures for at least 2 d. Three cocultures have been maintained aseptically for periods ranging from 2 mo. to over 2 yr. The coculture of R. padi on C. tinctorius cv N10 (N10-Rp) was used to study morphological and biochemical responses of roots under aphid herbivory. Aphid herbivory caused browning of cultures, reduced root vegetative growth, and increased production of polyacetylenes in C. tinctorius cv N10 roots. Our results suggest that this coculture system may improve our understanding of interactions between aphids and plant roots.     

Composition of root exometabolites of the symbiotically effective pea cultivar triumph and its parental forms

The qualitative and quantitative composition of low-molecular exometabolites in roots of pea (Pisum sativum L.) was studied with a cultivar Triumph and its parental forms (a symbiotically effective variety k-8274 and a modern highly productive cv. Classic). A relationship between root exudation and the ability of cultivars to establish symbiosis was analyzed. In the early stages of plant growth, the roots of cv. Triumph exhibited low exudation of organic acids, sugars, and amino acids. The quantitative composition of organic acids in the root exudates of cv. Triumph was close to that of cv. k-8274, whereas the composition of sugars and amino acids was similar to that of cv. Classic. In the field experiment, the effect of inoculation with a mixture of rhizobium strains and mycorrhizal fungus on plant growth was more evident in cv. Triumph than in cvs. Classic and k-8274. The results suggest that the high symbiotic potential of cv. Triumph is related to exudation of pyruvic and succinic acids that were the major components of root exometabolites both in Triumph and k-8274 cultivars.     

A fluorescent compound in oat root plasma membrane

Homogenates of 7-day-old oat (Avena sativa L. cv. Brighton) roots were highly fluorescent (excitation and emission maxima around 360 and 440 nm, respectively). Less than ¹/₁₀ as much fluorescence per g fresh weight was found in oat shoots or in wheat (Triticum aestivum L. cv. Drabant) roots or shoots. Most of the fluorescence of oat roots was found in the soluble fraction (150 000g supernatant). However, some could be detected in the plasma membrane fraction (excitation and emission maxima 365 and 417 nm, respectively), which contained a 3-fold higher fluorescence per mg protein than the homogenate. Growth of oat or wheat in a medium containing, 10-^?5M scopoletin (6-methoxy-7-hy-droxy coumarin), a fluorescent compound previously reported to be present in both wheat and oat roots, caused the disappearance of scopoletin from the medium (proportional to the amount of roots) and the appearance of increased fluorescence in the root homogenates but not in the shoot homogenates. In both oat and wheat roots ail of the extra fluorescence was recovered in the soluble fraction and at least in wheat it consisted of unconverted scopoletin. The concentration of scopoletin in wheat roots grown in 10-^?5M scopoletin was around 50 nmol (g fresh weight)^?1, or about five times the concentration in the growth medium. Scopoletin in the growth medium (10-^?5M) or in the assays (up to 10-^?4M) did not affect Mg²⁺-, Mg²⁺+K⁺- or Ca²⁺-ATPase activities in wheat or oat roots. The fluorescence properties of the oat plasma membrane were different from those of authentic scopoletin. Either the surroundings modify the fluorescence of membrane-associated scopoletin or the endogenous fluorescent compound is not scopoletin but a glycoside-derivative of scopoletin or some completely unrelated compound.     

Wheat germ agglutinin in wheat seedling roots: induction by elicitors and fungi

Summary Treatment of wheat (Triticum aestivum L.) seedlings with elicitors originating from either plant or fungal cell walls induces about a 2-fold increase of wheat germ agglutinin (WGA) in the roots. While the WGA content in roots of healthy plants normally decreases as a function of germination time, a transient accumulation of WGA could be observed in plants challenged with different fungi, including Rhizoctonia solani, Fusarium culmorum, Pythium ultimum and Neurospora crassa. Peak levels in challenged roots were 2 to 5 times as high as in control plants. Most of this induced WGA could be released from the roots by soaking them in a solution of the hapten N-acetylglucosamine. On the basis of the results obtained it is postulated that WGA may be involved in the defence of wheat against fungal attack.     

Relationship between osmotic stress and polyamines conjugated to the deoxyribonucleic acid-protein in wheat seedling roots

The contents of covalently conjugated polyamines (CC-PAs) and noncovalently conjugated polyamines (NCC-PAs) to deoxyribonucleic acid-protein (DNP) isolated from wheat (Triticum aestivum L.) seedling roots under osmotic stress were detected. Results showed that after osmotic stress treatment for 7 d, the levels in NCC-spermine (NCC-Spm) and NCC-spermidine (NCC-Spd) of drought-tolerant Yumai No. 18 cv. increased more markedly than that of drought-sensitive Yangmai No. 9 cv., while the NCC-putrescine (NCC-Put) could not be statistically detected in two cultivars. Exogenous Spm treatment alleviated osmotic stress injury to Yangmai No. 9 cv. seedlings, coupled with marked increases of NCC-Spm and NCC-Spd levels of this cultivar. Under PEG osmotic stress, the concomitant treatment of drought-tolerant Yumai No. 18 cv.seedlings with methylglyoxyl-bis (guanylhydrazone) (MGBG), an inhibitor of S-adenosylmethionine decarboxylase (SAMDC), aggravated osmotic stress injury to this cultivar, coupled with market decreases of the NCC-Spm and NCC-Spd levels. The levels in CC-Put and CC-Spd of drought-tolerant Yumai No. 18 cv. increased more markedly than that of drought-sensitive Yangmai No. 9 cv. Under osmotic stress. The treatment of drought-tolerant Yumai No. 18 cv. seedlings with phenanthrolin (o-Phen), an inhibitor of transglutaminase (TGase), aggravated osmotic stress injury to this cultivar, coupled with a reduction of sum contents of CC-Put+CC-Spd. These results suggested that NCC-Spm and NCC-Spd, together with CC-Put and CC-Spd, in DNP of roots could enhance tolerance of the wheat seedlings to osmotic stress.     

Effects of chlorosubstituted indoleacetic acids on root growth, ethylene and ATP formation

7-Chloroindoleacetic acid and dichloroindoleacetic acids with a Cl in the 7 position showed anti-auxinic activity and promoted root growth in wheat ( Triticum aestivum L. cv. Diamant II). In contrast, 4-, 5- and 6-chloroindoleatetic acids acted as strong auxins inhibiting the growth of wheat roots. Flax ( Linum usitatissimum L. cv. Concurrent) and cucumber ( Cucumis sativus L. cv. Favör) roots showed similar, but less clear-cut responses. 7-Chloroindoleacetic acid and 4,7-dichloroindoleacetic acid alleviated root growth inhibition in wheat caused by IAA, monochloroindoleacetic acids and benzyladenine. 2,4-D, 4- and 6-chloroindoleacetic acids strongly induced ethylene formation in cucumber seedlings; 4,7- and 6,7-dichloroindoleacetic acids did not, except at high concentrations. The more lipid-soluble dichloroindoleacetic acids were stronger inhibitors of ATP formation in cucumber mitochondria than monochloroindoleacetic acids, while IAA itself had only a very slight effect.     

Phenylalanine Ammonia-lyase Activity in Barley After Infection with Bipolaris sorokiniana or Treatment with its Purified Xylanase

Phenylalanine ammonia-lyase (PAL) activity was determined from leaves and roots of two barley (Hordeum vulgare L.) cultivars after infection with a necrotrophic pathogen, Bipolaris sorokiniana (Sacc.) Shoem., and treatment with its purified xylanase. PAL activity increased in leaves of both cultivars 16 h after fungal inoculation but two phases, with activity peaks at 24–32 h and 40 h, were recorded only for the more resistant cultivar, Agneta. Attempts to use a PAL inhibitor, χ-amin, ooxyacetic acid, to increase susceptibility to B. sorokiniana in barley leaves were unsuccessful. Treatments of leaves with purified xylanase resulted in more rapid (4–12 h after injection), although reduced, induction of PAL compared with fungal injection. The higher the concentration of xylanase applied the earlier the activity peaks were detected. Fungal inoculation only slightly increased PAL activity in barley roots while xylanase treatment had no effect. The basal level of PAL was however much higher in roots than in leaves. In wheat, Triticum aestivum L. resistant to B. sorokiniana, the time-course of PAL induction after fungal infection and xylanase treatment resembled that for cv. Agneta, while in oats, Avena sativa L. (non-host) PAL activity did not change after the treatments. The results suggest that the second phase of PAL induction, associated only with responses of barley cv. Agneta and wheat, is linked with their resistance to B. sorokiniana infection. The possible role of xylanase as an elicitor of PAL is discussed.     

Osmotic adjustment in Lycopersicon esculentum and L. Pennellii under NaCl and polyethylene glycol 6000 iso–osmotic stresses

Changes in leaf solute contents in response to saline (NaCl) and osmotic (polyethylene glycol, PEG, 6000) stresses were measured in three different salt tolerant cultivars of Lycopersicon esculentum (L.) Mill. (Pera, P-73 and Volgogradskij), and its wild relative L. pennellii (Correll) D'Arcy accession PE-47. Iso-osmotic stresses (–0. 5 MPa) of NaCl (140 mM) and PEG 6000 (150 g l^-1) were applied to one-month old plants for 3 weeks. Decreasing leaf dry weight was similar in L. pennellii or L. esculentum cv. P-73 and Volgogradskij under both stresses, while leaf dry weight of L. esculentum cv. Pera decreased more under PEG stress than under NaCl stress. Water contents decreased in all the PEG treated populations, while their calculated solute potential (Ψ_s increased. Under osmotic stress, the total ion contents decreased in relation to control, whereas organic solutes (sugars, amino acids and organic acids) markedly increased in both tomato species, specially in the tomato cultivars, where these solutes represented 50% of the Ψ₅ calculated. Soluble sugar increase was three times higher in leaves of L. esculentum than in the leaves of L. pennellii. Free proline increased under both stresses and its content was highest in L. esculentum and in L. pennellii, respectively, under NaCl and PEG stresses. Nevertheless, the contribution of this metabolite to Ψ_s did not exceed 5%, irrespective of treatment and species. The greater organic solute accumulation in L. esculentum than in L. pennellii– which was not reflected in their Ψ₅ values – was not correlated with the tolerances of the two species to osmotic stress. Therefore, osmotic adjustment may not be the only process influencing salt and drought tolerances in tomato; the ability of plants to regulate their metabolic and physiological functions could also play an important role under these harmful conditions. The possible roles of inorganic solutes and metabolites in osmotic adjustment, energetic metabolism and redox regulation are discussed     

Organic acids enhance the uptake of lead by wheat roots

The uptake and bioavailability of lead (Pb) in soil–plant systems remain poorly understood. This study indicates that acetic and malic acids enhance the uptake of Pb by wheat (Triticum aestivum L.) roots under hydroponic conditions. The net concentration-dependent uptake influx of Pb in the presence and absence of organic acids was characterized by Michaelis–Menten type nonsaturating kinetic curves that could be resolved into linear and saturable components. Fitted maximum uptake rates (V _max) of the Michaelis–Menton saturable component in the presence of acetic and malic acids were, respectively, 2.45 and 1.63 times those of the control, while the Michaelis–Menten K _m values of 5.5, 3.7 and 2.2 μM, respectively, remained unchanged. Enhanced Pb uptake by organic acids was partially mediated by Ca²⁺ and K⁺ channels, and also depended upon the physiological function of the plasma membrane P-type ATPase. Uptake may have been further enhanced by an effectively thinner unstirred layer of Pb adjacent to the roots, leading to more rapid diffusion towards roots. X-ray absorption spectroscopic studies provided evidence that the coordination environment of Pb in wheat roots was similar to that of Pb(CH₃COO)₂·3H₂O in that one Pb atom was coordinated to four oxygen atoms via the carboxylate group.