Manganese status of New Zealand pastures 1. Toxicity in ryegrass, white clover, and lucerne

Abstract

Increasing supplies of manganese (Mn) depressed DM yields of lucerne (Medicago sativa L. cv. Wairau), ryegrass (Lolium perenne L. cv. Grasslands Nui), and white clover (Trifolium repens L. cv. Grasslands Pitau) grown in silica sand, with the roots more affected than the shoots. Lucerne was less tolerant of excess Mn than either white clover or ryegrass. In all instances, the Mn treatments had a greater effect on DM yield at pH 5.5 than at pH 6.5. Mn was readily absorbed by the 3 plant species with the concentration in their tissues reflecting the amounts applied in the nutrient solutions. In general Mn concentrations were higher in the roots than in the shoots and plants grown at pH 5.5 had higher concentrations in both shoots and roots, than those grown at pH 6.5. Apart from Ca, Mn had little or no effect on the concentration of major elements and iron in the shoots and roots of the 3 species. Transport of Ca from the roots to shoots was significantly reduced by excess Mn in lucerne and white clover. Mn also reduced the chlorophyll content of lucerne and white clover but not of ryegrass. From this data critical concentrations of Mn in the shoot which were sufficiently toxic to reduce growth by 10070 were established for the 3 species|the concentrations in mg Mn/kg DM were lucerne 340, white clover 570, and ryegrass 1110.