Nitrite Reduction in The Roots And Leaves of Pisum Sativum

Abstract

The location of the nitrate/nitrite reducing mechanisms in plants is distributed between the root and the shoot in proportions which vary according to species. At one extreme, nitrate may be reduced almost entirely in the shoot, e.g. Beta and Chenopodium, while at the other extreme most reduction occurs in the root system as in some woody species. In the majority of species, however, including many important crop plants, both root and shoot participate in reduction. This subject has been discussed in some detail (Pate 1973). The mechanisms of function of both nitrate and nitrite reduction are better understood in leaves than roots. Root nitrogen assimilation has been reviewed by Oaks and Hirel (1985). Shortly after methods became available for the extraction and assay of nitrite reductase (Hageman et al. 1962), the enzyme was shown to be active in tomato roots (Sanderson and Cocking 1964). A few years previously an interesting study by Butt and Beevers (1961), approaching the subject from the direction of carbon-nitrogen relations had shown that glucose-6-phosphate could increase the rate of disappearance of nitrite in contact with preparations from maize roots. The association of both nitrite reductase and the enzymes of the pentosephosphate pathway (PPP) with root plastids, has been developed in a number of studies in the intervening years (see, e.g. Emes and Fowler 1979 a,b). The present paper describes a dual approach to the subject of nitrite reduction by roots, in which studies were made of the purified nitrite reductase (NIR) enzyme and of carbohydratenitrite relationships as found in root plastids. Pea was chosen for this work as a species in which nitrate was thought to be reduced in roots, and from which well authenticated root plastids can be extracted. A large number of labelling techniques was used in this study. Details of these may be found in Bowsher et al. (1988,1989).