The Occurrence of Nitrate Reductase in Leaves of Prunus Species

Abstract

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).

Influence of Epicuticular Waxes on Foliar Absorption of Nitrate Ions by Apricot Leaf Disks

“…to foliar nitrate sprays under field conditions (Leece and Kenworthy 1971) is not caused by an inability of Prunusleaves to metabolize nitrate ions (Leece, Dilley, and Kenworthy 1972). This is analogous to the response ofPrunus spp.…”

“…In an earlier study (Leece, Dilley, and Kenworthy 1972), nitrate uptake via the petiole was measured, using the nitrate reductase assay. In that system, steps (2), (3), and (4) were operating as above, but step (1) became petiole uptake, which was controlled by the rate of leaf transpiration.…”

“…The dishes were incubated in a water-bath at 25°C for 15 hr and at a light intensity (from fluorescent tubes) of 8600 lux at leaf level. Following incubation, the glass cylinders were peeled off the disks, the disks were rinsed with distilled water, blotted dry, then assayed for nitrate reductase as previously reported (Leece, Dilley, and Kenworthy 1972).…”

“…The dishes were incubated in a water-bath at 25°C for 15 hr and at a light intensity (from fluorescent tubes) of 8600 lux at leaf level. Following incubation, the glass cylinders were peeled off the disks, the disks were rinsed with distilled water, blotted dry, then assayed for nitrate reductase as previously reported (Leece, Dilley, and Kenworthy 1972).Where required, disruption and partial removal of epicuticular waxes was achieved either by brushing (10 strokes in one direction with a camel-hair brush), or with 80% (v/v) aqueous acetone (leaf wiped with acetone-treated tissue paper then dried immediately with untreated paper). Chloroform proved unsuitable for wax removal as it produced leaf senescence within 3 hr.…”

Partitioning of nitrate reductase activity in Coffea arabica L. and its relation to carbon assimilation under different irradiance regimes

“…Studies on both nitrate concentration in xylem sap exudates and in vivo and in vitro NRA showed that woody plants reduce their nitrate almost exclusively in the roots (Cruz et al, 1991(Cruz et al, , 1993Gojon et al, 1991;Lee and Titus, 1992;Thomas and Hilker, 2000). However, under high exogenous nitrate availability, NRA has also been detected in the leaves of woody plants (Klepper and Hageman, 1969;Leece et al, 1972;Smirnoff et al, 1984;Lee and Titus, 1992;Olsson and Falkengreen-Grerup, 2003). In addition to environmental factors, nitrate reduction also shows changes during organ and plant development (Andrews, 1986;Kenis et al, 1992).…”