Longitudinal variation in cadmium influx in sunflower (Helianthus annuus L.) roots as depending on the growth substrate,root age and root order

Aims

In a previous work, we observed a longitudinal decrease in Cd²⁺ influx starting from the root tip in first order lateral roots of sunflower (Helianthus annuus L.) grown in hydroponics. This variable influx was expected to impact the total Cd²⁺ uptake depending on the root system architecture and on how steep was the decrease of the influx. Here, we examined the influence of the culture substrate, of the age and order of lateral roots on the longitudinal variation of Cd²⁺ influx.

Methods

By using short-term exposures to ¹⁰⁹Cd-labelled solution (5 to 200 nM), we compared the longitudinal variations in Cd²⁺ roots influx depending on the growth substrate (hydroponics or sand), on the root age and order.

Results

In second order laterals, Cd²⁺ influx decreased from the apex to the root base, as for first order laterals. For sand cultures compared to hydroponics, the mean Cd²⁺ influx was lower and decreased more steeply with the distance from the apex. The influx also decreased with increasing root age and order, markedly in hydroponics but less for sand cultures.

Conclusion

Results suggested that for a given root surface area, the Cd²⁺ uptake by a root system should increase with increasing number of root tips and decreasing individual root length.     

Cadmium uptake kinetics and plants factors of shoot Cd concentration

Background and aims

Accumulation of Cd in the shoots of plants grown on Cd contaminated soils shows considerable variation. A previous preliminary experiment established that one major reason for this variation was the rate of Cd influx into the roots (mol Cd cm^?2 root s^?1). However, this experiment did not distinguish between solubilization of soil Cd on the one hand and difference in Cd uptake kinetics on the other. The main objectives of the present study were thus to characterize Cd uptake kinetics of plants continuously exposed to Cd concentrations similar to those encountered in soils. Furthermore we determined the factors responsible for differences in shoot Cd concentration such as net Cd influx, root area-shoot dry weight ratio, shoot growth rate and proportion of Cd translocated to the shoot.

Materials and methods

Maize, sunflower, flax and spinach were grown in nutrient solution with five constant Cd concentrations varying from 0 to 1.0 μmol?L^?1. Root and shoot parameters as well as Cd uptake were determined at two harvest dates and from these data Cd net influx and shoot growth rates were calculated.

Results and conclusions

Cadmium uptake kinetics, i.e. the net Cd influx vs. Cd solution concentration followed a straight line. Its slope is the root absorbing power, α, $ \left( {\alpha ={{{\mathrm{Cd}\;\mathrm{net}\;\mathrm{influx}}} \left/ {{\mathrm{Cd}\;\mathrm{solution}\;\mathrm{concentration}}} \right.}} \right) $ . The α values of spinach and flax were about double that of maize and sunflower (5?×?10^?6?cm?s^?1 vs. 2.5?×?10^?6?cm?s^?1). Spinach and flax had a 3–5 times higher shoot Cd concentration than maize and sunflower. The difference in shoot Cd concentration was partly due to the higher Cd influx but also to a higher translocation of Cd from root to shoot and also to a slower shoot growth rate.     

Mechanistic studies of perfluorooctane sulfonate, perfluorooctanoic acid uptake by maize (Zea mays L. cv. TY2)

Aims

There is a need to predict trace metal concentration in plant organs at given development stages. The aim of this work was to describe the Cd hyperaccumulation kinetics in the different plant organs, throughout the complete cultivation cycle, independently of a possible soil effect.

Methods

Plants of Noccaea caerulescens were exposed in aeroponics to three constantly low Cd concentrations and harvested at 6 to 11 dates, until siliquae formation.

Results

Dry matter allocation between roots and shoots was constant over time and exposure concentrations, as well as Cd allocation. However 86 % of the Cd taken up was allocated to the shoots. Senescent rosette leaves showed similar Cd concentrations to the living ones, suggesting no redistribution from old to young organs. The Cd root influx was proportional to the exposure concentration and constant over time, indicating that plant development had no effect on this. The bio-concentration factor (BCF), i.e. [Cd]_plant/[Cd²⁺]_solution for the whole plant, roots or shoots was independent of the exposure concentration and of the plant stage.

Conclusions

Cadmium uptake in a given plant part could therefore be predicted at any plant stage by multiplying the plant part dry matter by the corresponding BCF and the Cd²⁺ concentration in the exposure solution.     

Contribution of Cd-EDTA complexes to cadmium uptake by maize: a modelling approach

Background and aims

Chelant-enhanced phytoextraction has given variable and often unexplained experimental results. This work was carried out to better understand the mechanisms of Cd plant uptake in the presence of EDTA and to evaluate the contributions of Cd-EDTA complexes to the uptake.

Method

A 1-D mechanistic model was implemented, which described the free Cd²⁺ root absorption, the dissociation and the direct absorption of the Cd-EDTA complexes. It was used to explain Cd uptake by maize in hydroponics and in soil.

Results

In hydroponics, the addition of EDTA caused a decrease in Cd uptake by maize, particularly when the ratio of total EDTA ([EDTA] _T ) to total Cd ([Cd] _T ) was greater than 1. At [Cd] _T = 1 μM, when [EDTA] _T /[Cd] _T < 1, the model indicated that Cd uptake was predominantly due to the absorption of free Cd²⁺, whose pool was replenished by the dissociation of Cd-EDTA. When [EDTA] _T /[Cd] _T > 1, the low Cd uptake was mostly due to Cd-EDTA absorption. In soil spiked with 5 mg Cd kg^?1, Cd uptake was not affected by the various EDTA additions, because of the buffering capacity of the soil solid phase.

Conclusions

Addition of EDTA to soil increases Cd solubility but dissociation of Cd-EDTA limits the availability of the free Cd²⁺ at the root surface, which finally reduces the plant uptake of the metal.     

Temporal variability of solution Cd2+ concentration in metal-contaminated soils as affected by soil temperature: consequences on lettuce (Lactuca sativa L.) exposure

The aim of this study was to assess how the solubility and the speciation of Cd in soil solution were affected over time by the soil temperature for three metal-contaminated soils. The changes of solution Cd concentration (either total or free ionic) and other physico-chemical parameters (e.g. pH, ionic strength, the concentrations of ${\text{NO}}_3^ - $ , ${\text{SO}}_4^{2 - } $ , Ca, Mg and dissolved organic carbon) were monitored over a 28-day culture of lettuce (Lactuca sativa L.) in soils incubated at 10°C, 20°C or 30°C. The major result of this study was that Cd²⁺ concentration greatly varied over time in soil solution. The Cd²⁺ concentration declined over time in soil solution as did the concentration of cations that may compete for adsorption (Ca²⁺, Mg²⁺). The rise in soil temperature primarily impacted on the concentration of Cd²⁺ via promoting the microbial C-degradation and, thus, the complexation of Cd in soil solution. The integration of the temporal variations in Cd²⁺ concentration through the calculation of the root exposure to solution Cd (E _Cd) provided a fairly close and robust prediction of Cd concentration in lettuce roots. The present work thus provided new insights on the fate of Cd in contaminated soils that may be relevant for predicting the root uptake of Cd.     

Elevated CO2 improves root growth and cadmium accumulation in the hyperaccumulator Sedum alfredii

Aims

This study examined the effect of elevated CO₂ on plant growth, root morphology and Cd accumulation in S. alfredii, and assessed the possibility of using elevated CO₂ as fertilizer to enhance phytoremediation efficiency of Cd-contaminated soil by S. alfredii.

Methods

Both soil pot culture and hydroponic experiments were carried out to characterize plant biomass, root morphological parameters, and cadmium uptake in S. alfredii grown under ambient (350 μL L^?1) or elevated (800 μL L^?1) CO₂.

Results

Elevated CO₂ prompted the growth of S. alfredii, shoot and root biomass were increased by 24.6–36.7% and 35.0–52.1%, respectively, as compared with plants grown in ambient CO₂. After 10 days growth in medium containing 50 μM Cd under elevated CO₂, the development of lateral roots and root hairs were stimulated, additionally, root length, surface area, root volume and tip number were increased significantly, especially for the finest diameter roots. The total Cd uptake per pot was significantly greater under elevated CO₂ than under ambient CO₂. After 60 d growth, Cd phytoextraction efficiency was increased significantly in the elevated CO₂ treatment.

Conclusions

Results suggested that the use of elevated CO₂ may be a useful way to improve phytoremediation efficiency of Cd-contaminated soil by S. alfredii.     

Root morphology and cadmium uptake kinetics of the cadmium-sensitive rice mutant

To understand the physiological mechanism that confers Cd sensitivity, root morphology and Cd uptake kinetics of the Cd-sensitive mutant and wild type rice were investigated. The root length, root surface area, and root number of mutant rice decreased more significantly with increasing Cd concentration in growth media compared with the wild type rice. The uptake kinetics for ¹⁰⁹Cd²⁺ in roots of both the mutant and wild type rice were characterized by a rapid linear phase during the first 6 h and a slower linear phase during the subsequent period. Concentration-dependent Cd²⁺ influx in both species could be characterized by the Michaelis-Menten equation, with similar apparent K_m values for mutant and wild type rice (2.54 and 2.37 μM, respectively). However, the V_max for Cd²⁺ influx in mutant root cells was nearly 2-fold higher than that for wild type rice, indicating that enhanced absorption into the root is one of the mechanisms involved in Cd sensitivity in mutant rice.     

Short-term partitioning of Cd recently taken up between sunflowers organs (<Emphasis Type="Italic">Helianthus annuus</Emphasis>) at flowering and grain filling stages: effect of plant transpiration and allometry

Aims

This work concentrated on understanding the allocation of Cd recently taken up between the organs of sunflower at early and middle reproductive growth stages. The roles of transpiration and allometry were investigated.

Methods

Sunflowers were grown hydroponically in greenhouse, being exposed to low concentrations of Cd (pCd²⁺ = 11.03). At flower bud and grain filling stages, plants were exposed for three days to ¹¹¹Cd and at the same time, subjected or not to fans to increase the transpiration. The partitioning of ¹¹¹Cd between plant organs measured by high resolution ICP-MS was then modelled.

Results

Although the use of fans increased the plant water uptake and transpiration by about 20%, there were no significant effects on the partitioning of recent Cd. Most of the recent Cd was recovered in roots (60%) and only 2.8% were found in seeds (0.8% for the husk and 2.0% for the almonds). The sequestration of recent Cd in a plant organ was successfully explained by its biomass and except for leaves, by the biomass of other organs acting as competitive sinks.

Conclusions

This work proposes a modelling approach for the partitioning of the labelled Cd between plant organs in sunflower.

     

Identification of hot pepper cultivars containing low Cd levels after growing on contaminated soil: uptake and redistribution to the edible plant parts

Aims

This study aimed to screen and identify low-cadmium (Cd) hot pepper (Capsicum annuum L.) cultivars and to clarify the mechanisms of low Cd accumulation in fruits.

Methods

A pot experiment was conducted to investigate the variations of fruit Cd concentration among 30 hot pepper cultivars and to determine the differences in uptake and translocation of Cd between low- and high-Cd cultivars in the control and two Cd treatments.

Results

There are significant differences among the cultivars in their ability to accumulate Cd in fruits. Fruit Cd concentrations are positively and significantly correlated with the translocation of Cd from roots to aboveground parts and the Cd concentrations of leaves and stems. However, no correlation was observed between the fruit’s Cd concentration and the root’s Cd uptake ability.

Conclusions

Two hot pepper cultivars, Yeshengchaotianjiao (No. 16) and Heilameixiaojianjiao (No. 23), were identified as low-Cd cultivars, and two, Jinfuzaohuangjiao (No. 13) and Shuduhong (No. 18), were treated as high-Cd cultivars. The difference in fruit Cd concentrations between low- and high-Cd cultivars is attributable to the difference in Cd translocation from roots to aboveground parts and from leaves and stems to fruits, rather than to the root’s Cd uptake ability.     

Role of roots in cadmium accumulation of two water spinach cultivars: reciprocal grafting and histochemical experiments

Background and Aims

Cultivars of water spinach (Ipomoea aquatica Forsk.) differ widely in their shoot cadmium (Cd) concentration. Previously, we suggested that low-Cd cultivars are better able to retain Cd in their roots and thus prevent root-to-shoot Cd translocation. In this study, we explored the roles of roots and shoots in Cd accumulation in a high-Cd (T308) and low-Cd cultivar (QLQ).

Methods

We used reciprocal grafting to determine the importance of roots and shoots in Cd accumulation, and a dithizone histochemical method to investigate Cd distribution in the roots.

Results

The T308 scion with QLQ rootstock accumulated less Cd than the shoot of non-grafted T308. The QLQ scion with T308 rootstock showed a significantly higher Cd concentration than that in the shoot of non-grafted QLQ. Cadmium induced thicker phellem formation in the main roots of QLQ than in those of T308 and only QLQ showed thickening of the outer cortex cell walls in lateral roots.

Conclusions

Shoot Cd accumulation was primarily determined by root-to-shoot Cd translocation, not root Cd uptake. The thicker phellem and outer cortex cell walls in QLQ than in T308 may be one reason why QLQ roots were able to retain more Cd, and thus reducing Cd translocation to shoots.     

Spatial variation in root system activity of tomato (Solanum lycopersicum L.) in response to short and long-term waterlogging as determined by 15N uptake

Background and aim

Root system activity is affected by abiotic stresses, which often creates spatial differences in root conditions. This is expected to influence plants ability to cope with suboptimal conditions.

Methods

Changes in root system activity were determined as ¹⁵N root uptake in top and bottom layers of potted tomato plants (Solanum lycopersicum L.), after waterlogging the bottom layer for 24 h or 5 d. The plants were grown in peat-based media; non-compacted or highly-compacted, resulting in differences in gas diffusion, air permeability and oxygen availability.

Results

The roots were affected by short-term waterlogging (24 h) by decreasing uptake in the bottom layer and increasing uptake in the pot top layer. Long-term waterlogging (5 d) decreased the ¹⁵N root uptake more in both layers. Root uptake recovered fast (within 6 h) after short-term waterlogging, whereas recovery of long-term waterlogged roots took more than 24 h, suggesting production of new root biomass. Despite affecting physical properties, medium compaction did not affect root uptake. Aboveground biomass was affected by waterlogging by increasing the dry matter percentage, decreasing nitrogen (N) percentage and increasing starch content.

Conclusions

The results confirmed that root uptake in different layers of small pots could be distinguished by the ¹⁵N technique, which was applicable under potentially denitrifying conditions. The results demonstrated that during short-term stress in part of the root system plants increased uptake from the non-affected parts of the root system, probably as compensation for suboptimal conditions.     

Silicon modifies root anatomy, and uptake and subcellular distribution of cadmium in young maize plants

Background and Aims

Silicon (Si) has been shown to ameliorate the negative influence of cadmium (Cd) on plant growth and development. However, the mechanism of this phenomenon is not fully understood. Here we describe the effect of Si on growth, and uptake and subcellular distribution of Cd in maize plants in relation to the development of root tissues.

Methods

Young maize plants (Zea mays) were cultivated for 10 d hydroponically with 5 or 50 µm Cd and/or 5 mm Si. Growth parameters and the concentrations of Cd and Si were determined in root and shoot by atomic absorption spectrometry or inductively coupled plasma mass spectroscopy. The development of apoplasmic barriers (Casparian bands and suberin lamellae) and vascular tissues in roots were analysed, and the influence of Si on apoplasmic and symplasmic distribution of ¹⁰⁹Cd applied at 34 nm was investigated between root and shoot.

Key Results

Si stimulated the growth of young maize plants exposed to Cd and influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root. Si did not affect the distribution of apoplasmic and symplasmic Cd in maize roots, but considerably decreased symplasmic and increased apoplasmic concentration of Cd in maize shoots.

Conclusions

Differences in Cd uptake of roots and shoots are probably related to the development of apoplasmic barriers and maturation of vascular tissues in roots. Alleviation of Cd toxicity by Si might be attributed to enhanced binding of Cd to the apoplasmic fraction in maize shoots.     

Impact of active transport and transpiration on nickel and cadmium accumulation in the leaves of the Ni-hyperaccumulator Leptoplax emarginata: a biophysical approach

Aims

The primary aim of this study was to investigate the impact of active nickel and cadmium transport, transpiration and shoot biomass production on Ni and Cd accumulation in the leaves of the Ni-hyperaccumulator Leptoplax emarginata. A secondary objective was to observe the effects of various concentrations of nickel and cadmium in solutions on the plant growth and ecophysiological characteristics of these plants. Finally, the study sought to identify possible nickel and cadmium concentration gradients in solution as a function of the root distance.

Methods

The Intact Plant Transpiration Stream Concentration Factor (TSCF=xylem/solution solute concentration ratio) was determined for both Ni and Cd and for the selected intact transpiring Ni-hyperaccumulator Leptoplax emarginata, cultivated on two contrasting fertilized and Ni-Cd-contaminated sandy porous media (rhizotrons with central root compartments, linked to Mariotte tubes operated at ?1?kPa). IPTSCF_Ni and IPTSCF_Cd were calculated as the ratios between the hyperaccumulator plant’s nickel or cadmium mass in the leaves and the nickel or cadmium concentration in solution by the volume of water transpired during the period of culture. Plant growth characteristics and gas exchanges were also recorded.

Results

IPTSCF values were much greater than 1 (IPTSCF_Ni?=?5.2?±?0.9 and IPTSCF_Cd?=?4.4?±?0.6) whatever the amount of available Ni and Cd. This characterized a predominantly active plant metal uptake. Moreover, biological regulation was reported: plant growth and transpiration were significantly lower for hyperaccumulator plants cultivated in sand which was rich in available Ni and Cd, than for hyperaccumulator plants cultivated in topsoil, poor in available Ni and Cd. In the soil rhizosphere, capillary flow was related to transpiration and a depletion pattern was developed for Ni and sometimes for Cd.

Conclusions

Overall, the Intact Plant Transpiration Stream Concentration Factor appeared to be a relevant metal bioconcentration factor taking into account the predominant type of metal transport from roots to leaves, plant growth and transpiration coupling and metal availability. IPTSCF_Ni and IPTSCF_Cd values were much greater than 1 and similar whatever the amount of available Ni and Cd. This characterized a predominantly active plant combining Ni and Cd uptake and biological regulations dependent of the Ni and Cd concentrations in solution.     

Isotopic fractionation of Zn in tomato plants suggests the role of root exudates on Zn uptake

Aims

Phytosiderophore-chelated Zn can be absorbed in grasses. Root exudates of dicotyledonous plants can mobilize soil Zn but it is unclear how this affects Zn bioavailability. Stable Zn isotope shifts can indicate exudate-facilitated Zn uptake, since complexation of Zn²⁺ by organic ligands in solution yields a small, but detectable, enrichment of the heavy Zn isotope due to thermodynamic fractionation.

Methods

Tomato seedlings were grown in resin-buffered nutrient solution in which free Zn²⁺ concentrations are buffered, in a factorial design of two Zn levels and two solution volumes. The latter factor allowed altering the exudate concentrations in the solution. Dissolved Cu concentrations in the resin buffered system were used as a sensitive index of metal mobilization resulting from root activity. In addition, seedlings were grown in Zn deficient soil with and without Zn addition.

Results

The dissolved Cu concentration increased with Zn deficiency and was highest at the lowest solution volume, suggesting metal mobilization by root exudates. At low Zn supply, Zn in the plant was enriched in heavy Zn (⁶⁶Zn) and this was most pronounced at small solution volume. Similarly, Zn deficiency in soil enriched tomato shoot Zn with heavy isotope in this plant.

Interpretation

Zinc deficiency increases the contribution of Zn-exudate complexes, which are enriched in the heavy isotope compared to the free ion, to Zn uptake by transporting Zn from the bulk solution or soil to the roots where they likely dissociate and release Zn²⁺.     

Comparison of cadmium subcellular distribution in different organs of two water spinach (Ipomoea aquatica Forsk.) cultivars

Aims

Mechanisms of low cadmium (Cd) accumulations in cultivars of water spinach are poorly investigated. We aimed to improve understanding of the subcellular biochemical properties of the mechanisms involved.

Methods

A pot experiment was conducted to investigate the subcellular distributions of Cd in lateral and main roots, stems, and young and old leaves of a high-Cd (T308) and a low-Cd cultivar (QLQ).

Results

The ratio of main root:lateral roots Cd concentration in QLQ was lower (0.34–0.35) than that in T308 (0.39–0.55). The ratio of stem:main root Cd concentration in QLQ was much lower (0.60–0.73) than that in T308 (1.19–1.58). QLQ has higher capacity to sequester Cd in cell wall fractions of main and lateral roots than T308.

Conclusions

The difference in shoot Cd concentration between QLQ and T308 is attributable to the difference in Cd translocation from lateral to main roots and from roots to the stem. Fixation of large amounts of Cd in old leaves is beneficial to protect young leaves from Cd toxicity. Cadmium immobilization by the cell wall is important in Cd detoxification, especially in main and lateral roots of QLQ and the shoot of T308.