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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. 395Plant and Soil 63, 395-406 (1981).

Nitrate reductase was found in leaves of apricot Prunus armeniaca, sour cherry P. cerasus, sweet cherry P. avium, and plum P. domestica, but not in peach P. persica, from Grasmanis and Nicholas (8) extracted an active nitrate reductase from apple roots, but they were unable to determine its physiological electron donor. Then Klepper and Hageman (11), employing insoluble PVPW in the extraction medium, obtained a typical NADH-dependent nitrate reductase from apple roots. Adding excess nitrate ions to the root medium of apple seedlings, they also found nitrate ions and both nitrite and nitrate reductases in the leaves; the enzyme was readily induced by immersing excised leaves in 0.1 M KNO3. Trace amounts of nitrate and nitrate reductase were found in leaves of mature apple trees receiving low annual amounts of nitrogen.The situation in other plant species which normally reduce nitrate in their roots appears to be similar to that found in apple trees. Wallace and Pate (19) found that in the field pea all nitrate was metabolized in the roots and only reduced nitrogen was found in exuded sap, provided the supply of nitrate to the roots was normal. However, when plants were supplied with nitrate levels higher than 10 mg/ml, nitrate was transported to the shoot and nitrate reductase was induced. The present study was undertaken to determine if nitrate reductase would be induced in leaves of Prunus species when supplied with nitrate ions.Recent commercial attempts to supply part of the nitrogen requirement of stone fruit trees as nitrate foliar sprays (3) has raised questions regarding nitrate metabolism by the leaves of Prunus species. In deciduous fruit trees, the site of nitrate reduction classically is regarded to be the fine roots (14). Early evidence (5, 18) indicated that in apple trees reduction of nitrate and synthesis of amino acids took place mainly in the roots; however, nitrate ions were translocated, unreduced, to the buds during bud burst or to the leaves when excess nitrate was applied to the roots. The literature does not exclude reduction in the leaf in special circumstances. Eckerson (5) reported nitrate reductase activity in apple tree roots, buds, and leaves. However, as Beevers and Hageman (2) have pointed out, nonenzymatic conversion of nitrate to nitrite may have been responsible, because even boiled plant extracts effected the conversion.More recently Bollard (4) failed to find nitrate in tracheal sap extracted from one-year-old shoots of mature stone and pome fruit trees. The absence of nitrate in apple and peach scion tissues has been confirmed in mature trees (17).

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