Differential tolerance of canola to arsenic when grown hydroponically or in soil

Separate studies found canola tolerant to arsenic (As) when grown in hydroponic solutions and generally sensitive to As when grown in soil. Fourteen-day-old canola was transferred to pots containing either soil or nutrient solution and then grown for an additional 14 days in a growth chamber at different times for the two different media. Plants were grown in 0.25-strength Hoagland's solution containing either 0, 0.27, 6.67, or 13.3 micromolar As or in three soils with As added at rates of 0, 5, and 10 mg.kg-1. Soil-solution As concentrations were determined via column displacement and were the same or less (0.147 to 4.27 micromolar) than the hydroponic As concentrations. Soil-solution phosphorus (P) concentrations were determined in the same manner and averaged 9.28 micromolar P compared to 500 micromolar P from the hydroponic solutions. Chlorosis, wilting, and stunted growth-symptoms of As toxicity--occurred in canola at the highest As rate in two of the three soils used. Dry-matter yields from the third soil were low from all treatments and a lack of response to As additions was probably due to injury from other soil-related factors. Shoot As concentrations were generally similar from both experiments. Canola did not suffer a yield loss or exhibit As toxicity symptoms when grown in the hydroponic solutions. Leaf P was 8,000 mg.kg-1 in the hydroponically-grown canola and 100 mg.kg-1 for the soil-grown canola. These values are greater than (hydroponic) and lower than (soil media) sufficiency levels for plants similar to canola. High solution P concentrations in the hydroponic solution may have detoxified As by competing with As for uptake in the solution and during metabolism. Phosphate should be added to hydroponic solutions in As studies at levels close to normal soil P levels and added daily to replenish plant uptake.