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Univ. of Arkansas, Dep. of Crop, Soil, and Environmental Sciences, 1366 W. Altheimer Drive, Fayetteville, AR 72704
* Corresponding author (lpurcell{at}uark.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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2 mm diam. had approximately 3.5 times the 14C concentration of nodules 
4 mm diam. For plants of the water-deficit treatment, 14C concentration of nodules 2 mm diam. was only 1.6 times that of nodules 4 mm diam. Nodules from the plants of the water-deficit treatment had a greater 14C concentration than nodules from the well-watered treatment for all nodule size classes >2 mm diam. Additionally, 14C concentration for all nodule size classes was greater for Jackson than for KS4895. We conclude that drought tolerance of Jackson is partially due to the advantages of large nodules, but that drought tolerance in Jackson also results from an inherently greater supply of photosynthates to nodules.
Abbreviations: ARA, acetylene reduction activity • Co, oxygen conductance • DAS, days after sowing • FTW, fraction transpirable soil water • FWC, fractional water content • NDW, nodule dry weight • NFW, nodule fresh weight • NTW, nodule turgid weight • Oe, external oxygen concentration • Po, permeability to oxygen • RNR, root plus nodule respiration • RWC, relative water content • SA, surface area • TW, transpirable water
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The cultivar Jackson was identified as the most drought tolerant for N2 fixation of eight cultivars evaluated (Sall and Sinclair, 1991), and the drought tolerance of Jackson was confirmed in greenhouse (Serraj and Sinclair, 1996; Purcell et al., 1997) and field studies (Serraj et al., 1997). The decrease in N2 fixation during drought and differences among genotypes in sensitivity of N2 fixation to water deficits have been associated with one, or a combination of three mechanisms: limited oxygen supply to nodules, nodule C shortage, and potential feedback regulation from accumulation of various N-containing compounds (for review, see Serraj et al., 1999a). A key point linking these three mechanisms is that the nodule water supply is derived almost exclusively from the phloem (Walsh et al., 1989). Decreased volumetric flow in the phloem in response to water deficit may be responsible for eliciting the changes in each of these proposed mechanisms.
Nodule permeability to oxygen (Po) may regulate nitrogenase activity in response to water deficit (Pankhurst and Sprent, 1975; Weisz et al., 1985) as a result of an increased barrier to O2 diffusion in the outer cortex of nodules (Pankhurst and Sprent, 1975). Purcell and Sinclair (1995) proposed that, as nodules dried and nodule relative water content (RWC) decreased, an accumulation of solutes in the apoplast of the nodule cortex could cause water to fill intercellular air spaces and decrease nodule Po. A greater supply of phloem-derived water during water deficits would likely allow greater Po and continued N2 fixation by maintaining a high RWC in the nodule cortex (Purcell and Sinclair, 1995) and by providing adequate water for solute export from nodules.
It is not clear whether decreased nodule Po causes or is a result of decreased nitrogenase activity and nodule respiration in response to water deficits. Nitrogenase activity and nodule respiration decreased prior to a decrease in nodule Po in response to water deficits (Diaz del Castillo and Layzell, 1995; Purcell and Sinclair, 1995). During the initial stages of water deficit, N2 fixation activity of the drought-tolerant cultivar, Jackson, and of drought-sensitive ‘Biloxi’ was completely restored by doubling the ambient O2 concentration (Serraj and Sinclair, 1996), indicating an O2 limitation to nodule activity. Interestingly, under well-watered conditions, nodule activity of Jackson was stimulated approximately 50% by elevated O2 whereas there was no effect of increased O2 on Biloxi. Although the authors did not measure Po directly, they speculated that Po may be less in Jackson than in drought-sensitive cultivars, and through an unknown mechanism contribute to drought tolerance of Jackson. Differences in nodule Po between drought-tolerant and drought-sensitive cultivars for N2 fixation have not been directly evaluated.
Differences in photosynthate allocation and water supply to nodules during water deficits, as a result of differences in volumetric flow in the phloem to nodules, may contribute to differential sensitivity of N2 fixation to water deficits among soybean genotypes. Purcell et al. (1997) proposed that large nodules provided a greater sink for phloem-derived delivery of sugars and water than small nodules. In support of this hypothesis, the relative tolerance of N2 fixation to water deficit in Jackson was associated with greater individual nodule dry weight than in sensitive cultivars (Purcell et al., 1997, 2000; Serraj and Sinclair, 1998). During moderate drought stress, Jackson had four times the 14C concentration in nodules following exposure of leaves to 14CO2 and had twice the acetylene reduction activity (ARA) as the drought-sensitive cultivar KS4895 (Purcell et al., 1997).
The reason that large nodules may have a greater phloem supply than small nodules is that large nodules have proportionately greater amounts of N2-fixing tissue than do small nodules. Approximately 25% of the volume of a 2-mm-diam. nodule is comprised of infected tissue, whereas a 4-mm-diam. nodule has approximately 60% infected tissue (based on equations found in Weisz and Sinclair, 1988). The high energy demands associated with N2-fixing tissue may generate a greater sink capacity per unit nodule mass in large nodules, resulting in greater delivery of water and photosynthates to large nodules than to small nodules.
A third potential mechanism associated with decreased N2 fixation during water deficit is a negative feedback response triggered by elevated levels in the plant of N-containing compounds. It has been proposed that feedback inhibition of N2 fixation may be in direct response to elevated concentrations of ureides, the products of N2 fixation, in the nodule (Streeter, 1993). Decreased volumetric flow of phloem supply to nodules would decrease the amount of water available for ureide export in the xylem (Walsh et al., 1989). Nodule ureide concentrations increase in response to water deficits in drought-sensitive cultivars (Purcell and Sinclair, 1995; Serraj et al., 1999b), but no information is available comparing nodule-ureide levels in sensitive and tolerant cultivars in response to water deficit.
Other compounds that may cause feedback inhibition of nitrogenase activity in nodules are amino acids, such as asparagine (Oti-boateng and Silsbury, 1993; Bacanamwo and Harper, 1997; Serraj et al., 1999b); or glutamine (Neo and Layzell, 1997). Serraj et al. (1999a) proposed that decreased demand for amino acids during water deficit resulted in asparagine accumulation, which has been shown to inhibit leaf ureide breakdown (Lukaszewski et al., 1992). In this case, asparagine transported to nodules would decrease N2 fixation, and increased leaf ureides would reflect decreased breakdown due to leaf asparagine accumulation. Nevertheless, shoot ureide concentrations have been used as a screening tool to identify soybean genotypes that are potentially drought tolerant for N2 fixation (Sinclair et al., 2000). Low levels of ureides in shoots of well-watered plants and lower ureide accumulation during soil drying have been documented in Jackson compared to drought sensitive genotypes (Purcell et al., 1998, 2000; Serraj et al., 1997; Serraj and Sinclair 1996; Serraj and Sinclair, 1997), which supports the hypothesis that feedback inhibition may be in response to elevated shoot ureides either directly or indirectly. It appears likely that elevated levels of ureides or other N-containing compounds in soybean roots and/or shoots may play a role in regulating N2 fixation.
We tested the hypothesis that large nodules in combination with lower leaf ureide levels would confer drought-tolerant N2 fixation in the cultivar Jackson relative to the drought-sensitive cultivar KS4895. The advantage of large nodules during water deficit was hypothesized to result from a greater phloem supply to nodules, resulting in greater nodule permeability, an increased C and water supply, and a decreased accumulation of ureides in nodules.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil was inoculated with Bradyrhizobium japonicum (USDA 110) at sowing. A single plant was grown through a hole in the upper end cap of each pot, and plants were kept well watered until initiation of the drying cycle. Each pot received an additional 200 mL of N-free nutrients 7 d before the drying cycle began. The growth chamber was maintained at 24°C with a 16 h photoperiod (0600–2200 h) and PAR of 650 µmol m-2 s-1 at the top of the plant.
The amount of soil water available for transpiration by plants, transpirable water (TW), at pot capacity was defined as the difference between the pot-capacity weight and the pot weight when daily transpiration for water-deficit plants was <10% of the well-watered plants (Ritchie, 1981; Sinclair and Ludlow, 1986). According to this criterion, it was determined in previous experiments that transpirable water was zero at 22% of pot-capacity weight for both cultivars. The fraction of the total transpirable water (FTW) at a given pot weight was calculated:
 | (1) |
The drying cycle and gas exchange measurements were begun when plants were at the V6 developmental stage (Fehr and Caviness, 1977), approximately 30 d after sowing (DAS). Half the plants from each cultivar were designated either well watered or water deficit, and pots were watered to the desired target weight every 2 h during the photoperiod from 0600 to 2200 h. Well-watered plants were watered to 0.60 FTW, and water-deficit plants were watered to 0.60, 0.24, 0.19, 0.15, and 0.10 FTW on Day 1 through Day 5 of the drying cycle, respectively.
On Days 1 through 4, RNR was measured at 1030 h at 21 kPa O2 using a closed-system respirometer (Bacanamwo et al., 1997; Bacanamwo and Purcell, 1999) by following the linear depletion of O2 in the enclosed root system for 10 min. On Day 5, changes in RNR in response to O2 concentrations surrounding the roots, ranging from 6150 to 10 250 µmol L-1, were used to determine the conductance (Co, mm3 s-1) and permeability (Po, µm s-1) of nodules to O2 using a one-dimensional model of Fick's first law (Bacanamwo et al., 1997; Bacanamwo and Purcell, 1999). Two assumptions of this model are that O2 concentration inside the nodule is negligible relative to the concentration surrounding the nodule and that root respiration is constant over the concentration range used for measurements (Bacanamwo et al., 1997). The RNR was regressed against the O2 concentration surrounding the root system (Oe, µmol O2 mm-3), and the change in RNR (
RNR) with changes in Oe (Oe) was assumed to be due to changes in nodule-linked respiration (Bacanamwo et al., 1997). The slope of this relationship was defined as Co:
 | (2) |
Nodule conductance, therefore, is a coefficient equating nodule-linked respiration per plant with the O2 gradient between the air surrounding the nodule and the nodule interior. Po was calculated as the quotient of Co and total nodule surface area (SA, mm2), such that:
 | (3) |
Nodule permeability is a coefficient equating nodule-linked respiration per unit of nodule surface area with the O2 concentration gradient between the air surrounding the nodule and the nodule interior. For a given total nodule mass per plant and for similar rates of nodule-linked respiration per plant, a plant with large nodules would have a higher Po than a plant with small nodules because the total nodule surface area per plant would be less for a plant with large nodules relative to a plant with small nodules.
To evaluate the influence of water deficits on nitrogenase activity, ARA was measured each day using an in situ flow-through assay (Bacanamwo et al., 1997) by introducing a 10 kPa C2H2, 21 kPa O2, and 69 kPa N2 mixture from a gas mixing device (Parsons et al., 1992) into the sealed root chamber at 200 mL min-1. After 8 min, gas samples were collected from the exhaust of each pot, and C2H4 was quantified by gas chromatography. This short-term acetylene exposure prevented the decline in nodule activity (data not shown) associated with long-term acetylene exposure (Minchin et al., 1983). Plants were harvested on Day 5 and separated into shoots, roots, and nodules. Nodules removed from plants on Day 5 were photocopied, and from the photocopy, nodule number and surface area were determined (Weisz and Sinclair, 1988). Plant parts were dried at 65°C for 72 h and dry weights recorded.
Nodule Water Content Experiment
Seeds of Jackson and KS4895 were sown in 15-cm pots, on 1 July 1999 in a greenhouse at Fayetteville, AR (latitude 36° 5'N). The pots contained approximately 1.9 L of N-free potting mix. The potting mix was saturated with deionized water followed by 750 mL of N-free nutrient solution prior to sowing. Pots were allowed to drain overnight and pot-capacity weights were recorded. The potting mix was inoculated with B. japonicum (USDA 110) at sowing and plants were thinned to one per pot after emergence. Each pot received an additional 500 mL of nutrients at 20 DAS. Day/night temperatures were 29 and 24 (±3)°C, and natural illumination was supplemented with 1000 W metal halide lamps for a 16-h photoperiod.
Plants were maintained well watered with deionized water until initiation of water-deficit treatments at 23 DAS, when plants were at the V5 developmental stage. During the drying cycle, pots were watered to a specific target weight at 0900 h daily. Well-watered pots were maintained at 0.60 FTW, and water-deficit pots were dried stepwise over a 5-d period. Four plants of each cultivar and water treatment were harvested at 1300 h on each of the final 3 d, corresponding to FTW values for the water-deficit treatments of 0.23, 0.17, and 0.10. Plants were harvested in a laboratory so that nodule fresh weights (NFW) could be recorded immediately. Soil was removed from plant roots, and nodules were plucked prior to separating the root and shoot. Nodules were sorted using 2.4 and 4-mm wire-mesh sieves resulting in three nodule size classes (<2.4, 2.4–4, and >4 mm), and NFW was recorded by size class. Leaves were separated from the stem, and all plant tissues were dried at 65°C for 96 h and dry weights recorded, including nodule dry weights (NDW) by nodule size class.
The RWC of nodules in each size class was measured by first determining the fractional water content of nodules from well–watered plants (FWCww) (Purcell and Sinclair, 1995):
 | (4) |
It was assumed that nodules from well-watered plants were fully turgid, and FWCww was used to estimate the nodule turgid weight (NTW) for each water treatment and nodule size class:
 | (5) |
 | (6) |
By definition, the RWC of nodules from well-watered plants was 1.0.
Ureides were extracted from approximately 25 mg of dried nodule or leaf tissue in 1.25 mL of 0.2 M NaOH for 30 min at 100°C, and concentrations were determined colorimetrically using the procedure of Young and Conway (1942) modified as previously described (de Silva et al., 1996). Data were analyzed as a split plot, with cultivar by water regime as the main-plot and nodule size as the subplot.
The experiment was repeated two additional times, and sowing dates for Runs 2 and 3 were 9 Sept. and 28 Oct. 1999, respectively. In Runs 2 and 3, water deficits were established to allow all plants to be harvested on the same day by staggering the initial withholding of water over a 2- or 3-d period. At harvest, FTW values were 0.60, 0.23, 0.17, and 0.10 FTW for Run 2 and 0.60, 0.17, and 0.10 FTW for Run 3. Plant harvests and tissue analysis were the same as for Run 1 of the experiment.
Carbon 14 Labeling Experiment
Photosynthate transport to soybean root nodules of different size classes was evaluated in a completely random experiment design with a factorial arrangement of two soybean cultivars and two water regimes with six replications. Seeds of Jackson and KS4895 were sown on 2 Feb. 1999 in 3-L pots in a greenhouse at Fayetteville, AR, under conditions similar to those for the nodule water content experiment. Plants were thinned to one per pot after emergence. Each pot received 1 L of N-free nutrient solution at sowing and an additional 0.5 L at 5 wk after sowing.
Pots were well watered until plants reached the V6 developmental stage. At V6, half of the plants for each cultivar were maintained well watered by watering the soil to 0.60 FTW daily, and half of the plants were dried stepwise over a 4-d period to a final soil weight of 0.17 FTW. Pots were weighed and watered daily at 0900 h during the drying cycle and during the 14C labeling and translocation period.
At 1300 h on the day water-deficit treatments reached 0.17 FTW, the uppermost fully expanded trifoliolate leaf of each plant was exposed to 14CO2 for 15 min followed by a 24 h translocation period prior to harvest. To administer the 14CO2 pulse, a 1-L mylar bag was placed over the source leaf and sealed around the petiole with tape. A scintillation vial was attached to the side of the mylar bag so that 14CO2 generated inside the vial would equilibrate inside the mylar bag that was sealed around the treatment leaf. One-half mL of 0.2 M NaOH, containing 1.85 x 105 Bq of NaH14CO3 (Sigma Chemical Co., St. Louis, MO), was injected through a septum in the side of the vial, and 14CO2 was generated by injecting 4 mL of 1 M lactic acid into the vial. After 15 min, 4 mL of 2 M KOH was injected into the vial to trap free 14CO2, and the bag was removed from the treated leaf after an additional 15 min.
Twenty four h after exposure to 14CO2, plants were sectioned into treated leaf, the remaining leaves, stem, root, and nodules. All plant tissue, except nodules, were dried for 48 h at 65°C. Nodules were frozen at -50°C and then freeze dried, so that nodules maintained their harvest size. Freeze-dried nodules were separated into six size classes using wire-mesh sieves. Nodule size classes were <2, >2 to 2.4, >2.4 to 3.3, >3.3 to 4, >4 to 4.7 and >4.7 mm diam.
Dried plant sections were weighed and combusted in a biological material oxidizer (Model OX100, R.J. Harvey Instr. Corp., Hillsdale, NJ). Evolved CO2 was trapped in 15 mL of a scintillation cocktail that allows for direct counting of 14C (Carbon 14 Cocktail, R.J. Harvey Instr. Corp., Hillsdale, NJ). Radioactivity of all samples was quantified by liquid scintillation spectrometry (Tri-Carb 4530, Packard Instr. Co., Downers Grove, IL).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A key question is whether the greater Po of Jackson was simply a means of compensating O2 supply per unit nodule surface area or if a greater Po actually increased O2 availability to nodules. To address that question, we estimated nodule oxygen supply per unit nodule volume as the quotient of nodule dependent respiration and the sum of individual nodule volumes (mm3 plant-1). Nodule oxygen supply represents the rate of oxygen delivery per unit nodule volume. Nodule dependent respiration at 21 kPa O2 was calculated by rearrangement of Eq. [2] to solve for a constant respiration rate. The greater Po of Jackson resulted in the nodule oxygen supply being approximately 50% greater than that of KS4895, averaged over water treatments (Table 1).
Nodule Water Content
Fractional water content of nodules from well-watered plants averaged 0.79 to 0.80 regardless of cultivar or nodule size (data not shown). In Run 1 of the experiment (Table 2), the more drought-sensitive cultivar, KS4895, had a higher nodule RWC than Jackson at 0.23 and 0.17 FTW, but there was no difference between cultivars at the most severe stress level (0.10 FTW). For Runs 2 and 3 there was no cultivar by FTW interaction for nodule RWC.
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Accumulation of ureides in leaves and petioles during water deficits has been hypothesized to result in nitrogenous compounds such as asparagine (Serraj et al., 1999b; Bacanamwo and Harper, 1997) or glutamine (Neo and Layzell, 1997) being sent to nodules and resulting in feedback inhibition of N2 fixation (de Silva et al., 1996; Serraj et al., 1999b; Purcell et al., 1998, 2000). Furthermore, cultivar differences in drought tolerance of N2 fixation have been associated with differences in shoot-ureide concentrations, with drought-tolerant cultivars maintaining lower leaf and petiole ureide levels during water deficit (Serraj and Sinclair, 1996; Purcell et al., 1998, 2000; Sinclair et al., 2000). With the exception of Run 2, our results agreed with these previous reports in that Jackson generally maintained lower leaf ureides than KS4895 across water treatments (Table 3). There was a tendency for leaf ureide concentration to increase slightly with increased severity of water deficit (Table 3). Leaf ureides for well-watered plants at 0.60 FTW were significantly less than those at 0.10 FTW in Runs 1 and 3.
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Clearly, the data indicate that large nodule size by itself does not result in preferential photosynthate allocation compared to small nodules. Additionally, increased 14C concentrations in nodules in response to water deficit does not support the hypothesis that N2 fixation is limited by photosynthate supply during water deficit.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A second factor contributing to drought tolerant N2 fixation of Jackson was greater individual nodule size. Larger nodules tended to have greater RWC during water deficits than smaller nodules. Although there was no consistent difference between cultivars in nodule RWC for nodules within a size class, the greater proportion of large nodules for Jackson was a likely advantage in maintaining a favorable nodule water balance.
Thirdly, Jackson had a greater 14C concentration in nodules of all size classes during water-deficit stress than did KS4895 following exposure of leaves to 14CO2 (Table 5). The increased 14C concentration in nodules of Jackson relative to KS4895 in response to water deficit (Table 5) may be due to one or a combination of the following factors: (i) decreased nodule respiration, (ii) increased photosynthate allocation to nodules, (iii) decreased export of 14C from nodules in the xylem, and (iv) increased 14C incorporation into nodule structure. Although respiration was certainly decreased by water deficit in both cultivars, it appears unlikely that greater nodule 14C concentration in Jackson was because nodule respiration was decreased more in Jackson than in KS4895. The fact that Jackson maintained higher relative ARA throughout the water deficit in the gas exchange experiment than did KS4895 would indicate that nodule-linked respiration may have been greater for Jackson. In a previous experiment, Jackson had a similar increase in 14C concentration in nodules during water deficit relative to KS4895 as found in this report, and respiration of 14C photosynthates by roots and nodules was greater for Jackson than for KS4895 (Purcell et al., 1997). Associated with the greater amount of respired 14C was a significantly greater ARA activity for Jackson than for KS4895. The greater increase in 14C concentration in nodules of Jackson than of KS4895 during water deficit is presumed due to a greater amount of photosynthate being exported from the leaves and delivered to the nodules.
The role of nodule size on the greater photosynthate allocation to nodules of Jackson than of KS4895 is speculative. Surprisingly, for both cultivars, the 14C concentration was greatest for small nodules (<2.8 mm diam.) under well-watered and water-deficit conditions, but 14C concentrations increased more for large nodules than for small nodules during water deficit (Fig. 4B). The preponderance of large nodules for Jackson appeared to be one factor contributing to a greater 14C concentration during water deficit, but Jackson also appeared to have an inherently greater photosynthate allocation to nodules regardless of nodule size or water treatment (Fig. 4A). Greater O2 supply and photosynthate allocation to nodules during water deficit likely allowed continued respiratory activity to support N2 fixation in Jackson compared with KS4895.
Finally, leaf ureides were generally less for Jackson than for KS4895 during water-deficit stress, but nodule ureide concentration was greater. Lower shoot ureide concentrations for Jackson have been noted in several previous reports (Serraj and Sinclair, 1996; Purcell et al., 1998, 2000), and continued ureide degradation in leaves during water deficit has been hypothesized as a key point for preventing feedback inhibition of N2 fixation in response to delivery of ureides or other nitrogenous compounds to the nodules in the phloem (Purcell et al., 1998, 2000; Serraj et al., 1999a). The fact that Jackson maintained higher nitrogenase activity while nodule ureide concentrations increased during water deficit, relative to KS4895, indicates that feedback inhibition, if it occurs, is more likely triggered by compounds other than ureides that are delivered in the phloem from the shoot, possibly in response to elevated shoot ureide concentrations. Certainly, large nodules of Jackson would favor photosynthate and water allocation, maintain a favorable nodule RWC, and provide continued supply of water for export of ureides in the nodule xylem.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication September 11, 2000.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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