Root and Nodule Respiration in Relation to Acetylene Reduction in Intact Nodulated Peas

Self Cite

Pisum sativum L. cv. Trapper plants were inoculated and grown in a controlled environment on N-free nutrient solution. After 4 weeks N was supplied to treatment plants as NH4NO3, KNO3, or NH.C1 and rates of C2H2 reduction, root + nodule respiration, and leaf photosynthesis were determined 1 week later. The increase in respiration per unit of C2H2 reduction was not affected by either the fonn of N added or the light conditions during growth, although the basal respiation rate with no C2H2 reduction increased with iradiance level. The mean regression coefficient from plots of respiration versus C2H2 reduction was 0.23 + 0.04 (P . ; .01) mg of CO2 (jmol of C2H2 reduced)-1 which was very similar to the value for the coefficient of respiration associated with nitrogenase activity estimated by subtracting growth and maintenance respiration. Since the rate of N accumulation in N-free nutrient conditions was proportional to the rate of C2H2 reduction, it appears that the method gives a true estimate of the energy requirements for N fixation which for these conditions was equivalent to 17 grams of carbohydrate consumed per gram of N fixed.Because of recent evidence that symbiotic N fixation can be limited by energy supply from the host plant (8, 11,12,20) there is increasing interest in the efficiency of energy use by the N-fixing reactions (6,15,19). Although the theoretical energy requirements for N fixation and nitrate reduction are similarly high (5,8,15), the difficulties in estimating the in vivo energy consumption (1, 3, 5, 15) have prevented any critical examination of the relative efficiency of energy use by the two processes.In a previous report (13), it was suggested that an extension of the two-component respiration model (14,22) to include a fixation component could provide the basis for estimating the respiration directly related to the fixation process. After comparing the effects of several treatments on the rates of both root + nodule respiration and C2H2 reduction (13); it was concluded that addition of NH4NO3 to nodulated plants decreased the respiration associated with nitrogenase activity while causing little difference in growth and maintenance components as compared to N-free treated plants of similar age. A comparison of the changes in respiration and C2H2 reduction under these conditions should be useful in assessing the energy requirements of N fixation. However, difficulties with parallel changes in other respiration components, high variability within a single symbiotic system, and the indirect nature of the C2H2 reduction assay could decrease the utility of this method. Therefore before the method can be applied to comparison of different symbiont genotypes, these possible problems must be examined and minimized. Measuring System. All measurements were made in the laboratory gas exchange system described previously (13). Rates of C2H2 reduction were determined from gas chromatographic analysis of C2H4 10, 20, and 30 min after the injection of C2H2 to a final concentration of 0.02 at...

mentioning

confidence: 99%

mentioning

confidence: 99%

Respiration and the Energy Requirement for Nitrogen Fixation in Nodulated Pea Roots

Mahon¹

1977

Self Cite

“…Alternatively, changes in intracellular pH (or other parameters) may affect both enzyme systems directly. High levels of CO2 were found to inhibit nodule respiration but not acetylene reduction in peas (6). Coker and Schubert (2) also found no effect of high CO2 levels on acetylene reduction by soybean nodules.…”

Section: Methodsmentioning

confidence: 80%

“…In plants with nitrogen-fixing root nodules, the energy cost of nitrogen fixation and associated metabolism can be measured by the ratio between nodule respiration and nitrogenase activity. Many investigations of the respiration associated with nitrogen fixation have involved measurements of CO2 evolution from entire nodulated root systems (5)(6)(7). This approach has resulted in excellent measurements of long-term effects and of total root respiratory costs and minimized artifacts due to disturbance.…”

mentioning

confidence: 99%

Simultaneous Measurement of Acetylene Reduction and Respiratory Gas Exchange of Attached Root Nodules

Winship¹,

Tjepkema²

1982

A method was developed for the simultaneous measurement of acetylene reduction, carbon dioxide evolution and oxygen uptake by individual root nodules of intact nitrogen-fixing plants (Ahis rubra Bong.). The nodules were enclosed in a temperature-controlled leak-tight cuvette. Assay gas mixtures were passed through the cuvette at a constant, known flow rate and gas exchange was measured by the difference between inlet and outlet gas compositions. Gas concentrations were assayed by a combination of an automated gas chromatograph and a programmable electronic integrator. Carbon dioxide and ethylene evolution were determined with a coefficient of variation which was less than 2%, whereas the coefficient of variation for oxygen uptake measurements was less than 5%. Nodules subjected to repeated removal from and reinsertion into the cuvette and to long exposures of 10% v/v acetylene showed no irreversible decline in respiration or acetylene reduction. This system offers long-term stability and freedom from disturbance artifacts plus the ability to monitor continuously, rapidly and specifically the changes in root nodule activity caused by environmental perturbation.Nitrogen fixation is an energy-intensive process, requiring large inputs of reductant and ATP. In plants with nitrogen-fixing root nodules, the energy cost of nitrogen fixation and associated metabolism can be measured by the ratio between nodule respiration and nitrogenase activity. Many investigations of the respiration associated with nitrogen fixation have involved measurements of CO2 evolution from entire nodulated root systems (5-7). This approach has resulted in excellent measurements of long-term effects and of total root respiratory costs and minimized artifacts due to disturbance. But nodule respiration was not separated from root respiration, except by indirect means.Studies of excised nodules have provided the needed separation but at the risk of greater disturbance and reduced experimental time span. Under favorable conditions, the effects of detachment are small and nitrogenase activity is maintained for many hours (8). In other cases, activity decreases rapidly after detachment. Thus, experiments with many treatments, such as temperature response curves, must be done rapidly or with many replicate samples. Transient effects and variation among samples complicate the interpretation of such results. The influence of host plant activities, such as changes in photosynthesis, cannot be studied directly.To overcome these limitations, we measured nodule respiration by enclosing nodules in a cuvette while still attached to the root system of the actively growing plant. 02 were simultaneously and rapidly determined, permitting detailed investigation of the interaction of nitrogenase activity and respiration. Three main concerns were addressed in the design of this method. First, it was necessary to grow the plants so that the nodules were readily accessible and could be enclosed in the cuvette without damage or change in metabolism. This was a...

“…As two distinct nitrogen acquiring processes, N2 fixation and inorganic nitrogen metabolism probably generate two different sink strengths for reduced carbon (29). There is evidence that because of the maintenance of the bacteria and the nodules, N2 fixation generates 259 the greater sink for reduced carbon (6,21,23). However, this is not entirely certain (7).…”

mentioning

confidence: 99%

Photosynthesis and Photosynthate Partitioning in N₂-Fixing Soybeans

Veau¹,

Robinson²,

Warmbrodt³

et al. 1990

Leaf area, chlorophyll content, net CO2 photoassimilation, and the partitioning of fixed carbon between leaf sucrose and starch and soluble protein were examined in Glycine max (L) Merr Legumes have the capability to acquire nitrogen for growth through N2 fixation and inorganic nitrogen metabolism, and reduced carbon is needed by the plants to carry out both processes. Little is known about the effects of N2 fixation versus inorganic nitrogen metabolism on photosynthesis and photosynthate partitioning within the source leaves of the host plants of legume-Rhizobium symbioses, although the influence of photosynthesis and photosynthate production of the source leaves upon N2 fixation in legumes is well documented (1, 12, 13, 34). As two distinct nitrogen acquiring processes, N2 fixation and inorganic nitrogen metabolism probably generate two different sink strengths for reduced carbon (29). There is evidence that because of the maintenance of the bacteria and the nodules, N2 fixation generates 259 the greater sink for reduced carbon (6, 21, 23). However, this is not entirely certain (7).It is known that sink strength influences photosynthetic metabolism and photosynthate partitioning (24, 33). High sink strength elevates net photosynthetic rates (14, 33). If N2 fixation is the stronger sink, then N2-fixing legumes should have higher photosynthetic rates than non-N2-fixing counterparts. This laboratory has established that in some cases N2-fixing soybeans have higher leaf net photosynthesis than non-N2-fixing plants (27). This higher photosynthetic rate was evident in both intact trifoliolates and isolated mesophyll cells. However, others have shown that leafnet photosynthesis between the two groups ofplants are not significantly different (2), but this point needs further investigation.Reduced carbon is partitioned within a plant to meet various metabolic, storage, and structural demands. Photosynthate partitioning can be thought ofin terms ofthe amount of carbon transported out of the photosynthetic cells to meet the demands of nonphotosynthetic tissue versus that amount of carbon sequestered for later use. Photosynthate is stored usually in the form of starch or some other carbohydrate. The amount ofcarbon diverted to meet the metabolic needs versus starch storage is influenced by the strength of the sink. High sink strength will result in more photosynthate from the source leaves being synthesized for transport and less for storage (24). Source leaves from N2-fixing plants should have lower foliar starch levels than source leaves of plants assimilating inorganic nitrogen if the demands of N2 fixation for fixed carbon are greater than for inorganic nitrogen assimilation.A lower foliar starch content within nodulated legumes has been shown, indicating that N2 fixation is a stronger sink than inorganic nitrogen assimilation (16). However, there have been reports of N2-fixing soybeans having higher starch content within source leaves than non-N2-fixing soybeans (2,3,17). This suggests that nitrogen assimilat...