Limitations on Leaf Nitrate Reductase Activity during Flowering and Podfill in Soybean

The objective of this study was to identify factors which limit leaf nitrate reductase (NR) activity as decline occurs during flowering and beginning seed development in soybean (Glycine max [L.] Merr. cv Clark). Level of NR enzyme activity, level of reductant, and availability of NO₃⁻ as substrate were evaluated for field-grown soybean from flowering through leaf senescence. Timing of reproductive development was altered within one genotype by (a) exposure of Clark to an artificially short photoperiod to hasten flowering and podfill, and (b) the use of an early flowering isoline. Nitrogen (N) was soil-applied to selected plots at 500 kilograms per hectare as an additional variable. Stem NO₃⁻ concentration and in vivo leaf NR activity were significantly correlated (R² = 0.69 with nitrate in the assay medium and 0.74 without nitrate in the medium at P = 0.001) across six combinations of reproductive and soil N-treatment. The supply of NO₃⁻ from the root to the leaf tissue was the primary limitation to leaf NR activity during flowering and podfill. Levels of NR enzyme and reductant were not limiting to leaf NR activity during this period.     

Leaf Nitrate Reductase, d-Ribulose-1,5-bisphosphate Carboxylase, and Root Nodule Development of Genetic Male-Sterile and Fertile Soybean Isolines

The objectives of this study were to determine the effect of pod and seed development on leaf chlorophyll concentration, and on activities of leaf ribulose bisphosphate carboxylase, leaf nitrate reductase, and root nodule acetylene reduction in field-grown soybean (Glycine max [L.] Merr.). Two genetic male-sterile lines and their fertile counterparts (Williams and Clark 63) were compared in both 1978 and 1979. Two additional lines (Wells × Beeson and Wells × Corsoy) were compared in 1979.

The expression of male-sterile character was nearly complete as very little outcrossing due to insect pollinators was observed. Male-sterile plants showed a delayed late season decline in leaf chlorophyll content and ribulose bisphosphate carboxylase activity when compared with fertile plants. A slight delay in the loss of in vivo leaf nitrate reductase activity was also observed for male-sterile plants. Root nodule fresh weight and acetylene reduction activity declined slightly more rapidly for fertile lines than for male-sterile lines in both years with differences significant on the last two to three sampling dates as leaf loss occurred in the control plants.

Seed development was found to increase slightly, the rate of decline of metabolic activity in fertile lines compared with that of male-sterile lines. However, pod development was not an a priori requirement for leaf and root nodule senescence. Male-sterile plants also lost photosynthetic and nitrogen metabolic competence, but at a slower rate. These results support the concept that pod and seed development does not signal monocarpic senescence per se but rather affects the rate at which senescence occurs after flowering.

     

Effect of Atmospheric CO(2) 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 CO₂ 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.     

Nitrate Absorption and Acetylene Reduction by Soybeans during Reproductive Development

Nodulated and unnodulated soybeans (Glycine max [L.] Merr.) were grown in N-free or N-containing nutrient solutions, respectively. Starting at the initial flowering stage, and throughout reproductive growth, the NO₃- absorption capacity of roots of intact plants from both treatments was determined in short-term uptake experiments. Acetylene reduction activity was determined for nodulated plants. Nitrate absorption rate, expressed on a root dry weight basis, was greatest at early flowering for both nodulated and unnodulated plants. At 33 days after germination, the NO₃^- absorption rate of unnodulated plants was twice as great as that of nodulated plants. During the remainder of the sampling period, NO₃^- absorption rates of both nodulated and unnodulated plants decreased progressively and similarly. Maximum nodule specific activity occurred 30 days after germination, or initial flowering. However, maximum total C₂H₂ reduction activity, oner plant basis, was observed during the early stages of pod-filling. Compared to unnodulated plants dependent on NO₃^- assimilation, nodulated plants were smaller, had less N in vegetative tissues, and produced less seed per plant. We suggest that the higher NO₃^- absorption rate of unnodulated soybean roots, particularly during early reproductive growth, may have reflected a more favorable supply of photosynthate translocated to the roots from larger, more vigorous, non-N-stressed shoots.     

Ontogenetic variation of four cytokinins in soybean root pressure exudate

Cytokinins exported from the root may be involved in the correlative control of plant development. To test this hypothesis in soybean ((Glycine max [L.] Merr. cv. McCall, cv Chippewa 64, and cv Hodgson 78), cytokinins were intercepted en route from the root to the shoot by collecting root pressure exudate from detopped roots. The quantities of four cytokinins in the exudate were studied throughout the development of plants grown in the field and in controlled environment chambers. Zeatin, zeatin riboside, and their dihydro derivatives, dihydrozeatin and dihydrozeatin riboside, were isolated and quantitated using high-performance liquid chromatography.

Cytokinin fluxes (pmoles per plant per hour) were independent of exudate flux (grams per plant per hour). All fluxes are averages for a 6- or 8-h collection period. The ribosides accounted for the majority of the observed cytokinin transport. The fluxes of zeatin riboside and dihydrozeatin riboside increased from low levels during vegetative growth to maxima during late flowering or early pod formation. Before the seeds began rapid dry matter accumulation, zeatin riboside and dihydrozeatin riboside fluxes decreased and remained at low levels through maturation. The fluxes of zeatin and dihydrozeatin were low throughout development.

No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).

The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.

     

Effects of defoliation and shading on the physiological cost of reproduction in silky locoweed Oxytropis sericea

Background

The production of flowers, fruits and seeds demands considerable energy and nutrients, which can limit the allocation of these resources to other plant functions and, thereby, influence survival and future reproduction. The magnitude of the physiological costs of reproduction depends on both the factors limiting seed production (pollen, ovules or resources) and the capacity of plants to compensate for high resource demand.

Methods

To assess the magnitude and consequences of reproductive costs, we used shading and defoliation to reduce photosynthate production by fully pollinated plants of a perennial legume, Oxytropis sericea (Fabaceae), and examined the resulting impact on photosynthate allocation, and nectar, fruit and seed production.

Key Results

Although these leaf manipulations reduced photosynthesis and nectar production, they did not alter photosynthate allocation, as revealed by ¹³C tracing, or fruit or seed production. That photosynthate allocation to reproductive organs increased >190 % and taproot mass declined by 29 % between flowering and fruiting indicates that reproduction was physiologically costly.

Conclusions

The insensitivity of fruit and seed production to leaf manipulation is consistent with either compensatory mobilization of stored resources or ovule limitation. Seed production differed considerably between the two years of the study in association with contrasting precipitation prior to flowering, perhaps reflecting contrasting limits on reproductive performance.     

Effects of resource availability, and fruit and ovule position on components of fecundity in Alliaria petiolata (Brassicaceae)

In four field and glasshouse experiments designed to alter the supply of resources through manipulation of nutrients, root tissue, leaf area and fruit number in Alliaria petiolata (Brassicaceae), more than 99% of ovules per plant showed signs of fertilization, suggesting that seed production in this plant was not pollen limited. However, in all treatments a significant proportion of fruits and seeds did not develop to maturity. Total fruit and seed production did not differ significantly from controls when plants were given nutrient supplements at flowering. Removal of 50–75% of the root tissue in 1-yr-old plants significantly reduced fruit set, but had no effect on individual seed development. Removal of cauline leaves significantly reduced most measures of fruit and seed production, suggesting that current photosynthate is critical for fruit and seed filling. Seed maturation was significantly affected by both fruit position within an infructescence and ovule position within a fruit. Basally located fruits and ovules (within fruits) developed more mature seeds than distally positioned fruits and ovules. Plants responded to removal of basal fruits by re-allocating resources to distal fruits that would normally have aborted. Our results suggest that fruits and seeds act as reproductive sinks competing for parental photosynthate. Patterns of resource allocation within infructescences and fruits were also modified by our experiments.     

Effect of changes in shoot carbon-exchange rate on soybean root nodule activity

The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean (Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N₂ fixation. Shoot CER, root + nodule respiration, and apparent N₂ fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N₂ fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N₂ fixation was measured within 4 hours, and decreasing CO₂ concentration from 320 to 90 microliters CO₂ per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO₂ concentration around the shoot from 320 to 1,000 microliters CO₂ per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO₂-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO₂ enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N₂ fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N₂-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known.     

The response and recovery of nitrogen fixation activity in soybean to water deficit at different reproductive developmental stages

Soybean (Glycine max [L.] Merr.) N₂ fixation is a primary plant mechanism responsible for meeting plant-N demand during seed development. Nitrogen fixation is recognized as a drought-sensitive mechanism; however, N₂ fixation response to water deficit and N₂ fixation recovery at different reproductive stages are not well documented. We tested the hypothesis that water deficit during late reproductive stages would inhibit N₂ fixation and lead to the breakdown of essential leaf proteins and an inability to recover N₂ fixation. Acetylene reduction activity (ARA) and N redistribution response to a 5-d drought period at flowering (R2), early seed fill (R5), and late seed fill (R6) were evaluated in one genotype (Hendricks, maturity group 0). Control plants maintained high rates of nodule activity until late seed fill. Plants drought stressed at R2 and R5 recovered ARA after rewatering and in some cases had higher nitrogenase activity than control plants during mid-seed fill. Recovery of ARA on plants stressed at R2 and R5 was associated with higher shoot N concentration than control plants at maturity. Drought stress at R6 reduced ARA, and the inability to recover ARA after stress alleviation at R6 resulted in decreased individual seed mass, which was likely caused by an acceleration of leaf N redistribution and a shorter seed-fill period. Results emphasized the importance of soybean N₂ fixation during late seed development on seed yield and that the ability to recover N₂ fixation following drought is dependent upon crop developmental stage.     

Ontogenetic interactions between photosynthesis and symbiotic nitrogen fixation in pigeonpea

Total nodule nitrogenase activity (TNA, μmols ethylene plant^-1 h^-1) in pigeonpea (Cajanus cajari) increased with plant growth to reach maximum at flowering (75 days after sowing), decreasing thereafter until maturity (120 days after sowing). However, specific nodule nitrogenase activity (SNA, μmols ethylene g^-1 nodule fresh wt h^-1) reached its maximum earlier (45 days after sowing). The rate of photosynthesis and shoot and nodule respiration followed a similar pattern to TNA. However, higest rates of root respiration were observed at flowering and again immediately before final harvest. ¹⁴CO₂ feeding studies showed that assimilates produced in leaves before flowering were retained in the vegetative parts. Assimilates produced after flowering were exported to the reproductive structure at the expense of the nodules. It is suggested that the decreased availability of photosynthate to nodules decreased nitrogen fixation.     

Light and Shade Effects on Abscission and C-Photoassimilate Partitioning among Reproductive Structures in Soybean

Field experiments were conducted in 1981 and 1982 to study the effects of low-irradiance supplemental light on soybean (Glycine max [L.] Merr. cv Evans) flower and pod abscission. Cool-white and red fluorescent lights illuminated the lower part of the soybean canopy during daylight hours for 3 weeks late in flowering. At the same time, flowers and young pods on half the plants were shaded with aluminum foil. Flowers were tagged at anthesis and monitored through abscission or pod maturity.

Responses to red and white lights were similar. Supplemental light tended to reduce abscission and increase seed weight per node compared to natural light. Shading flowers and pods increased abscission and reduced seed weight per node. Number of flowers produced per node, individual seed weight, and seeds per pod were not affected by light or shade treatments.

Further studies examined the effects of shading reproductive structures on their capacity to accumulate ¹⁴C-photoassimilates. Individual leaves were pulse labeled with ¹⁴CO₂ 1, 2, and 4 weeks post anthesis. Flowers and pods in the axil of the labeled leaf were covered with aluminum foil 0, 24, 72, and 120 hours before pulsing.

Shading flowers and pods resulted in a 30% reduction in the relative amount of radiolabel accumulated from the source leaf. The reduction in ¹⁴C accumulation due to shading was evident regardless of the length of the shading period and was most pronounced when the shades were applied early in reproductive development. We conclude that light perceived by soybean flowers and young pods has a role in regulating both their abscission and their capacity to accumulate photoassimilates.

     

大豆-根瘤菌共生体系光合与固氮关系研究

水培大豆和田间生长的大豆,接种根瘤菌 Rhizobium B16-11C 后植株全氮含量、叶片叶绿素含量和净光合速率及种子产量都明显增加。比较 Clark 大豆的结瘤品系和不结瘤品系获类似结果。摘除根瘤后3天内叶片净光合速率无明显变化。大豆植株遮阴、去叶或切掉地上部导致根瘤活性明显下降。但去豆荚不能提高根瘤固氮的比活性。根瘤活性的日变化不能用根瘤蔗糖、淀粉含量或周围温度的变化来解释,其控制因子尚待深入研究。     

Studies on the Relationship Between Photosynthesis and Nitrogen Fixation in the Symbiotic System of Soybean and Nodule Bacteria( Rhizobium)

The relationship between photosynthesis of soybean and nitrogen fixation of the nodules by symbiotic Rhizobium was studied. The contents of total nitrogen and chlorophyll, the net photosynthetic rate and seed yield of soybean were much higher in either hydroponically cultivated or field-grown plants inoculated with Rhizobium B16–11C (or Clark nodulating strain) than in control without inoculation (or Clark non-nodulating strain). These results show that the symbiotic nitrogen fixation has a beneficial effect on photosynthesis. However, the effect was indirect and slow so that there was no change in the net photosynthetic rate of the soybean leaves until three clays after removing nodules from the soybean roots. On the other hand, decreasing the photosynthate supply to nodule by shade, defoliation or shoot removal of the soybean, the nodule activity declined significantly. It seems that the supply of photosynthate to root nodule is a limiting factor for symbiotic nitrogen fixation. However, the diurnal variation of the nodule activity could not be explained by change neither in the contents of sucrose and starch of the root nodules nor in the ambient temperature. The factor controlling the diurnal variation deserves further study.     

Effect of pod removal on leaf photosynthesis and soluble protein composition of field-grown soybeans

Well nodulated, field-grown soybeans (Glycine max [L.] Merr. var Williams) were depodded just prior to seed development and near mid pod-fill. Both treatments caused a considerable increase in leaf dry weight, suggesting continued photosynthate production following pod removal. Moreover, depodding had a marked effect on leaf soluble protein without affecting total proteolytic activity. Early depodding caused a 50% increase in leaf protein, and both early and late depodding caused the retention of protein for several weeks following the decline in control leaves. But despite this retention of protein, leaves of depodded plants showed no difference in the onset of the irreversible decline in photosynthesis. Therefore, although depodding delayed the loss of leaf chlorophyll and protein, it did not delay the onset of functional leaf senescence and in fact, actually appeared to enhance the rate of decline in photosynthesis. There was a good correlation between the irreversible decline in ribulose bisphosphate carboxylase (activity and amount) and that of photosynthesis. In contrast, the correlation did not seem as good between stomatal closure and the onset of the irreversible decline in photosynthesis. The reason total soluble protein remained high following depodding while carboxylase, which normally comprised 40% of the soluble protein, declined was because several polypeptides increased in amounts sufficient to offset the loss of carboxylase. This change in leaf protein composition indicates a change in leaf function; this is discussed in terms of other recent findings.     

Soybean lectin as a component of a composite biopreparation involving Bradyrhizobium japonicum 634b

The effects of the composite biopreparation Bralec (involving the soybean-specific root nodule bacterium Bradyrhizobium japonicum strain 634b and soybean lectin at concentrations of 500, 50, and 5 μg/ml as major components) on the development and functional activity of soybean-rhizobium symbiosis (development phases of one leaf, four true leaves, and budding) was studied. It was demonstrated that pretreatment of seed with this preparation stimulated the development of both the macro-and microsymbionts. The experimental plants displayed an active accumulation of biomass (4–42% higher compared with the variant with inoculation), development of root nodules (the number increased by 11–110% and the weight by 27–157%), and elevated nitrogen-fixing activity (by 45–204%). The soybean yield increased by 8–10% upon treatment with Bralec 500 and Bralec 5 as compared with the traditional seed bacterization with root nodule bacteria.     

Effect of light and atmospheric carbon dioxide concentration on nitrogen fixation by herbage legumes

Summary Lucerne, red clover and white clover were grown at two atmospheric concentrations of CO₂ (300 and 1000 μl l⁻¹) and the effects on N₂ fixation, nodule mass/number and root/shoot dry matter production determined. Pea plants were similarly evaluated as a comparison with grain legumes. CO₂ enrichment increased N₂ fixation activity in all cases but activity/unit nodule mass was significantly increased only in the pea. The enhancement of N₂ fixation in herbage legumes by CO₂ enrichment reflected an increase in nodule mass which in turn was attributed to increased nodule number, and results show that under the experimental conditions obtaining here photosynthate supply did not limit nodule N₂ fixation in these plants though it was limiting in the case of peas. White clover growing in a 6 and 14 hour photoperiod was studied for response of the N₂ fixing system to light. Long photoperiod (14 hour) plants assayed at constant temperature (20°C) did not show a significant response to light at the end of the dark period either in terms of fixation per plant or per unit nodule mass, in contrast with short photoperiod (6 hour) plants which showed significant responses. Short photoperiod plants compensated for reduced photosynthates by maintaining only half the root nodule mass and fixation activity of 14 hour photoperiod plants though plants in both systems supported similar rates of N₂ fixation per unit mass of nodule during the photoperiod. Comparison of N₂ fixation activities in whole and decapitated plant systems indicates the importance of shoot reserves for sustaining nitrogenase activity in white clover during short-term interruption of photosynthesis. These results support the conclusion of the CO₂ enrichment studies, that herbage legumes have the potential for supplying their nodule photosynthate requirements for sustaining optimum rates of N₂ fixation and excess carbon supply is used solely to promote further nodulation. Nodules of short photoperiod white clover plants were less efficient in N₂ fixation in that they evolved more H₂ relative to N₂ (C₂H₂) reduced than did long photoperiod plants.     

Studies on Genetic Male-Sterile Soybeans : II. Effect of Nodulation on Photosynthesis and Carbon Partitioning in Leaves

Soybean (Glycine max L. Merr.) germplasm, essentially isogenic except for loci controlling male sterility (ms₁) and nodulation (rj₁), were developed to study the effects of reproductive development and nitrogen source on certain aspects of photosynthesis. Plants were sampled from flowering (77 days after transplanting) until maturity (150 days after transplanting). With all four genotypes, net carbon exchange rates were highest at flowering and declined thereafter. Photosynthetic rates of the sterile genotypes (nodulated and non-nodulated) declined more rapidly than the fertile genotypes, and after 105 days, both sterile genotypes maintained low but relatively constant carbon exchange rates (<3 milligrams CO₂/gram fresh weight per hour). Photosynthetic rates and starch accumulation (difference between afternoon and morning levels) declined with time. The sterile genotypes attained the highest morning starch levels, which reflected reduced starch mobilization. After 92 days, the proportion of photosynthetically fixed carbon that was partitioning into starch (relative leaf starch accumulation) in the sterile genotypes increased dramatically. In contrast, relative leaf starch accumulation in the fertile genotypes remained relatively constant with time. Throughout the test period, all four genotypes maintained leaf sucrose levels between 5 and 15 micromoles glucose equivalents per gram fresh weight.

The activities of sucrose phosphate synthase (SPS) in leaf extracts of the four genotypes declined from 77 to 147 days. Nodulated genotypes tended to maintain higher activities (leaf fresh weight basis) than did the non-nodulated genotypes. In general, relative leaf starch accumulation was correlated negatively with the activity of SPS (normalized with leaf net carbon exchange rate) in leaf extracts for all four genotypes during early reproductive development, and for the fertile genotypes at all sampling dates. In contrast, leaf sucrose content was correlated positively with SPS activity during early reproductive development. These results suggested that a direct relation existed between the activity of SPS and starch/sucrose levels in soybean leaves. However, the interaction between these processes also may be influenced by other factors, particularly when leaf photosynthetic rates and plant demand for assimilates is low, as in the sterile genotypes.

     

Soybean root and nodule nitrate reductase

Nitrate reductase (NR) activity was followed in root and nodule from Glycine max (L.) Merr. (Cv. Tracy) inoculated with Rhizobium japonicum . Initially, a plus NO^3- in vivo assay was used. When chlorate-resistant mutants were used as inoculum, nodule NR activity was reduced by about 90%. indicating that the bacteroid accounts for much of the normal nodule's NR. With plants 3 to 15 weeks of age nodule NR activity (g fresh weight)^-1 was highest in young plants and root activity highest in old plants. Root and nodule total NR activity increased with plant age and were often not greatly different. Root NR activity correlated with plant NO^3- supply and increased from 0.8 to 11.4 μmol plant^-1 h^-1 as NO^3- was increased from 0 to 3 m M . In contrast, nodule NR activity was high in plants grown without NO^3- and did not appear to increase as nitrate supply to the plant was increased. Nodule activity was 6 to 14 μmol NO^2- plant^-1 h^-1. Use of a minus NO^3- in vivo assay had little affect on root NR activity, but greatly reduced nodule activity. Root tissue was found to have 5 to 38 times more NO^3- than nodule tissue. It is concluded that low nitrate levels within the nodule limit NR activity and that it is improbable that the nodule is a major site of plant nitrate reduction.     

Distribution and Metabolism of sym-Homospermidine and Canavalmine in the Sword Bean Canavalia gladiata cv Shironata

The unusual polyamines, sym-homospermidine (homoSPD) and canavalmine (CAN), were found in the seed of Canavalia species such as C. gladiata, C. ensiformis, and C. brasilensis, but not in those of other leguminous crops. To examine the distribution and metabolism of homoSPD and CAN in sword bean, C. gladiata cv Shironata, polyamine analysis was carried out throughout the life cycle of this plant. During seed germination, putrescine (PUT), spermidine (SPD), and spermine (SPM) were accumulated in the radicle and hypocotyl. HomoSPD and CAN were, however, maintained at very low levels over a 6-day period of germination. In nodulated sword bean plants, a large quantity of homoSPD was found in the root nodule. CAN was detected exclusively in the senescent nodule at very low concentrations. These polyamines were not detected in any other organs including root, stem, leaf, vine, flower, and pod, while PUT, SPD, and SPM were always found in those organs. As plants reached the reproductive stage, homoSPD and CAN appeared in the immature seed and their concentrations increased as seed formation progressed. By contrast, the level of SPM continuously decreased during seed development. In developing seeds, considerable accumulation of canavanine, an analog of arginine, which is a precursor in polyamine biosynthesis, was also observed.     

The Glycine-Glomus-Bradyrhizobium symbiosis. IX. Nutritional, morphological and physiological responses of nodulated soybean to geographic isolates of the mycorrhizal fungus Glomus mosseae.

The objective of the work was to determine differences in plant response to geographic isolates of a vesicular-arbuscular mycorrhizal (VAM) fungus, and to demonstrate the need for such determinations in the selection of desirable host-endophyte combinations for practical applications. Soybean ( Glycine max (L.) Merr.) plants were inoculated with Bradyrhizobium japonicum and isolates of the VAM-fungal morphospecies Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe, collected from an arid (AR), semiarid (SA) or mesic (ME) area. Inoculum potentials of the VAM-fungal isolates were determined and the inocula equalized, achieving the same level of root colonization (41%, P >0.05) at harvest (50 days). Plants of the three VAM treatments (AR, SA and ME) were evaluated against von VAM controls. Significant differences in plant response to colonization were found in dry mass, leaf K, N and P concentrations, and in root/shoot, nodule/root, root length/leaf area and root length/root mass ratios. The differences were most pronounced and consistent between the AR and all other treatments. Photosynthesis and nodule activity were higher ( P <0.05) in all VAM treatments, but only the AR plants had higher ( P <0.05) photosynthetic water-use efficiency than the controls. Nodule activity, evaluated by H₂ evolution and C₂H₂ reduction, differed significantly between treatments. The results are discussed in terms of nutritional and non-nutritional effects of VAM colonization on the development and physiology of the tripartite soybean association in the light of intraspecific variability within the fungal endophyte.