Acetylene Reduction (Nitrogen Fixation) and Metabolic Activities of Soybean Having Various Leaf and Nodule Water Potentials

Abstract

An apparatus was designed that permitted acetylene reduction (N2 fixation) by root nodules to be ineasured in situ simultaneously with net photosynthesis, dark respiration, and transpiration of the shoot in soybean plants (Glycine max [L.] Merr. var. Beeson). Tests showed that acetylene reduction was linear with time for at least a hours, except for the first 30 to 60 minutes. Endogenous ethylene production did not affect the measurements. Successive determinations of acetylene reduction could be made without apparent aftereffects onl the plant.This apparatus was used to investigate the effects of soil flooding and desiccation on acetylene reduction under conditions where soil, nodule, and leaf water potentials could be measured. No acety-lene reduction was detectable in flooded soil or in soil desiccated to a water potential of -19.5 bars. Between these extremes, acetylene reduction displayed a sharp optimum. Removing the soil eliminated the inhibitory effects of flooding, suggesting that rates of gas exchange were restricted between the nodules and thie atnosphere at soil water potentials above -2 bars.As the soil desiccated further, acetylene reduction decreased, and the decrease was correlated with decreases in photosvnthesis and transpiration. Although dark respiration was inhibited, it was not affected to thie extent that acetylene reduction, photosynthesis, or transpiration were. Consequently, it was concluded that photosynthesis, transpiration, or some direct effect on the nodules other than that caused by respiration were most likely to account for the inhibition of acetylene reduction at soil water potentials below -2 bars. (11,13,22,29,30). Generally, plant moisture status was not determined, but in those cases where soil water potential was measured, one study showed a decrease (22) whereas the other showed an enhancement (13) in acetylene reduction (N2 fixation) at low soil water potentials. Sprent (29) observed a decrease in acetylene reduction and nodule respiration when detached nodules lost 20 % of their fresh weight during drying in air. In intact soybean, acetylene reduction decreased at soil water contents below "field capacity" (30), and Sprent suggested that direct effects of low water potentials on the nodules may account for the decrease in activity for acetylene reduction, probably because of an inhibition of nodule respiration (11,29,30).Nitrogen fixation depends on a number of factors which must be supplied either by the host plant or by the soil. Photosynthates are required in order for nodule respiration to supply the reductant and ATP necessary for the reduction of N2, and for the carbon chains that combine with NH3 before it is exported by the nodules (3, 12). Since the amino acid products of N2 fixation move to the shoot in the transpiration stream (26-28), transpiration may be required, particularly since the accumulation of NH3 in the nodules might cause a repression of the synthesis of nitrogenase, as it does in free-living organisms (10, 25). In addition to water from...

Response to Drought Stress of Nitrogen Fixation (Acetylene Reduction) Rates by Field-Grown Soybeans

“…While some researchers have concluded that this loss of nodule activity is due to an inhibition ofphotosynthesis (8,9,13), Sprent (22) concluded that nitrogen fixation is more sensitive than photosynthesis to drought stress. This is supported by the findings of Bennett and Albrecht (1) who found that nitrogen fixation was closely correlated with nodule water potential which, in turn, was more sensitive to drought stress than was either leaf water potential or diffusive conductance.…”

Effects of Short-Term Waterlogging on Soybean Nodule Nitrogen Fixation at Different Soil Reductions and Temperatures

“…In particular, SPAD value in WR markedly decreased. Waterlogging causes inhibition of not only the nitrogen fi xation of the nodule (Huang et al, 1975;Shimamura et al, 2002) but also the nitrogen absorption by the root system (Sugimoto, 1994), as shown by the decreased SPAD value in the leaf (Bacanamwo and Purcell, 1999a) or the nitrogen contents in the leaf (Rhine et al, 2010). This indicates that the adverse effect of waterlogging was promoted by a lower ORP in the soil, a higher temperature of the soil and a longer duration of waterlogging.…”

“…Nitrogen fixation in the nodule is often evaluated using acetylene reduction activity (ARA). After short-term waterlogging, ARA was restored to the same value as that in the control plant after the waterlogging in some studies (Huang et al, 1975;Huang and LaRue, 1985), but not in other studies (Andreeva et al, 1987;Sung, 1993). Thus, it is uncertain whether ARA is restored after waterlogging.…”

“…However, short-term waterlogging for several days (i.e., less than a week) in farm fi elds, did not lead to the development of adventitious roots or the formation of aerenchyma. Consequently nodule nitrogen fi xation is almost stopped due to the severe restriction of gas exchange between the soil and atmosphere (Huang et al, 1975;Shimamura et al, 2002Thomas et al, 2005. Therefore, the recovery of nitrogen fixation from the reduction in waterlogging after the removal of excess water is important for the acquisition of nitrogen in soybean under short-term waterlogging stress.…”

“…Concerning the effect of waterlogging lasting less than a week, there have been reports on stomatal conductance (SC) (Kozlowski, 1984;Ahmed et al, 2002), photosynthesis (Huang et al, 1975;Oosterhuis et al, 1990;Sugimoto, 1994) and nitrogen fixation of the nodule (Andreeva et al, 1987;Huang et al, 1975;Huang and LaRue, 1985;Sugimoto, 1994;Sung, 1993). Nitrogen fixation in the nodule is often evaluated using acetylene reduction activity (ARA).…”