Nitrogenase Activity and Nodule Gas Permeability Response to Rhizospheric NH₃ in Soybean

Abstract

This study was conducted on soybean (Glycine max L. Merr.) nodules to determine if exogenous NH3 exerts a controlling influence over nitrogenase activity through changes in nodule gas permeability (P), and if decreasing carbohydrate availability, as a result of low-light treatment, increases the sensitivity of root nodules to NH3. Nodulated root systems of intact plants were exposed to one of several NH3 concentrations ranging from 0 to 821 microliters per liter for an 8-hour period. Treatments were conducted under high-light (2300 micromoles per square meter per second) or low-light (800 micromoles per square meter per second) conditions. Increasing the NH3 concentration and length of exposure of NH3 caused a progressive decline in acetylene reduction activity (ARA). There was generally a greater reduction in ARA under the low-light treatment compared to the high-light treatment at a particular NH3 concentration. The NH3 concentration necessary to decrease P was greater than that needed to decrease ARA, and there was no evidence of a causal relationship between P and ARA in response to NH3.The inability to increase nitrogenase activity in legumes on a nodule weight basis through increased photosynthate availability (14,19) has led researchers to explore other potential limitations to N2 fixation. Recent work has indicated that N2 fixation rates are closely linked quantitatively to (P') for O2 diffusion (7,16). Despite the observations of P adjustments in response to the environment (7,10,12,16), there is a virtual void in understanding how P changes are achieved.Since NH3 is the initial product of N2 fixation, environmental factors which alter P and N2 fixation also affect NH3 production in nodule bacteroids. Treatments that prevent NH3 production, such as prolonged exposure to 1O% C2H2 (10, 12) or replacement of the N2:02 atmosphere surrounding nodules with an Ar:02 atmosphere (7, 10), decrease P. These reports have led to the suggestion that maintenance of steady-state P is dependent upon the continual production of NH3 (10, 12). However, this hypothesis does not account for either the increase in P under conditions of low NH3 production (as indicated by decreased ARA) associated with decreasing the rhizosphere PO2 from 0.21 to 0.10 MPa, or the decrease in P Abbreviations: P, nodule gas permeability; PO2, partial pressure of 02, ARA, acetylene reduction activity.following an increase in rhizosphere PO2 (16). A causal relationship between NH3 production and P remains to be determined.There have been no reports characterizing the effects of exogenous NH3 on nodule permeability. The addition of NH4' salts to the growth media of pea (Pisium sativium, L.) results in a decreased ARA of detached nodules (4, 5), excised nodulated root systems (1), and intact plants (4, 6) but has no short-term effect on the ARA of isolated bacteroids (4, 6). In addition, NH4' toxicity in plant tissues may be overcome by increasing the light intensity (1, 6) or by the addition of organic acids to the culture media (9). The obj...

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