Estimation of physiologically active zinc in maize by biochemical assay

Summary The enzymes ribonuclease, aldolase and carbonic anhydrase were evaluated as biochemical assays for physiologically active zinc in maize. Seedlings were cultured for 14 or 30 days on a black-earth soil with factorial combinations of phosphorus and zinc fertilizers so as to produce in the leaves varying levels of active zinc at a constant level of total zinc. Enzyme activity was correlated with plant growth, leaf nutrient composition and the occurrence of visual symptoms of zinc deficiency. Ribonuclease and aldolase activities in leaf tissues were insensitive to the changes in active zinc concentration, though aldolase activity was sensitive to phosphorus deficiency. In contrast, leaf carbonic anhydrase activity correlated well with the onset and correction of zinc deficiency symptoms and, as early as 14 days after emergence, was more sensitive to the deficiency than was plant growth. The sensitivity of carbonic anhydrase to changes in active zinc and its specificity for such changes were further examined in 10 to 30 day-old plants grown in solution culture. Zinc was added to plants which had been cultured without zinc for 16 days. A 2.5-fold increase in carbonic anhydrase activity preceded responses by either zinc concentration or plant growth. We propose the use of leaf carbonic anhydrase as an index of active zinc in maize, particularly to supplement inorganic analysis in the diagnosis of zinc deficiency when much of the zinc in the plants is inactive. The assay is simple, is sensitive to and specific for zinc status, and enables early detection of a deficiency before irreversible biochemical events predispose a large yield reduction.