Comparative responses to copper of some tropical and temperate pasture legumes

Abstract

Five tropical and five temperate pasture legumes were grown in pots containing an acutely copper-deficient soil, and in water culture. Growth responses and copper contents of the tops, roots, and seed are reported. In soil culture, the relative dry matter yields of the tops of Desmodium uncinatum (Sacq.) DC., Phaseolus lathyroides L., Centrosema pubescens Benth., Indigofera spicata Forsk., and Stylosanthes bojeri Vog. on the nil treatments were 60, 46, 45, 21, and 7% respectively of their maximum yields obtained with optimum copper additions to the soil. Yields of Trifolfum repens L., Trifolium fragiferum L., Trifolium alexandrinum L., Medicago sativa L., and Medicago tribuloides Benth. on the nil treatments were 38, 31, 3, 17, and 14% respectively of their maxima. Thus Trifolium alexandrinum and Stylosanthes were the most sensitive to copper deficiency, Desmodiurm and Trifolium repens the least sensitive, the remaining species showing intermediate responses. The sensitive species required additions of copper of up to the equivalent of 4 lb copper sulphate per acre to produce maximum growth, while the relatively insensitive species required only the equivalent of ½ lb per acre. Similar results were obtained in the water culture experiment. The copper concentrations in the dry plant material from the various species did not vary significantly at any one level of application. The range of copper concentration over all treatments was 1.7–11.3 p.p.m., but the species did not differ greatly in their copper requirements for normal growth. The relatively sensitive species such as Trifolium alexandrinum and Stylosanthes were much less efficient at extracting copper in both soil and water culture than Desmodium, Trifolium repens, and Trifolium fragiferum. However, on the basis of copper uptake per unit weight of root tissue Stylosanthes had a high value, which was largely controlled by top/root ratios. Copper accumulated more in the roots than in the tops of most species, particularly at high rates of copper application; however, Stylosanthes and Trifolium alexandrinum also had relatively high copper contents in their roots on the low treatments. The seeds of these two species had higher copper contents per unit weight than the other species. It is considered that, for legumes, a copper concentration above 5 p.p.m. in the plant material is satisfactory, while 4–5 p.p.m. is marginal and less than 4 p.p.m. indicates copper deficiency. The occurrence of high copper contents in some plants showing foliar symptoms and yield depression as a result of copper deficiency is discussed.