Copper nutrition of subterranean clover (Trifolium subterraneum L. cv. Seaton Park). III. Effects of phosphorus supply on the relationship between copper concentrations in plant parts and yield

Abstract

The effects of phosphorus supply on the relationship of copper supply with copper concentrations in various plant parts and yield of Seaton Park subterranean clover were examined. Plants were grown in a glasshouse for 40 and 74 days in pots with four levels of potassium phosphate (0, 13, 39, 65 mg phosphorus/pot) and six levels of copper sulfate (0, 50, 100, 200, 400, 800 pg copper/pot) added, in factorial combination, to a sand deficient in both phosphorus and copper. By increasing the phosphorus levels copper deficiency was induced partly by promoting growth and diluting copper concentrations in plants; and also by depressing copper absorption. Increasing phosphorus changed the distribution of copper in plant tops and the shape of curves relating copper concentration in whole plant tops to yield. At 39 mg phosphorus/pot, the relationship at Day 74 had a marked 'Piper-Steenbjerg' curvature, largely as a result of unusually high copper concentrations in the stems plus petioles of severely copper-deficient plants. At 65 mg phosphorus/pot, the relationship had no 'Piper-Steenbjerg' curvature for whole tops and only a relatively small curvature for stems plus petioles. The data suggests that 'Piper-Steenbjerg' curves in subterranean clover result primarily from high concentrations of copper in the stems plus petioles of severely deficient plants. At both harvests, young leaf blades had critical copper concentrations of around 3 ¦g copper/g at both 39 and 65 mg phosphorus/pot. However, copper-deficient plants with severe phosphorus deficiency did not respond to copper, and generally had copper concentrations below this critical level in all plant parts. The results confirm the value of copper analysis of young leaf blades for diagnosing copper deficiency in subterranean clover with moderately deficient to luxury supplies of phosphorus.