Enzymic diagnosis of copper deficiency in subterranean clover. I. Relationship of ascorbate oxidase activity in leaves to plant copper status

Abstract

Ascorbate oxidase activity in leaf blades of various ages was examined in relation to copper supply to subterranean clover cv. Seaton Park grown in the glasshouse on a copper-deficient sand. At each copper level, ascorbate oxidase activity per g fresh weight was highest in the youngest leaf blades and declined steeply with age; activity per leaf remained relatively constant with leaf age. On either basis, and in leaf blades of each age category, ascorbate oxidase activity followed copper level. The activity per young folded leaf (YFL) was chosen as the most suitable criterion for a diagnostic test of copper deficiency. Ascorbate oxidase activity per YFL correlated closely with the response to copper of the dry matter yield of plant tops. Activity per YFL also correlated closely with copper concentrations in the youngest open leaf (YOL). These correlations held for Seaton Park subterranean clover when nitrogen was supplied either as NH4NO3 or by symbiotic fixation. Ascorbate oxidase activity in YFL of Mt Barker subterranean clover grown at varying copper levels in the field also correlated closely with YOL copper concentrations in the same plants. Moreover, subterranean clover plants deficient in any one of six other nutrients (nitrogen, sulfur, phosphorus, potassium, manganese, zinc) all had ascorbate oxidase activities above the suggested critical value for adequate copper supply. It is suggested that ascorbate oxidase activity in YFL can be used as a specific and sensitive test for the copper status of subterranean clover, with activities of = 50 nmol O2/min per leaf indicating deficiency, = 100 indicating sufficiency, and 50-100 indicating a marginal copper supply.