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Background and aims: Cereals can be made safer and more nutritious by reducing cadmium (Cd) and enhancing zinc (Zn) levels. To respectively regulate the accumulation of these chemically similar elements in grains, it is essential to understand the differences between Cd and Zn allocation to grains. Methods: In durum wheat (Triticum durum), dual-isotope (111Cd and 67Zn) labeling was used to trace the post-anthesis uptake fluxes separately from the remobilization of pre-anthesis vegetative pools. Laser ablation inductively coupled mass spectrometry was used to investigate the spatial distribution of Cd and Zn in the uppermost node. Results: Among the shoot organs, both pre- and post-anthesis derived Cd was more allocated to the high-transpiring organs (i.e., bracts and flag leaves) whereas Zn was more to the grain. Cadmium was likely less efficiently transferred from the xylem to the phloem as suggested by the elemental maps which showed that Cd was more abundant than Zn in the xylem of the uppermost node. Furthermore, unlike Zn, Cd was not significantly remobilized from the high-transpiring organs, which further limited the allocation of Cd to the grain. Conclusion: High-transpiring organs are sources of grain Zn but irreversible sinks of Cd. Agronomic strategies that enhance Cd sequestration and Zn remobilization in high-transpiring organs could contribute to producing grains with low Cd and high Zn concentrations. [ABSTRACT FROM AUTHOR]

Aims: This study investigated the possible source organs delivering several trace elements to seeds (root uptake versus net remobilization), by studying changes in biomass and element contents in the plant organs.Methods: Sunflowers were grown in a greenhouse using a nutrient solution enriched with Cd. Four samplings were performed from the early flowering to the seeds physiological maturity.Results: The low grain Ca indicated that phloem was likely the main route for transporting the elements to seeds. Excluding roots, the mass balance of the elements indicated the following contribution of the net remobilization to the total quantities in seeds at maturity: Mg = 50%, Cd = 14%, Cu = 35%, Fe = 29%, Mn = 19%, Zn = 12%. Source organs were mainly the receptacle and the stem. No significant net remobilization was observed for Ca, K and Mo.Conclusions: The amount of trace elements accumulated in vegetative parts can be redistributed to seeds in an extent that depended on the element. Due to the important contribution of root uptake to the content in seeds at maturity, the availability of elements in soil during the reproductive stages is an important point to consider. [ABSTRACT FROM AUTHOR]

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 (pCd = 11.03). At flower bud and grain filling stages, plants were exposed for three days to Cd and at the same time, subjected or not to fans to increase the transpiration. The partitioning of Cd 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. [ABSTRACT FROM AUTHOR]

Aims: The objective of this study was to rank the mechanisms influencing the phytoavailability of cadmium (Cd) in agricultural soils. Methods: We developed a model that simulates the transport by diffusion and convection, the kinetics of sorption and complexation in solution and the root uptake of Cd. The ranking of mechanisms was performed by simulating the Cd uptake by 1 cm root for 30 days for French agricultural soil characteristics. Results: The initial Cd concentration was the most influential parameter followed by the soil buffer power for Cd and by the soil water content and impedance factor. The Cd was generally strongly depleted at the root surface and the convection was almost negligible. In general, the Cd complex dissociation contributed little to the uptake due to a strong kinetics limitation. Conversely, the kinetics of sorption was little influential. Conclusions: The initial concentration and diffusion of Cd were the dominant processes governing the phytoavailability in non-polluted soils. A model considering only the transport and sorption of Cd without kinetics would be adequate to predict the phytoavailability. The particular situations where these simplifications do not hold (relative error >10 %) corresponded to a high supply of labile Cd complex toward plant roots. [ABSTRACT FROM AUTHOR]

Aims: To rank the contributions of roots and shoots to Cd uptake by the hyperaccumulator Noccaea caerulescens, two hypotheses were tested: Shoots control the uptake through detoxification reactions; alternatively, roots do this through absorption and translocation. Methods: In a first experiment, plants with the same final age were exposed for 1, 2 or 3 weeks to 0.5 μM Cd in hydroponics. In another experiment, either roots or shoots or both were pruned before the plants were exposed for 1 week to Cd at 0.5 μM in the nutrient solution. Results: The first experiment showed that the Cd accumulation was driven by the root absorption combined with xylem loading, keeping the root Cd concentration constant. After root or/and shoot pruning, all the plants accumulated the same total quantity of Cd. After root pruning, the remaining roots increased their Cd absorption influx. Shoot pruning alone did not affect absorption or storage in roots and the pruned shoots accumulated as much Cd as the unpruned ones. Similar results were obtained for all major and minor nutrients. Conclusions: Root absorption and xylem loading are the major processes controlling Cd hyperaccumulation in N. caerulescens, the shoot being a compartment in which the metal exported by the roots is stored. [ABSTRACT FROM AUTHOR]

Aims: In a previous work, we observed a longitudinal decrease in Cd 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 Cd 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 Cd influx. Methods: By using short-term exposures to Cd-labelled solution (5 to 200 nM), we compared the longitudinal variations in Cd roots influx depending on the growth substrate (hydroponics or sand), on the root age and order. Results: In second order laterals, Cd influx decreased from the apex to the root base, as for first order laterals. For sand cultures compared to hydroponics, the mean Cd 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 Cd uptake by a root system should increase with increasing number of root tips and decreasing individual root length. [ABSTRACT FROM AUTHOR]

Aims: Contamination of sunflower ( Helianthus annuus L.) by cadmium (Cd) is a concern for food and feed safety as this species accumulates Cd to a greater extent than other crops. We examined the relationships between root architecture and Cd uptake by roots. Methods: We determined and mathematically modelled the longitudinal variation of Cd influx in first order roots of sunflower grown in hydroponics by using short-term exposure to Cd-labelled solutions (0.8 to 500 nM). Thereafter, by taking into account the longitudinal variation of the influx, we simulated the uptake of Cd for 24 h by cohorts of roots characterised by various architectural characteristics. Results: Cd influx at the root tip was on average 2.9 times that of the basal region close to the taproot. The simulations indicated that the total Cd uptake by root cohorts mainly depends on 1/ the root diameter and the number of roots, 2/ the value of the Cd influx at the basal region 3/ the stronger influx at the root tip. Conclusion: Considering a higher Cd influx at the root tip may be important to understand the relationship between root architecture and Cd uptake by the root system. [ABSTRACT FROM AUTHOR]

This study tested the hypothesis that Cd uptake is correlated with the shoot or root growth of Arabidopsis thaliana ecotype Columbia cultivated hydroponically at environmentally relevant Cd concentrations: 20, 100 and 350 nmol L−1. Growth of both roots and shoots were delayed at 350 nmol L−1. The rate of Cd uptake determined by spiking the nutrient solution with 109Cd for 24 h, was significantly correlated with the root growth rate. The fraction of Cd absorbed that was allocated to shoots was constant with time but decreased with increasing exposure to Cd. Autoradiography and gamma counting showed that Cd was preferentially allocated to developing leaves. Hence, the quantity of Cd in shoots depended both on the root growth, which probably governed the uptake, and on the maturity of the leaves, which may have determined the Cd allocated to shoots through changes in the transpiration stream. [ABSTRACT FROM AUTHOR]

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth.Lolium multiflorumLam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of14C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of14C to roots. In parallel, N fertilisation increased the root soluble14C and the14C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration). [ABSTRACT FROM AUTHOR]

Describes a method to follow rate of {14}CO—{2} and total CO—{2} production from rhizosphere respiration after plant shoots had been pulse-labelled with ∧{14}CO*mdash;{2}. Use of radioactivity detector for flow detection of beta radiation; Counting efficiency of radioactive flow cell; Applications used to trace photoassimilates within the plant-soil system.