The Use of Nitrate Reductase Activity for the Appraisal of Nitrogen Status and Productivity of Grapefruit Orchard Trees

Abstract

Abstract Mature Marsh seedless grapefruit trees on Sour orange stock planted in a loamy soil were subjected to three levels of nitrogen fertilization with and without phosphate or chicken manure added. The details of the experimental plan and tree responses during the five-year experimental period will be described elsewhere. Briefly, the phosphate and manure treatments resulted in a significant increase in yield and in improved fruit quality. As far as the leaf composition is concerned, the phosphate and manure treatments caused an increase in the P and a decrease in the N content of the leaves, thus producing a completely inverse relationship between leaf N concentration and the yield of the corresponding trees. This situation served as an opportunity to test—under orchard. conditions—the validity of NaR (nitrate reductase) assay in leaves as a measure for some “active fraction” of leaf nitrogen and as a parameter for tree productivity. The use of the NaR assay has been suggested for the determination of N requirements of citrus trees (1).