The low‐molecular‐weight organic acids (LMWOAs) secreted by plant roots modify the mobility of Cd through formation of soluble complexes in the soil rhizosphere; the kinetics of Cd release as influenced by organic acids and its impact on Cd bioaccumulation remains to be established. The influence of LMWOAs (10−3 and 10−2 M), viz., acetic, citric, oxalic, fumaric, and succinic acids, which are present in the soil rhizosphere, on the kinetics of Cd release from selected soils (Luseland, Waitville, and Jedbergh) of Saskatchewan, Canada, was investigated at 25°C and at an ionic strength of 0.1 M NaNO3. It was observed that the Cd release from the soils increased initially up to a reaction period of about 2 h and then slowly decreased with time. With time, the LMWOAs were apparently microbially degraded and the soil solution Cd2+, which was transformed from the Cd‐LMWOA complexes by microbial degradation, was adsorbed onto the negatively charged soil particles. Increasing amounts of Cd were released from the soils to the soil solution with the renewal of LMWOAs after every 2 h. The short‐term (reaction period of 0.25 to 1 h) kinetics of Cd release from the soils was described by a parabolic diffusion kinetic model. The overall diffusion coefficient of the Cd release from the soil and the Cd released by the renewal of LMWOAs followed the same trend as the Cd availability index of the soils: Luseland >> Waitville > Jedbergh. Thus, the results presented in this investigation indicate the importance of the kinetics of the Cd release from the soils by the LMWOAs excreted by root exudates in understanding the availability of soil Cd.

Accumulation of cadmium (Cd) in crop plants is of great concern due to the potential for food chain contamination through the soil-root interface. Although Cd uptake varies considerably with plant species, the processes which determine the accumulation of Cd in plant tissues are affected by soil factors. The influence of soil type on Cd uptake by durum wheat (Triticum turgidum vat. durum L.) and flax (Linum usitatissimum L.) was studied in a pot experiment under environmentally controlled growth chamber conditions. Four cultivars/lines of durum wheat (Kyle, Sceptre, DT 627,and DT 637) and three cultivars/lines of flax (Flanders, AC Emerson, and YSED 2) were grown in two Saskatchewan soils: an Orthic Gray Luvisol (low background Cd concentration; total/ABDTPA extractable Cd: 0.12/0.03 mg kg -l, respectively) and a Dark Brown Chernozem (relatively high background Cd concentration; total/ABDTPA Cd: 0.34/0.17 mg kg-~ respectively). Plant roots, stems, newly developed heads, and grain/seeds were analyzed for Cd concentration at three stages of plant growth: two and seven weeks after germination, and at plant maturity. The results showed that Cd bioaccumulation and distribution within the plants were strongly affected by both soil type and plant cultivar/line. The Cd concentration in roots leaves and stems varied at different stages of plant growth. However, all cultivars of both plant species grown in the Chernozemic soil accumulated more Cd in grain/seeds than plants grown in the Orthic Gray Luvisol soil. The different Cd accumulation pattern also corresponded to the levels of ABDTPA extractable and metal-organic complex bound soil Cd found in both soils. Large differences were found in grain Cd among the durum wheat cultivars grown in the same soil type, suggesting the importance of rhizosphere processes in Cd bioaccumulation and/or Cd transport processes within the plant. Distribution of Cd in parts of mature plants showed that durum grain contained up to 21 and 36% of the total amount of Cd taken up by the plants for the Orthic Gray Luvisol and Chernozemic soils, respectively. These results indicate the importance of studying Cd speciation, bioaccumulation and cycling in the environment for the management of agricultural soils and crops.

Many high-pressure liquid chromatographic methods used for the determination of low molecular weight organic acids in root exudates lack specificity and cannot completely separate mixtures of mono-, poly-, and hydroxycarboxylic acids simultaneously. Hence, a new gas chromatographic (GC) method that is specific for the identification and quantitation of low molecular weight dicarboxylic acids as methyl esters was developed. Samples collected from root exudates of selected durum wheat and flax cultivare grown in sterile hydroponic solutions were extracted and concentrated by anion exchange membranes. Samples were then subjected to methylation in acidified methanol at elevated temperatures and injected into a GC. The five acids investigated in this study-oxalic, malonic, succinic, fumaric, and maleic-required 7 min for elution from a GC column and were quantified using methylmalonic acid as an internal standard. Good recovery and reproducibility as well as a low detection limit of ca. 1 ppm acid in the membrane eluate make this method very suitable for dicarboxylic acid determination in root exudates.

Three strawberry (Fragaria × ananassa Duch.) cultivars Rainier, Totem and Selva were grown under greenhouse conditions in a Parkhill sandy loam soil with a background DTPA-extractable Cd concentration of 0.18 mg kg -1 and a pH of 5.

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