Tomato roots have a functional silicon influx transporter but not a functional silicon efflux transporter

Silicon (Si) accumulation in shoots differs greatly with plant species, but the molecular mechanisms for this interspecific difference are unknown. Here, we isolated homologous genes of rice Si influx (SlLsi1) and efflux (SlLsi2) transporter genes in tomato (Solanum lycopersicum L.) and functionally characterized these genes. SlLsi1 showed transport activity for Si when expressed in both rice lsi1 mutant and Xenopus laevis oocytes. SlLsi1 was constitutively expressed in the roots. Immunostaining showed that SlLsi1 was localized at the plasma membrane of both root tip and basal region without polarity. Furthermore, overexpression of SlLsi1 in tomato increased Si concentration in the roots and root cell sap but did not alter the Si concentration in the shoots. By contrast, two Lsi2-like proteins did not show efflux transport activity for Si in Xenopus oocytes. However, when functional CsLsi2 from cucumber was expressed in tomato, the Si uptake was significantly increased, resulting in higher Si accumulation in the leaves and enhanced tolerance of the leaves to water deficit and high temperature. Our results suggest that the low Si accumulation in tomato is attributed to the lack of functional Si efflux transporter Lsi2 required for active Si uptake although SlLsi1 is functional.     

Uptake and transport of foliar applied zinc (65Zn) in bread and durum wheat cultivars differing in zinc efficiency

Using two bread wheat (Triticum aestivum) and two durum wheat (Triticum durum) cultivars differing in zinc (Zn) efficiency, uptake and translocation of foliar-applied ⁶⁵Zn were studied to characterize the role of Zn nutritional status of plants on the extent of phloem mobility of Zn and to determine the relationship between phloem mobility of Zn and Zn efficiency of the used wheat cultivars. Irrespective of leaf age and Zn nutritional status of plants, all cultivars showed similar Zn uptake rates with application of ⁶⁵ZnSO₄ to leaf strips in a short-term experiment. Also with supply of ⁶⁵ZnSO₄ by immersing the tip (3 cm) of the oldest leaf of intact plants, no differences in Zn uptake were observed among and within both wheat species. Further, Zn nutritional status did not affect total uptake of foliar applied Zn. However, Zn-deficient plants translocated more ⁶⁵Zn from the treated leaf to the roots and remainder parts of shoots. In Zn-deficient plants about 40% of the total absorbed ⁶⁵Zn was translocated from the treated leaf to the roots and remainder parts of shoots within 8 days while in Zn-sufficient plants the proportion of the translocated ⁶⁵Zn of the total absorbed ⁶⁵Zn was about 25%. Although differences in Zn efficiency existed between the cultivars did not affect the translocation and distribution of ⁶⁵Zn between roots and shoots. Bread wheats compared to durum wheats, tended to accumulate more ⁶⁵Zn in shoots and less ⁶⁵Zn in roots, particularly under Zn-deficient conditions. The results indicate that differences in expression of Zn efficiency between and within durum and bread wheats are not related to translocation or distribution of foliar-applied ⁶⁵Zn within plants. Differential compartementation of Zn at the cellular levels is discussed as a possible factor determining genotypic variation in Zn efficiency within wheat.     

Zinc deficiency-inducible OsZIP8 encodes a plasma membrane-localized zinc transporter in rice

Zinc is an essential micronutrient for several physiological and biochemical processes. To investigate its transport in rice, we characterized OsZIP8, a rice ZIP (Zrt, Irt-like Protein) gene that is strongly up-regulated in shoots and roots under Zn deficiency. OsZIP8 could complement the growth defect of Zn-uptake yeast mutant. The OsZIP8-GFP fusion proteins were localized to the plasma membrane, suggesting that OsZIP8 is a plasma membrane zinc transporter in rice. Activation and overexpression of this gene disturbed the zinc distribution in rice plants, resulting in lower levels in shoots and mature seeds, but an increase in the roots. Field-grown transgenic plants were shorter than the WT. Under treatment with excess zinc, transgenics contained less zinc in their shoots but accumulated more in the roots. Altogether, these results demonstrate that OsZIP8 is a zinc transporter that functions in Zn uptake and distribution. Furthermore, zinc homeostasis is important to the proper growth and development of rice.     

Silicon nutrition alleviates the negative impacts of arsenic on the photosynthetic apparatus of rice leaves: an analysis of the key limitations of photosynthesis

Silicon (Si) plays important roles in alleviating various abiotic stresses. In rice (Oryza sativa), arsenic (As) is believed to share the Si transport pathway for entry into roots, and Si has been demonstrated to decrease As concentrations. However, the physiological mechanisms through which Si might alleviate As toxicity in plants remain poorly elucidated. We combined detailed gas exchange measurements with chlorophyll fluorescence analysis to examine the effects of Si nutrition on photosynthetic performance in rice plants [a wild‐type (WT) cultivar and its lsi1 mutant defective in Si uptake] challenged with As (arsenite). As treatment impaired carbon fixation (particularly in the WT genotype) that was unrelated to photochemical or biochemical limitations but, rather, was largely associated with decreased leaf conductance at the stomata and mesophyll levels. Indeed, regardless of the genotypes, in the plants challenged with As, photosynthetic rates correlated strongly with both stomatal (r² = 0.90) and mesophyll (r² = 0.95) conductances, and these conductances were, in turn, linearly correlated with each other. The As‐related impairments to carbon fixation could be considerably reverted by Si in a time‐ and genotype‐dependent manner. In conclusion, we identified Si nutrition as an important target in an attempt to not only decrease As concentrations but also to ameliorate the photosynthetic performance of rice plants challenged with As.     

Dynamics of zinc uptake and accumulation in the hyperaccumulating and non-hyperaccumulating ecotypes of Sedum alfredii Hance

Sedum alfredii Hance has been identified as a Zn-hyperaccumulating plant species native to China. The characteristics of Zn uptake and accumulation in the hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of S. alfredii were investigated under nutrient solution and soil culture conditions. The growth of HE was normal up to 1000 μM Zn in nutrient solution, and 1600 mg Zn kg⁻¹ soil in a Zn-amended soil. Growth of the NHE was inhibited at Zn levels ≥250 μM in nutrient solution. Zinc concentrations in the leaves and stems increased with increasing Zn supply levels, peaking at 500 and 250 μM Zn in nutrient solution for the HE and the NHE, respectively, and then gradually decreased or leveled off with further increase in solution Zn. Minimal increases in root Zn were noted at Zn levels up to 50 μM; root Zn sharply increased at higher Zn supply. The maximum Zn concentration in the shoots of the HE reached 20,000 and 29,000 mg kg⁻¹ in the nutrient solution and soil experiments, respectively, approximately 20 times greater than those of the NHE. Root Zn concentrations were higher in the NHE than in the HE when plants were grown at Zn levels ≥50 μM. The time-course of Zn uptake and accumulation exhibited a hyperbolic saturation curve: a rapid linear increase during the first 6 days in the long-term and 60 min in the short-term studies; followed by a slower increase or leveling off with time. More than 80% of Zn accumulated in the shoots of the HE at half time (day 16) of the long-term uptake in 500 μM Zn, and also at half time (120 min) of the short-term uptake in 10 μM ⁶⁵Zn²⁺. These results indicate that Zn uptake and accumulation in the shoots of S. alfredii exhibited a down-regulation by internal Zn accumulated in roots or leaves under both nutrient solution and soil conditions. An altered Zn transport system and increased metal sequestration capacity in the shoot tissues, especially in the stems, may be the factors that allow increased Zn accumulation in the hyperaccumulating ecotype of S. alfredii. Section Editor: F. J. Zhao     

Uptake and transport of zinc in the hyperaccumulator Thlaspi caerulescens and the non-hyperaccumulator Thlaspi ochroleucum

Growth and zinc uptake of the hyperaccumulator species Thlaspi caerulescens J. & C. Presl and the non-hyperaccumulator species Thlaspi ochroleucum Boiss. & Heldr. were compared in solution culture experiments. T. caerulescens was able to tolerate 500 mmol m^?3 (32.5 g m^?3) Zn in solution without growth reduction, and up to 1000 mmol m^?3 (65 g m^?3) Zn without showing visible toxic symptoms but with a 25% decrease in dry matter (DM) yield. Up to 28 g kg^?1 of Zn in shoot DM was obtained in healthy plants of T. caerulescens. In contrast, T. ochroleucum suffered severe phytotoxicity at 500 mmol m^?3 Zn. Marked differences were shown in Zn uptake, distribution and redistribution between the two species. T. caerulescens had much higher concentrations of Zn in the shoots, whereas T. ochroleucum accumulated higher concentrations of Zn in the roots. When an external supply of 500 mmol m^?3 Zn was withheld, 89% of the Zn accumulated previously in the roots of T. caerulescens was transported to the shoots over a 33 d period, whereas in T. ochroleucum only 32% was transported. T. caerulescens was shown to have a greater internal requirement for Zn than other plants. Increasing the supply of Zn from 1 to 10 mmol m^?3 gave a 19% increase in the total DM of this species. liven the shoots from the 1 mmol m^?3 Zn treatment which showed Zn deficiency contained 10 times greater Zn concentrations than the widely reported critical value for Zn deficiency to occur in many other plant species. The results obtained suggest that strongly expressed constitutive sequestration mechanisms exist in the hyperaccumulator T. caerulescens, which detoxify the large amount of Zn present in shoot tissues and decrease its physiological availability in the cytosol. Both T. caerulescens and T. ochroleucum had constitutively high concentrations of malate in shoots, which were little affected by different Zn treatments. Although malate may play a role in Zn chelation because of the high concentrations present, it cannot explain the species specificity of Zn tolerance and hyperaccumulation.     

Mechanism of phosphorus-induced zinc deficiency in cotton. II. Evidence for impaired shoot control of phosphorus uptake and translocation under zinc deficiency

Cotton (Gossypium hirsutum L. cv. Deltapine 15/21) plants were precultured for 19 to 25 days under controlled climatic conditions in nutrient solutions with different levels of Zn. With the onset of visual Zn-deficiency symptoms the pH of the nutrient solution decreased from 6.0 to about 5.0. In contrast, Zn-sufficient plants raised the pH of the nutrient solution to about 7.0. In short-term studies it could be demonstrated that the Zn nutritional status of the plants remarkably influenced the uptake and translocation rates of mineral nutrients. Compared to Zn-sufficient plants, P uptake rate in severely Zn-deficient plants was increased by a factor of 2 to 3, whereas the uptake rates of K, Ca and particularly NO₃ decreased. The accumulation of P in the roots of Zn-deficient plants was either not affected or even lower than in Zn-sufficient plants. Thus, Zn deficiency had a specific enhancement effect on root to shoot transport of P. This enhancement effect of Zn deficiency on uptake and transport of P was similar at nutrient solution pH values of 7.0 and 5.8; i.e. it was not the result of acidification of the nutrient solution. After application of ³⁶CI, ⁸⁶Rb and ³²P to plant stems, basipetal transport of ³⁶CI and ⁸⁶Rb was not affected by the Zn nutritional status of the plants. However, in Zn-deficient plants, only 7.8% of the ³²P was translocated basipetally compared to 34% in the Zn-sufficient plants. A resupply of Zn for 19 h to Zn-deficient plants enhanced basipetal ³²P transport. The results indicate that a feedback mechanism in the shoots is impaired in Zn-deficient plants which controls the P uptake by roots and especially the P transport from roots to shoots. As a result of this impairment toxic concentrations of P accumulate in the leaves. The mechanism responsible is likely the retranslocation of P in the phloem from shoots to roots.     

Interaction between zinc nutritional status of cereals and Rhizoctonia root rot severity

An inverse correlation between plant Zn concentration and the severity of Rhizoctonia root rot, described in an earlier paper, was examined in two experiments in a growth chamber. In the first experiment, wheat (Triticum aestivum cv Songlen) was planted in a Zn deficient soil with and without added Zn, and combined factorially with different inoculum densities of Rhizoctonia solani anastomosis group 8. When Zn was added, the percentage of seminal roots infected with R. solani was significantly lower compared to the treatments without added Zn, showing that low Zn potentiated the disease. A subsequent factorial experiment of four inoculum densities and six Zn levels, (0, 0.01, 0.04, 0.1, 0.4 and 6.0 mg Zn kg^–1 soil) was conducted to investigate the Zn effect in more detail. Disease severity was markedly decreased by the higher Zn applications; the disease score dropped sharply between treatments of Zn_0.04 and Zn_0.1, a difference which was reflected in the plant yield response to Zn. For both experiments the Zn concentrations in shoots were significantly different only among Zn treatments, not among the inoculum treatments. This indicated that inoculum density or disease severity did not reduce Zn concentration in the plant. Thus, disease did not exaggerate Zn deficiency, but rather, Zn sufficiency suppressed disease severity. A potentiating link between Zn nutrition and disease severity is thereby established, although this type of experiment did not indicate the mechanism of the Zn effect.     

Silicon does not always mitigate zinc toxicity in maize

We have investigated the influence of silicon on higher zinc concentration reducing the growth of aboveground parts by ca 50 % in young maize plants (hybrid Novania) grown in hydroponics. Eight different treatments were used: control, Zn (800 μM ZnSO₄·7H₂O), Si1/Si2.5/Si5 (1/2.5/5 mM Na₂SiO₇) and Zn+Si (combination of zinc and all silicon concentrations). The concentration of Zn and Si and their distribution in plants was determined. The growth parameters (length of primary seminal root, leaf area of first and second leaves, fresh and dry weight of below- and above-ground plant parts) of plants grown in various Zn+Si treatments were significantly decreased in comparison to all other treatments. Increasing concentration of Si in combination with Zn treatment and selected hybrid (Novania) resulted in increased physiological stress in comparison to Zn treatment. However, roots and shoots of all Zn+Si treated plants contained significantly lower amount of Zn than Zn treatment. The Si concentration in roots was the same in Si and Zn+Si plants. In general, higher amount of Si was observed in shoots than in roots of Si1- and Si2.5-treated plants and opposite was observed in Si5-treated plants. In spite of significantly decreased root and shoot accumulation of Zn in the presence of Si, no positive effect of Si on Zn toxicity in young maize plants under experimental conditions used in this work and used maize hybrid was observed.     

Influence of silicon on cobalt, zinc, and magnesium in baker's yeast, Saccharomyces cerevisiae

Silicon (Si, as silicate) is involved in numerous important structure and function roles in a wide range of organisms, including man. Silicate availability influences metal concentrations within various cell and tissue types, but, as yet, clear mechanisms for such an influence have been discovered only within the diatoms and sponges. In this study, the influence of silicate on the intracellular accumulation of metals was investigated in baker's yeast (Saccharomyces cerevisiae). It was found that at concentrations up to 10 mM, silicate did not influence the growth rate of S. cerevisiae within a standard complete medium. However, an 11% growth inhibition was observed when silicate was present at 100 mM. Intracellular metal concentrations were investigated in yeast cultures grown without added silicate (−Si) or with the addition of 10 mM silicate (+Si). Decreased amounts of Co (52%), Mn (35%), and Fe (20%) were found within +Si-grown yeast cultures as compared to −Si-grown ones, whereas increased amounts of Mo (56%) and Mg (38%) were found. The amounts of Zn and K were apparently unaffected by the presence of silicon. +Si enhanced the yeast growth rate for low-Zn²⁺ medium, but it decreased the growth rate under conditions of a low Mg²⁺ medium and did not alter the growth rates in high Zn²⁺ and Co²⁺ media. +Si doubled the uptake rate of Co²⁺ but did not influence that of Zn²⁺. We propose that a possible explanation for these results is that polysilicate formation at the cell wall changes the cell wall binding capacity for metal ions. The toxicity of silicate was compared to germanium (Ge, as GeO₂), a member of the same group of elements as Si (group 14). Hence, Si and Ge are chemically similar, but silicate starts to polymerize to oligomers above 5 mM, whereas Ge salts remain as monomers at such concentrations. Ge proved to be far more toxic to yeast than Si and no influence of Si on Ge toxicity was found. We propose that these results relate to differences in cellular uptake.     

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

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

Effect of silicon application on Sorghum bicolor exposed to toxic concentration of zinc

The aim of this study was to determine the effect of exogenously applied Si on the growth and physiological parameters of sorghum [Sorghum bicolor (L.) Moench] cultivated in hydroponics with elevated zinc concentrations (75 μM and 150 μM Zn). Increased concentrations of Zn inhibited root growth and biomass production of roots and shoots. Application of Si individually showed a positive effect on root growth but negatively affected production of fresh and dry biomass of roots and shoots. On the other hand, silicon in combination with Zn significantly reduced the inhibitory effect of Zn on root growth but did not positively affect biomass production of roots and shoots. Accumulation of Zn in plant tissues increased with increasing Zn concentration in nutrient solution, but application of Si in combination with Zn did not significantly influence Zn accumulation in roots. Completely opposite results were found in Si accumulation in roots treated with Si in combination with Zn. Interaction of these ions resulted in considerable increase of Si accumulation in roots which almost doubled in comparison to individal Si treatment. Impact of Zn on the activity of some antioxidant enzymes was equivocal and differences were observed also between two Zn concentrations. Individual application of Si resulted in significant increase in the activity of all studied antioxidant enzymes but Si in combination with Zn mostly negatively affected their activity except the activity of catalase (CAT) which was the highest in roots grown in solution containing both Si and Zn ions. Comparing all obtained data we can assume that Si applied in combination with Zn did not significantly alleviate Zn toxicity in young sorghum except the growth of primary seminal root and further experiments are required for better understanding of their interaction.     

Influence of subsoil zinc on dry matter production,seed yield and distribution of zinc in oilseed rape genotypes differing in zinc efficiency

The effects of Zn supply (+Zn: 1 mg kg^–1 soil, -Zn: no Zn added) in subsoil were examined in three genotypes of Brassica napus (Zhongyou 821, Xinza 2, Narendra) and one genotype of Brassica juncea (CSIRO-1) in a glasshouse experiment in pots (100 cm long, 10.5 cm diameter). The topsoil (upper 20 cm soil in pots) was supplied with Zn in all treatments whereas Zn was either supplied or omitted from the subsoil. Supplying Zn to subsoil significantly increased the root growth in the lower zone, markedly decreased the number of aborted and unfilled pods plant^–1 and significantly increased the number of developed pods plant^–1, number of seeds pod^–1, individual seed weight and overall seed yield. Subsoil Zn also significantly increased the Zn concentration and Zn content of seed and improved the ratio of Zn uptake by seed to total Zn uptake by seed and shoots. These effects of subsoil Zn were more pronounced in Zhongyou 821, Xinza 2 and Narendra compared with CSIRO-1. CSIRO-I had 92% Zn efficiency (ratio of -Zn subsoil seed yield to +Zn subsoil seed yield expressed in percentage) compared with 63% for Zhongyou 821. Among the four genotypes, CSIRO-1 had the lowest Zn concentration in roots and shoots but highest Zn concentration and content in seed, suggesting it has a superior Zn transport mechanism from source (roots) to sink (seed). CSIRO-1 also significantly decreased the rhizosphere pH in lower rooting zone (20-93 cm) in -Zn subsoil treatment compared with +Zn treatment.     

Heavy metal uptake and chemical changes in the rhizosphere of Thlaspi caerulescens and Thlaspi ochroleucum grown in contaminated soils

Thlaspi caerulescens (J. and C. Presl) and Thlaspi ochroleucum (Boiss. ex Heldr) were grown in three different soils containing moderate to high amounts of heavy metals in a pot experiment, using a rhizobag technique. T. caerulescens accumulated significantly more Zn in the shoots than T. ochroleucum. The concentrations of Zn in the shoots of T. caerulescens ranged from 3100 to 8100 mg kg^-1 dry matter (DM), but only from 800 to 1600 mg kg^-1 DM in T. ochroleucum. Total uptake of Zn in the shoots of T. caerulescens was about 5 times that of T. ochroleucum. In contrast, the differences between the two species in the uptake of Cd, Cr, Cu, Ni and Pb were generally small. Concentrations of mobile Zn (extractable with 1M NH₄NO₃) in the rhizosphere and non-rhizosphere soils decreased considerably after growth of both plants, but the decreases were greater with T. caerulescens than with T. ochroleucum. The decreases in the mobile fraction accounted for less than 10% of the total Zn uptake by T. caerulescens indicating that this species was effective in mobilising Zn from less soluble fractions in the soils. The rhizosphere soils tended to have higher concentrations of mobile Zn than the non-rhizosphere soils, probably because of the lower pH in the rhizosphere. The pH in the rhizosphere soils was 0.2-0.4 units lower than that in the non-rhizosphere soils at the end of the experiment. However, there were no significant differences between the two species in the degree of rhizosphere acidification. The results suggest that T. caerulescens has potential for removing Zn from moderately to highly contaminated soils, but that this ability was not related to the pH changes in the rhizosphere.     

Decrease in nitrate uptake and increase in proton release in zinc deficient cotton,sunflower and buckwheat plants

The effect of varied Zn supply on the pH of the nutrient solution and uptake of cations and anions was studied in cotton (Gossypium hirsutum L.), sunflower (Helianthus annuus L.) and buckwheat (Fagopyrum esculentum Moench) plants grown under controlled environmental conditions in nutrient solutions with nitrate as source of nitrogen. With the appearance of visual Zn deficiency symtoms, the pH of the nutrient solutions decreased from 6 to about 5 whereas the pH increased to about 7 when the plants were adequately supplied with Zn. In Zn deficient plants the pH decrease was associated with a shift in the cation-anion uptake ratio in favour of cation uptake. Of the major ions, uptake of Ca²⁺ and K⁺ was either not affected or only slightly lowered whereas NO₃ ^- uptake was drastically decreased in Zn deficient plants. Although the Zn nutritional status of plants hardly affected the NO₃ ^- concentrations in the plants, the leakage of NO₃ ^- from roots of Zn deficient plants into a diluted CaCl₂ solution was nearly 10 times higher than that of plants adequately supplied with Zn. In contrast to Zn deficiency, Mn deficiency in cotton plants neither affected NO₃ ^- uptake nor the pH of the nutrient solution.The results indicate that, probably as a consequence of the role of Zn in plasma membrane integrity and nitrogen metabolism, when Zn is deficient in dicotyledonous species net uptake of NO₃ ^- is particularly depressed which in turn results in an increase in cation-anion uptake ratio and a corresponding decrease in external pH. The ecological relevance of this rhizosphere acidification is discussed.     

Organic acids are not specifically involved in the nitrate-enhanced Zn hyperaccumulation mechanism in Noccaea caerulescens

Nitrogen form has been shown to affect Zn uptake, translocation and storage in the Zn-hyperaccumulating plant Noccaea caerulescens but the biochemical processes are not fully understood. Organic acids and amino acids have been implicated in Zn transport and storage. This study aimed to examine the effect of N form on concentrations of organic acids and amino acids and how these metabolites correlated with Zn hyperaccumulation. Plants were grown in nutrient solution with NO₃⁻, NH₄NO₃ or NH₄⁺, supplied with 50 or 300 μM Zn, and buffered at either pH 4.5 or 6.5. The metabolomic profile was determined by gas chromatography mass spectroscopy. The concentration of Zn in shoots, xylem and roots was greatest for the NO₃⁻, pH 6.5 and 300 μM Zn treatments. For all N forms, the lower growth-medium pH raised xylem sap pH but had no influence on Zn concentration or exudation rate of the xylem sap. Nitrate enhanced organic acid production while NH₄⁺ increased amino acid production. Organic acids in the xylem were more responsive to changes in growth-medium pH than N form, and did not correlate with Zn concentration in shoots, roots or xylem. Serine might be directly involved in Zn hyperaccumulation. Phosphoric acid was associated with reduced Zn accumulation in the shoots. Malic acid was not detected in the shoots but responded to cation uptake more than to Zn specifically in the roots. Citric acid responded to cation uptake more than to Zn specifically in the shoots but did not correlate with Zn concentration in the roots or the xylem sap, or any other cations in the roots. In conclusion, organic acids in N. caerulescens are not specifically involved in Zn hyperaccumulation but are involved in regulating pH in the xylem and cation–anion balance in plants.     

Zinc compartmentation in root, transport into xylem, and absorption into leaf cells in the hyperaccumulating species of Sedum alfredii Hance

Sedum alfredii Hance can accumulate Zn in shoots over 2%. Leaf and stem Zn concentrations of the hyperaccumulating ecotype (HE) were 24- and 28-fold higher, respectively, than those of the nonhyperaccumulating ecotype (NHE), whereas 1.4-fold more Zn was accumulated in the roots of the NHE. Approximately 2.7-fold more Zn was stored in the root vacuoles of the NHE, and thus became unavailable for loading into the xylem and subsequent translocation to shoot. Long-term efflux of absorbed ⁶⁵Zn indicated that ⁶⁵Zn activity was 6.8-fold higher in shoots but 3.7-fold lower in roots of the HE. At lower Zn levels (10 and 100 μM), there were no significant differences in ⁶⁵Zn uptake by leaf sections and intact leaf protoplasts between the two ecotypes except that 1.5-fold more ⁶⁵Zn was accumulated in leaf sections of the HE than in those of the NHE after exposure to 100 μM for 48 h. At 1,000 μM Zn, however, approximately 2.1-fold more Zn was taken up by the HE leaf sections and 1.5-fold more ⁶⁵Zn taken up by the HE protoplasts as compared to the NHE at exposure times >16 h and >10 min, respectively. Treatments with carbonyl cyanide m-chlorophenylhydrazone (CCCP) or ruptured protoplasts strongly inhibited ⁶⁵Zn uptake into leaf protoplasts for both ecotypes. Citric acid and Val concentrations in leaves and stems significantly increased for the HE, but decreased or had minimal changes for the NHE in response to raised Zn levels. These results indicate that altered Zn transport across tonoplast in the root and stimulated Zn uptake in the leaf cells are the major mechanisms involved in the strong Zn hyperaccumulation observed in S. alfredii H.     

Nitrate nutrition enhances zinc hyperaccumulation in Noccaea caerulescens (Prayon)

Nitrate fertilization has been shown to increase Zn hyperaccumulation by Noccaea caerulescens (Prayon) (formerly Thlaspi caerulescens). However, it is unknown whether this increased hyperaccumulation is a direct result of NO₃ ^? nutrition or due to changes in rhizosphere pH as a result of NO₃ ^? uptake. This paper investigated the mechanism of NO₃ ^?-enhanced Zn hyperaccumulation in N. caerulescens by assessing the response of Zn uptake to N form and solution pH. Plants were grown in nutrient solution with 300 μM Zn and supplied with either (NH₄)₂SO₄, NH₄NO₃ or Ca(NO₃)₂. The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO₃ ^?-fed plants than in NH₄ ⁺-fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO₃ ^?, whereas the Zn concentration in the roots was mainly enhanced by pH 6.5. Nitrate increased Zn uptake in the roots at pH 6.5 and increased apoplastic Zn at pH 4.5. Zinc and Ca co-increased and was found co-localized in leaf cells of NO₃ ^?-fed plants. We conclude that NO₃ ^? directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.     

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.     

The roles and interactions of ethylene with gibberellins in the far-red enriched light-mediated growth of <Emphasis Type="Italic">Solanum lycopersicum</Emphasis> seedlings

Wild type (WT) tomato seedlings responded to a low red to far-red (R/FR) ratio with increased stem elongation, similar leaflet area expansion and lower shoot ethylene levels. The levels of endogenous growth-active GA₁ and its immediate precursor GA₂₀ were decreased by low R/FR ratio, whereas the levels of GA₁ catabolite, GA₈, increased. To examine the interaction of ethylene with GAs in regulating tomato shoot growth under low R/FR ratio, transgenic (T) seedlings bearing Le-ACS2 and Le-ACS4 antisense mRNA were utilized. Low R/FR ratio increased stem elongation and decreased ethylene levels in T tomato shoots, as it did in WT shoots. However, T stems were significantly taller than the WT stems under low R/FR ratio. Leaflet areas were significantly larger for T, than WT seedlings under both R/FR ratios. Low R/FR ratio did not decrease endogenous levels of GA₁ and GA₂₀ in T shoots, but did increase GA₈ levels, which were higher than in WT shoots. These results, and hormone/inhibitor application studies, showed that in tomato shoots subjected to low R/FR ratio, GAs play a growth-promotive role in stem elongation, whereas ethylene is growth-inhibitory. Further, these results may imply that decreasing ethylene production under low R/FR ratio causes increases in stem elongation and GA levels.