Effect of Water Stress on the Reduction of Nitrate and Nitrite by Soybean Nodules

Abstract

The effects of water stress on nitrate reductase and nitrite reductase activities in symbiotic nodules were examined in field-grown soybean plants (Glycine max L Merr. cv Clark). The in vitro assays of enzyme activity indicated that the nodule cytosol and bacteroids contained both nitrate reductase and nitrite reductase activities. The reduction of nitrate in bacteroids increased significantly as nodule water potential declined from -0.6 to -1.4 megapascals, and then decreased when -1.8 megapascals water potential was reached. On the contrary, the reduction of nitrate in nodule cytosol was inhibited as water stress progressed. Increases in water stress intensity also caused a significant inhibition in nitrite reductase activities of bacteroids and nodule cytosol within soybean nodules. The results show that nitrate reduction occurred both in the cytosol and bacteroids of water-stressed soybean nodules. The reduction of nitrate functioned at different physiological modes in these two fractions.Soybean plant can utilize both soil NO3-and atmospheric N2.Although the majority of absorbed N03-was reduced in the aerial portion of bean plant, NO3-reduction was also detectable in symbiotic nodules (1,2,4,8,13 (7), Russell et al. (9), and Ishizawa (4) suggested that the reduction of NO3-in soybean nodules could also be assimilatory, and, thus contributed to the nitrogen economy ofthe bean plant.The NO2-, produced as a result of NO-reduction, is further reduced by NiR2. Daniel and Appleby (2) were unable to detect NiRA in bacteroids of soybean nodules, even though enzyme activity could be found in Rhizobium japonicum grown anaerobically. However, NiR existed in the cytosol fraction of soybean nodules ( 12).It is well documented that the NRA in the leaves of higher plants is very sensitive to changes in the water status ofthe plant. But little attention seems to have been given to the effect ofwater stress on NRA in root nodules. Aparicio-Tejo and Sanchez-Diaz (I) reported that the nodule NRA in Medicago sativa L. was stimulated by water stress. There are no reports on the relationship between nodule NRA and water stress in the soybean plant. The present study reports the effect of water stress on NRA and NiRA using detached soybean nodules.

Allelochemical stress produced by aqueous leachate of Nicotiana plumbaginifolia Viv.

“…This might be the strong reason for declination of NR activity under allelochemicals stress. Another possible reason of reduced NR activity may be the reduced synthesis or induction of enzymes (Chen and Sung 1983). It is correlated with decreased absorption of nitrate by roots due to allelochemicals and transport of nitrate from roots to leaves, consequently decreased foliar nitrate (Abd-El Baki et al 2000).…”

Denitrification in lucerne nodules and bacteroids supplied with nitrate

“…This is somewhat unexpected because antagonistic N-transformations are involved (Zumft 1990). Moreover, bacteroid etizymes of inorganic nitrogen metabolism, mainly NR, are induced not only by nitrate itself but also in nodules subjected to drought (Chen and Sung 1983). With free-living bacteria, Daniel and Gray (1976) showed that NR induction is controlled by oxygen rather than by nitrate.…”

Nitrate and nitrite reduction in the plant fraction of alfalfa root nodules

“…Most of these studies have focused on the enzymes from soybean. It has been reported that the soybean nodule cytosoi possesses sig-nificant activities of NR (Streeter 1982, Cheo and Sung 1983, Stephens and Neyra 1983, Streefer 1985 and NiR (Chen and Sung 1983, Stephens and Neyra 1983, Hunter 1984, Streeter 1985. However, oniy scant information is avaiiable with respect to the nodules' ability to reduce NO, and NO; in other legume species.…”