Total Nodule Activity Research Articles

The effect of Frankia and multiple ectomycorrhizal fungil species on Alnus growing in low fertility soil

Plant interactions with single symbionts of nitrogen fixing bacteria or mycorrhizal fungi are well studied. However, less is known about how plants interact with multiple symbiont species. The aim of this study was to assess whether multiple symbionts had a synergistic effect on host plant performance compared with a single symbiont and whether increasing the number of non-specific fungal species would increase plant performance due to niche complementarity. Green alder (Alnus viridis ssp. crispa) were inoculated with non-specific ectomycorrhizal fungi (Lactarius torminosus, Lactarius theiogalus, Hebeloma crustuliniforme) alone or in combination, with and without Frankia, and grown in nutrient-poor soil. Frankia significantly increased plant growth by 48.5%, doubled the proportion of plants forming ectomycorrhizae, as well as increased root extracellular phosphatase activity by 8.8% compared to non-Frankia treatments. However, increasing number of fungal species decreased nodulated plant biomass. Plants that formed a Hartig net had reduced nodule number and nodule biomass allocation, as well as total nodule activity compared with plants with no Hartig net. Our results indicated that Frankia inoculation provided benefit for both host plants and ectomycorrhizal fungi in the nutrient-poor soil, while ectomycorrhizal fungi did not. These negative effects of mycorrhizal fungi are likely dependent on soil nutrient P availability.

Influence of Nodule Development on the Long-term Photosynthetic Response of Beans to Elevated CO2 and Temperature

The widely observed reduction in photosynthetic (Pn) capacity following long-term exposure to elevated CO2 is believed to result from an imbalance in source–sink status. We hypothesized that nitrogen fixation in root nodules would provide a strong sink for photosynthate and lead to a sustained positive photosynthetic response to elevated CO2. Bean plants (Phaseolus vulgaris L., cv Redkloud) were grown in poly chambers at one of four combinations of temperature (35/21 or 26/15°C day/night), and CO2 (350 or 700 ppm). Half the plants in each chamber were inoculated with Rhizobium and fertilized with a complete nutrient solution lacking nitrogen; control plants received a similar solution with nitrogen. Total nitrogenase activity (acetylene reduction assay; 8 weeks after planting) of excised whole root systems was stimulated (up to 4-fold) by elevated CO2, but this response was only significant for 26/15°C-grown plants. Inoculated plants also accumulated more biomass (10%) than control plants. Nodule abundance and size were significantly higher in high CO2-grown plants than ambient CO2 plants, but the Pn capacity of inoculated plants was only slightly greater than that of control plants. Averaged across other treatments, high CO2-grown plants accumulated more biomass (42%) and had higher Pn rates (50%) than ambient CO2 plants. Treatment effects on leaf carbohydrate levels and Pn acclimation to CO2 were not consistent. The results suggest that the higher total nodule activity was due to increased nodule number and size in proportion with increased plant size under high CO2, rather than an increase in nitrogenase activity per nodule. It is also evident that plants with symbiotic nitrogen fixation capability can benefit from elevated CO2, even with reduced input of inorganic nitrogen.

Use of 13C and 15N isotopes to investigate O 3 effects on C and N metabolism in soybeans. Part II. Nitrogen uptake, fixation, and partitioning

Short- and long-term N uptake/partitioning dynamics were studied using stable isotope techniques to investigate the uncertain mechanism(s) of O 3 action on plant yield and photosynthate partitioning. Glycine max [L.] Merr.(soybean) plants were grown in 15N enriched soil within open-top chambers and exposed to one of three O 3 regimes: half-ambient, ambient, or 2 × ambient. The seasonal 7 h average O 3 concentrations (nl l −1) were 25, 43, and 76 nl l −1, respectively. Nitrogen fixation was estimated using the 15N isotope dilution method utilizing a non-nodulating soybean isoline as the control. Macro-kjeldahl technique was used for determining N concentration. Short-term plant responses were investigated by evaluating the following parameters: % N, total N, total N fixed, total N fixed per organ dry weight, the proportion of N-fixed/soil N, and the fraction of N derived through rhizobial N-fixation on an individual organ (leaves, stems, roots, pods, and nodules) and whole plant basis at two reproductive growth stages. Long-term plant responses were investigated by characterizing the same N parameters of the mature grain. Ozone significantly affected both short- and long-term N uptake/partitioning dynamics. Ozone exposure reduced the amount of N derived from N-fixation, but did not significantly affect total N or % N for organs and whole plants. For mature grain, O 3 significantly decreased seed yield and all N parameters except N-fixed/soil N, but the responses were dependent upon year. Our results suggest that total nodule activity was affected rather than specific activity. Total N uptake was maintained despite significant decreases in % N-fixed and N-fixed/soil N. We conclude that N-fixation was inhibited by reduced photosynthate translocation to nodules. The photosynthate translocated was sufficient to maintain moderate rates of soil N uptake, but not adequate to maintain high rates of N-fixation, the latter costing more energy. Thus, soybeans damaged by the exposures imposed here, relied more heavily on soil N to meet their total N requirements when photosynthate translocation was inhibited. The long-term negative effects for mature seed also indicate a significant reduction in photosynthate and total N translocated to nodules, and an increased reliance on soil N. In summation, these findings and those of our companion carbon study, support the hypothesis that the mechanism of chronic O 3 action involves an inhibition of carbon translocation from leaves to other organs.

Dinitrogen fixation and growth responses to phosphorus and aluminum application in pigeon. pea (Cajanus cajanL.)

Abstract Responses to aluminum phosphate (Al-P) and superphosphate (Sup-P) application in terms of dry matter production by pigeon pea and soybean were compared under field conditions. The effect of aluminum on growth and symbiotic dinitrogen fixation was examined under hydroponic conditions. In a field experiment, Al-P application significantly increased whole plant dry weight and P amount in pigeon pea, whereas in soybean, Al-P application had no significant effect on the two parameters. The results indicate that P absorption ability from Al-P is higher in pigeon pea compared to soybean. With Al-P application, leaf and whole plant Al concentration in pigeon pea significantly increased unlike in soybean. Under hydroponic conditions, 50 nig L−1 Al application increased the plant weight of pigeon pea while it decreased that of soybean. This treatment decreased the total nodule activity of soybean more appreciably than that of pigeon pea. Under the Al treatment conditions, whole plant P amount increased in ...

Response of short-duration pigeonpea to nitrogen application after short-term waterlogging on a Vertisol

Short-duration pigeonpea suffers from waterlogging damage following heavy rainfall at the pre-flowering stage on soils with high clay content, such as Vertisols. Effects of short-term waterlogging (3 d) on shoot and root growth of short-duration pigeonpea grown on a Vertisol field were quantified, and the alleviation of waterlogging damage by top-dressing of nitrogen fertilizer was examined. Reduced leaf chlorophyll, increased senescence and abscission of lower leaves were observed within 3 d of the initiation of waterlogging. Root growth and symbiotic N 2 fixation were also severely impaired. Root distributions of waterlogged plants were shallower than those of the control during the subsequent recovery period because new adventitious roots were formed in the shallow rather than deep soil layer. Yield of waterlogged crops was significantly smaller than the control. Top-dressing of nitrogen at one day after the termination of waterlogging alleviated waterlogging damage either completely or partially. Leaf chlorophyll and shoot dry mass of waterlogged plants were 78 and 84% of the values in control plants immediately after waterlogging but recovered to 92 and 98% of the control values at the pod-filling stage with a top-dressing of 50 kg N ha −1 . Nitrogen application promoted root growth in the shallow soil layers during the first 11 d after application, and in the deeper soil layers during the subsequent 16 d. Total nodule activity was significantly reduced by 100 kg N ha −1 , but increased by 50 kg N ha −1 around one month after the top-dressing. The reduction in seed yield was largely compensated for by 50 kg N ha −1 , because of recovery from waterlogging damage to shoot and root growth involving increased nodule activity.

Molecular and whole-plant responses to selection for enzyme activity in alfalfa root nodules: evidence for molecular compensation of aspartate aminotransferase expression

The enzyme aspartate aminotransferase (AAT) plays a key role in the assimilation of fixed-N in alfalfa (Medicago sativa L.) root nodules. AAT activity in alfalfa nodules is due to the activity of two dimeric isozymes, AAT-1 and AAT-2, that are products of two distinct genes. Three forms of AAT-2 (AAT-2a, -2b, and-2c) have been identified. It was hypothesized that two alleles occur at the AAT-2 locus, giving rise to the three AAT-2 enzymes. In a prior study bidirectional selection for root nodule AAT and asparagine synthetase (AS) activities on a nodule fresh weight basis in two diverse alfalfa germ plasms resulted in high nodule enzyme activity subpopulations with about 20% more nodule AAT activity than low enzyme activity subpopulations. The objectives of the study presented here were to determine the inheritance of nodule AAT-2 production and to evaluate the effect of bidirectional selection for AAT and AS on AAT-2 allelic frequencies, the relative contributions of AAT-1 and AAT-2 to total nodule activity, nodule enzyme concentration, and correlated traits. Two alleles at the AAT-2 locus were verified by evaluating segregation of isozyme phenotypes among F1 and S1 progeny of crosses or selfs. Characterization of subpopulations for responses associated with selection was conducted using immunoprecipitation of in vitro nodule AAT activity, quantification of AAT enzyme protein by ELISA, and AAT activity staining of native isozymes on PAGE. Results indicate that selection for total AAT activity specifically altered the expression of the nodule AAT-2 isozyme. AAT-2 activity was significantly greater in high compared to low activity subpopulations, and high AAT subpopulations from both germ plasms had about 18% more AAT-2 enzyme (on a nodule fresh weight basis). No significant or consistent changes in AAT-2 genotypic frequencies in subpopulations were caused by selection for AAT activity. Since changes in AAT activity were not associated with changes in AAT-2 genotype, selection must have affected a change(s) at another locus (or loci), which indirectly effects the expression of nodule AAT.

Analysis of factors controlling dinitrogen fixation in wild and cultivated soybean (Glycine max) plants by reciprocal grafting

Abstract This study was carried out to examine the factors controlling dinitrogen fixation in soybean. Reciprocal grafting in all combinations for the wild type (Glycine soja Sieb and Zucc line Nakei-No.l) and the cultivated type (Glycine max (L) Merr. cv. Tamahoniare) was performed. The results indicated that: 1. irrespective of root genotype, the plant combination with the shoot of the cultivated type, gave a significantly higher whole plant weight than that with the shoot of the wild type, suggesting the importance of the contribution of the shoot growth rate compared with that of the root, for the overall plant growth determination. 2. Total nodule activity was consistently and significantly higher in plants with shoot of the cultivated type than in plants with shoot of the wild type, and it was also slightly greater in plants with roots of the wild type than in plant combinations with the same shoot genotype. This finding also suggests that the shoot genotype is the major determinant of dinitrogen fi...

The relationship between foliar injury, nitrogen metabolism, and growth parameters in ozonated soybeans.

A single 12 h ozone exposure peaking at 0.20 ppm proved phytotoxic to greenhouse-grown 'Cutler 71' soybeans at each growth stage tested from V5 to R6. Visible injury occurred within 40 h on the unifoliodate leaves and middle-aged and older trifoliolates while the younger leaves were free from toxicity symptoms. In some instances visible injury was accompanied by a decrease in chlorophyll and an increase in leaf diffusive resistance. Although nitrogen fixation was not significantly altered except at early pod formation (R3), and nitrate reductase activity was significantly reduced only if the ozone exposure occurred at the time of maximal enzyme activity (V5), nitrogen content of the leaves was reduced by ozone treatment. Shoot dry weight was not affected 40 h after ozone treatment, but root dry weight was significantly reduced. Plants grown with supplemental NO(3)(-) were more sensitive to ozone than those dependent on fixed nitrogen. At plant maturity, there was no evidence of an ozone effect on shoot, root, or seed dry weight, NO(3)(-) -grown plants showed a significant increase in growth and yield over N(2)(-) plants; but no ozone effect was observed, despite the increased foliar sensitivity. Multiple ozone exposures at growth stages V3, R1 and R3 exacerbated the effects noted with a single episode and also reduced nitrogenase activity (reflected in specific and total nodule activity) and shoot and root dry weight. At plant maturity, there was again no evidence of a significant effect of multiple ozone treatment on shoot dry weight or seed yield although root weight remained low. The results would tend to support the hypothesis that older leaves of soybean do not make a significant contribution to seed yield. Although they may be injured by ozone during the reproductive phases of growth, seed yield may not be affected if the younger O(3)-tolerant leaves remain functional.

White Clover N2-Fixation in Response to Root Temperature and Nitrate

Established, nodulated white clover plants were transferred to eight tanks of a flowing culture apparatus with solution temperatures of 5, 11, 17, and 25 °C (two tanks per temperature). Shoot temperature and light environment were common to all plants. After 7 d, NO3- (10 mmol m−3) was continuously supplied to one tank at each temperature while in the remaining four tanks (one at each temperature) the plants were completely dependent on nodule N2-fixation. Plants were randomly selected at intervals during the following 14 d period in order to measure root and nodule respiration and acetylene reduction activity (ARA) in a flow-through system set at the adapted root temperature. Additional plants were assayed for in vitro nitrate reductase activity in leaves, roots, and nodules. Apparent nitrogenase activity (ARA) and respiration associated with it were each markedly affected by temperature in two ways; (1) Activity per unit weight of nodule was reduced at lower temperatures; (2) Development of the plant, and thus also nodule mass, was restricted at lower temperatures which, in turn, restricted total nodule activity per plant. The presence of nitrate significantly reduced ARA of nodules, particularly at higher temperatures. However, significant discrepancies were found when N2-fixation rates, estimated from the acetylene reduction assay, were compared with N2-fixation rates calculated from curves fitted to N accumulation data (minus the rate of NO3- uptake in the case of nitrate-treated plants). Carbon use efficiency (CO2 respired per C2H4 produced) was not significantly affected by temperature or the presence of nitrate. Nitrate reductase activity (NRA) developed in all plant parts at the three highest temperatures, but not at 5 °C. We calculate that leaf NRA may account for 82, 75, and 68% of total nitrate reduction at 11, 17, and 25 °C respectively.

Herbicidal Effects on Crownvetch Rhizobia and Nodule Activity

Two strains of crownvetch (Coronilla variaL. # CZRVA) rhizobia were cultured in vitro with various rates of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate]. Growth, measured turbidimetrically over 48 h, was similar for both strains. Atrazine and bifenox significantly reduced bacterial growth after 14 and 36 h, respectively, only at the highest concentrations tested (463 μM atrazine and 292 μM bifenox). Since growth of crownvetch rhizobia was apparently not affected by rates of atrazine or bifenox above reasonable soil solution concentrations, it is likely that herbicidal effects on nodulation were due to toxicity to the host plant rather than toxicity to these bacteria. In a growth chamber experiment, total nodule activity (TNA) and carbon dioxide exchange rate (CER) were measured simultaneously in an effort to distinguish direct atrazine effects on nodule function from indirect effects due to inhibition of photosynthesis and a resulting decrease in photosynthate supply to nodules. When 5 and 50 mg atrazine per kg soil were applied to intact plants, CER was severely reduced within 24 h, but similar reductions in TNA were not observed until 48 h after treatment. Total nodule activity was reduced similarly by atrazine and defoliation; the application of atrazine to defoliated plants did not inhibit TNA more than did defoliation alone. The data indicate that reductions in crownvetch nodule activity by atrazine are due to inhibition of photosynthesis or other processes rather than direct toxicity to N fixation.

Nodule Activity in Soybean Cultivars Exposed to Ozone and Sulfur Dioxide

AbstractIn an effort to obtain a greater understanding of the mechanisms of action for air pollutants on yield‐limiting processes in crop plants, studies were conducted in the field and greenhouse in 1979 to investigate the effects of O3 and SO2 exposures on nodule activities in roots and chlorophylls a and b contents in leaves of soybeans [Glycine max (L.) Merr.]. Six cultivars of soybeans were exposed 4 h d−1 for 5 successive days during flowering and pod development to the combination of 6.7 µmol m−3 O3 and 11.1 µmol m−3 SO2 in activated charcoal‐filtered air. Acetylene reduction procedures were used to assay nodule activities.Significant differences (P < 0.1) were observed in total nodule activity (TNA) per plant for treatments and cultivars in combined analysis of variance (ANOVAs) for two greenhouse studies and two exposure dates in the field; however, treatment × cultivar interactions were nonsignificant in all cases. Compared with filtered air controls, the air pollution treatment reduced TNA by an average of 16.4 and 27.4% in the greenhouse and field studies, respectively. Specific nodule activities (SNA) were unaffected by the treatments in both studies. In the greenhouse studies, the O4 and SO2 treatment lowered the chlorophyll a content by 14.1%, but no significant differences were observed in the field study. Chlorophyll b contents were unaffected by treatments in both investigations. In these studies, the sensitivity of changes in chlorophyll contents in soybean leaves following exposures to O3 and SO2 at concentrations found in the atmosphere were found to be too low to be of practical value for screening plants for tolerance to air pollutants.

Influence of source-sink alterations on photosynthesis and nitrogen fixation in pigeon pea

The treatment of pigeon pea (Cajanus cajan L.) designed to enhance the photosynthetic source-sink ratio (depodding) resulted in increased total and specific nodule activities by 122 and 159%, respectively. Conversely, treatments designed to reduce the photosynthetic source-sink ratio, i.e. shading and defoliation, reduced total nodule activity by 33 and 39%, respectively. Compared with control, depodding, shading and defoliation decreased shoot respiration, but root and nodule respiration was increased by these treatments. Depodding had no effect on the rate of photosynthesis but this was deshaded in defoliated and shaded plants. Interestingly, during the later growth stages, shaded plants maintained higher photosynthetic rates compared with control because of greater leaf area. No significant effect was observed on dry weight and protein content of seeds at harvest, which indicated that no advantage accrued to the plant as a result of enhanced nitrogen fixation by depodding.

Effects of Solar Radiation Regimes on Growth and N2 Fixation of Soybean, Cowpea, and Bushbean1

AbstractLegumes are commonly grown in the tropics under reduced solar radiation due to dense cloud cover and shading from plantation or tall intercrops. This paper examines the growth and N2 fixation of several grain legumes common to the tropics on an Oxic Haplustoll in Hawaii. Plantings were made in April (warm season) and November (cool season) of soybean (Glycine max L. Merr.) ‘Kahala’; cowpea (Vigna unguiculata Walp.) ‘TVu 1190’ (April) and ‘TVu 4557’ (Nov.); and bushbean (Phaseolus vulgaris L.) ‘Burpee Tenderpod’. Polypropylene netting was used to obtain 27, 45, 70, and 100% of the incoming solar radiation through the entire crop cycle. Consistent shading effects were with the three legumes during both warm and cool seasons even though absolute yields were lower in the cool season due to daylength and temperature sensitivity in soybean and cowpea. Nitrogen fixation parameters generally followed dry matter (DM) and seed yields. Total nodule activity (TNA) was very low in all the legumes at 27% sun. With the exception of bushbean at 27% sun, shading caused an 34% average reduction in DM and grain yields, vs a 50% reduction in TNA, and a 76% reduction in nodule mass. Soybean TNA peaked 3 to 4 weeks after flowering, cowpea (cool season) TNA peaked at flowering, and bushbean TNA peaked slightly before or at flowering. There was a tendency for a delay (≈ 2 weeks) at 70, 45, and 27% sun in the final stages of TNA, for summer and cool‐season soybean and bushbean. These delays were not noted for cowpea or cool‐season bushbean. Of the three legumes, cowpea was the least shade tolerant. It produced more DM at full sun than either soybean or bushbean, but at 27% sun, DM yields were similar for all three crops. The response of soybean to shade was intermediate with no reduction in DM and grain yield at 70% sun. Bushbean was the least affected by shading; in fact, maximum yields were obtained at 70% sun. At full sun, their leaves were yellow and seed weight was low; this problem could be corrected by N fertilization.

Effect of 2,4-DB on Soybeans (Glycine max)

Field and greenhouse studies conducted in North Carolina in 1981 and 1982 evaluated the effect of the dimethylamine salt of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] on soybeans. In the field, 2,4-DB at and above 0.07 kg ae/ha reduced soybean shoot mass, leaf nitrogen levels, and nodule activity. Soybean seed yields were reduced by 2,4-DB at and above 0.07 kg/ha. In greenhouse studies, total nodule activity was reduced by 0.07 and 0.14 kg/ha of 2,4-DB. Soybeans treated with 2,4-DB at initiation of flowering (R1 stage) or when pods were 1.9 cm long (R4 stage) yielded progeny with visible injury symptoms. Seedling emergence was also reduced in progeny of soybeans.

Effect of gibberellins and the growth retardant CCC on the nodulation of soya

The effect of exogenous applications of gibberellins (GAs) or the growth retardant β-chloroethyltrimethylammonium chloride (CCC) on root nodule formation and activity (C2H2-reduction) in soya was studied. Daily foliar application of GA3 (2.89×10−6 M) delayed the formation of nodule initials and reduced the numbers mass nodule−1 and specific activity of nodules by 43%, 31% and 47% respectively, without affecting plant growth. Similar effects on nodulation were produced by foliar application of GA4 (3.01×10−5 M) or GA7 (3.03×10−5 M), or by the addition of GA3 (2.89×10−6 M) to the rooting medium. GA effectiveness in reducing nodule numbers was decreased by delaying its application until after the initial infection process had occurred, but the nodules formed were smaller and less active than those of the untreated control plants. The GA effect on nodulation and nodule activity was not associated with alterations in root exudate or due to a direct inhibitory effect of the hormone on the nitrogenase system. When the endogenous root content of GA-like substances was reduced (86% decrease) by foliar application of CCC (6.30×10−5 M), nodule numbers were increased by 56%, but nodule size and total nodule activity were similar to those of control plants. The GA and CCC treatments had no effect on rhizobial growth in liquid culture nor on root colonisation by rhizobia. The results suggest that the endogenous content of root GA may have a regulatory role in both the infection process and in subsequent nodule morphogenesis, thus controlling both the number and effectiveness of the root nodules formed.

EFFECT OF ROOT, SHOOT AND RHIZOBIUM STRAIN ON NITROGEN FIXATION IN FOUR ASIATIC VIGNA SPECIES

SUMMARYReciprocal root: shoot grafts of Vigna angularis, Vigna mungo, Vigna radiata and Vigna umbellata were used to study the effects of root and shoot genotype and Rhizobium strain on growth, nodulation and nitrogen uptake of plants grown with nitrogen‐free nutrient solution in sand culture.There were large and significant effects of root genotype, shoot genotype and Rhizobium strain on plant and nodule characters. Interaction effects were invariably small.Effects of both shoot and root genotype on total nodule activity and nitrogen fixation appeared to be due to effects on nodule fresh wt, while those of Rhizobium strain were associated primarily with specific nodule activity. It is suggested that shoot genotype differences in nodule fresh wt arose because of differences in leaf area. Some implications for genetic manipulation of the symbiosis in the Vigna species are discussed.

Effect of Atmospheric CO2 Enrichment on Growth, Nonstructural Carbohydrate Content, and Root Nodule Activity in Soybean

The objective of this study was to determine whether the supply of current photosynthate was limiting root nodule activity. Both short-term (36 hours) and long-term (16 days) periods of CO(2) enrichment were imposed on vegetative, growth chamber-grown soybean plants (Glycine max. [L.] Merr. cv. ;Clay') to increase the supply of current photosynthate and to observe the effects on photosynthate partitioning in the plants, plant growth, and root nodule activity.Neither total nor specific nodule activities were increased during exposure to short-term (36 hours) CO(2) enrichment. Dry weight of the leaves increased after 12, 24, and 36 hours of CO(2) enrichment and dry weight of the stems plus petioles increased after 36 hours of CO(2) enrichment. Dry weights of the roots and nodules were not altered by short-term CO(2) enrichment. Short-term CO(2) enrichment increased the total nonstructural carbohydrates in the leaves and stems plus petioles, but not in the roots and nodules. Analyses of the separate pools of carbohydrate reserves indicated that the majority of the additional carbohydrate provided by short-term CO(2) enrichment was stored as leaf starch with relatively little being partitioned to the roots and nodules.Long-term CO(2) enrichment (16 days) did not enhance specific nodule activity. Shoot, root, and nodule dry weights were increased 109, 34%, and 56% respectively. Total nodule activity per plant was significantly enhanced only after 16 days of treatment and was related to increased nodule mass. These results indicate that the increased total nodule activity in response to CO(2) enrichment is a consequence of a general growth response of the plant.Results of both studies indicate that nodule activity was not directly limited by current photosynthesis but rather by the partitioning and utilization of photosynthate in the plant.

Water Stress Effects on CO2 Assimilation, Photosynthate Partitioning, Stomatal Resistance, and Nodule Activity in Soybean1

14CO2 assimilation, photosynthate distribution, stomatal resistance, and acetylene reduction activity of vegetative soybean plants [Glycine max (L.) Merr., var. ‘Clay’] were examined 3 hours after pulse labeling the leaves with 14CO2. The plants had previously been water stressed for 20 hours by polyethylene glycol additions to the culture solution. Photosynthetic CO2 exchange rate (CER) and acytelene reduction activity of control and water stressed soybean plants were also monitored for 6 hours following polyethylene glycol induced water stress.14CO2 assimilation and specific nodule activity (SNA) decreased and stomatal resistance (r8) increased with increasing water stress in the 14CO2 labeling experiment. There was a significant negative correlation (r=−0.71) between SNA and r8• Dry weights of leaves, of stems plus petioles, and of nodules were significantly decreased in the water stress treatments, while root dry weights increased. Water stress resulted in a redistribution of 14C with a greater proportion of the 14C being translocated to the roots and nodules at the expense of the leaves.In the time course study, CER decreased after the imposition of an abrupt water stress (−12 bar), but total nodule activity was not affected. It is suggested that in. hibition of nodule activity under long term water stress was not caused solely by the decreased CER, but also by changes in photosynthate pool sizes.

Shading and Temperature as Environmental Factors Affecting Growth, Nodulation, and Symbiotic N2 Fixation by Soybeans1

AbstractSoybeans [Glycine max (L.) Merr.] are subjected to varying amounts of light and temperature changes when grown in the field. A study was made on the effect of shading and different temperatures of the root medium upon growth and N2 fixation of soybeans. Four shading treatments (0, 18, 40, and 62%) were imposed by screen cloths and two fertilizer N levels were used. Soybeans were also grown with four root temperatures (15, 20, 25, and 30 C) and four fertilizer N rates (0, 15, 30, and 45 ppm N). Growth, total N, nonstructural carbohydrate, nodule count, and nodule mass were determined on all experiments and, in addition, acetylene reduction was done on the shading experiment.Plants with no shade produced more dry matter and had higher N content, higher nonstructural carbohydrate, higher nodule mass and number than when shaded. Total nodule activity (acetylene reduction) was greatest at 18% shading, however. Fertilizer N increased nonstructural carbohydrate and specific nodule activity but decreased nodule number and mass, and total nodule activity.Temperature increase from 15 to 30 C resulted in an increase in relative growth rate and N content of plant tops, whereas total N and nonstructural carbohydrate in roots was decreased. There was no significant effect of increasing temperature upon growth of roots or nodule number or nodule mass. The N content of plant tops was increased while nodule number and nodule mass were decreased by fertilizer N application.The most significant finding was that 18% shading produced the highest acetylene reducing activity of soybeans.

Symbiotic Nitrogen Fixation in Soybeans. II. Interrelationship Between Carbon and Nitrogen Assimilation1

Reciprocal intervarietal grafts were made in all root: shoot combinations using seedlings of eight soybean (Glycine max (L.) Merr.) genotypes, grown in sandfilled pots in the greenhouse. After grafting, the plants were inoculated with Rhizobium japonicum (USDA strain #138) and watered with a low‐nitrogen nutrient solution. Two replicate plants of each root:shoot combination were sampled at each of four times, 28, 38, 50, and 58 days after grafting, for photosynthetic rate of leaves; leaf area, shoot and root dry weight; and number, fresh weight, and acetylene reduction activity of nodules.Significant shoot genotype, sampling date, and shoot genotype ✕ sampling date effects occurred for all traits measured. Root genotype effects, independent of the shoot genotype, occurred for all root traits measured, except nodule fresh weight. No overall root effects occurred for any of the shoot traits. Shoot genotype effects were small at the first two sample dates relative to root genotype effects, but were large at the last two sampling dates due to differences in stage of physiological development. Shoot genotype effects on total nodule activity (TNA) per plant were strongly and positively associated with shoot effects on nodule fresh weight, but showed no association with shoot effects on specific nodule activity (SNA). Shoot genotype effects on nodule fresh weight per plant were, in turn, strongly and positively associated with shoot effects on photosynthesis per plant and shoot dry weight. Root genotype effects on TNA per plant were due to inherent differences in SNA.