Half Of Root System Research Articles

Transcriptomic Profiling Revealed Genes Involved in Response to Drought Stress in Alfalfa

Drought stress is a primary abiotic stress that causes the crop losses worldwide. Under drought conditions, root growth is determined by the action of environmental drought stress through specific genes that adapt root development to water-limited conditions. We observed that root length, root-to-shoot ratio, and ABA content increased under water stress. To precisely identify drought-responsive genes and depict the drought response of alfalfa roots at the transcriptomic level, alfalfa (Medicago sativa L.) was grown in a split-root system in the experiment. Half of the alfalfa root system was watered at field capacity, and the other half of the root system was subjected to drought conditions. Illumina sequencing using RNAs obtained from these two half-root systems was performed to identify the drought-responsive gene expression patterns of alfalfa. In total, 5484 differentially expressed genes (DEGs) were identified and DEGs were significantly enriched in 268 GO terms and 14 KEGG pathways. Gene response to microorganism and associated membrane components were determined to be strongly induced by drought conditions. The major pathways involved in the drought response were involved in plant-pathogen interactions, plant hormone signal transduction, MAPK, and phenylpropanoid biosynthesis pathways. In addition, 392 lncRNAs and 1538 TFs that participated in the alfalfa root response to drought were identified. The major TFs involved in regulating the drought response included NAC, WRKY, bHLH, MYB, and B3. The results of this study may provide a reference for further research investigating drought-responsive gene expression and regulation patterns in the alfalfa roots.

Sugarcane regrowth is dependent on root system size: an approach using young plants grown in nutrient solution

ABSTRACT The root system is essential for sugarcane regrowth and the vigor of ratoon cycles as it represents the unique source of carbon skeletons and energy for the initial plant development. However, root system dynamics after shoot harvesting and its role in sugarcane regrowth remains poorly known. Here, it was hypothesized that sugarcane plants with small volume of root system will accumulate less biomass after shoot harvesting than plants with larger volume and that such regrowth is dependent on root reserves. In sugarcane plants grown in nutrient solution, shoots were cut, and two root treatments were established: reference plants with the entire root system (100%); and plants with half of the root system (50%), randomly removing half of root system. After 37 days of shoot harvesting, plants with the entire root system showed higher shoot, root and total dry mass, root length, root diameter, root area and root volume, when compared with those with 50% of the root system. Sugarcane plants with the entire root system had higher root content of starch, soluble sugar and nonstructural carbohydrates as compared to plants with 50% of the root system. A significant positive correlation was found between the variation of shoot dry mass and the variation of root nonstructural carbohydrates. Interestingly, this data revealed a disproportionate effect of root system size on sugarcane regrowth, with plants with the entire root system accumulating almost three times more biomass than plants with half of the root system during regrowth.

Revisiting Fe/S interplay in tomato: A split-root approach to study the systemic and local responses

Based on our previous studies demonstrating an intriguing interplay between sulfur (S) and iron (Fe), a split-root experiment was performed to determine whether plant S status and/or S external concentration could modify plant capability to take up and accumulate Fe. This split-root system allowed the roots of each tomato plant to grow in two different compartments, both Fe-deficient, but one S-sufficient, and the other one S-free.Although S was freely available to half root system and thus plant S status was preserved, S-deficient part of root apparatus exhibited a decrease of total S, thiols and protein content, an enhanced activity of both ATPsulfurylase and O-acetylserine(thiol)lyase, and a higher expression of SlST1.1, as occurring under S deficiency. The side of the root apparatus exposed to combined S and Fe deficiency, showed an over induction of the FeIII-reducing capacity (+40%) and of the expression levels of the gene codifying for this protein (SlFRO1), with respect to the Fe-deficient part of the root system. Interestingly, the regulation pattern of the bHLH transcription factor SlFER, controlling the expression of both SlFRO1 and SlIRT1 genes, was very close to that of SlFRO1. SlIRT1 expression levels appeared unaffected by S supply, suggesting distinct regulatory processes targeting SlFRO1 and SlIRT1.

Effect of partial root-zone irrigation system on seed quality changes of Persian clover (Trifolium resupinatum L.) during seed development and maturation

A field experiment was conducted in Aleshtar, Iran during 2015-16 growing season to evaluate the effect of limited water supply during seed filling period on seed quality changes of Persian clover. The experiment was carried out as a 2×6 factorial with three replications. Irrigation systems including conventional (all root system was irrigated) and partial root-zone irrigation (half of root system exposed to dry soil and the other half was watered) and harvest time (initiated from 10 days after flowering (DAA) and continued on six occasions at 5-day intervals) were used as treatments. Germination percentage, seedling dry mass, electrical conductivity and 1000-seed mass were used as seed quality traits. Seed quality attributes were significantly affected by harvest time and irrigation system, but the interaction of irrigation system × harvest time was not significant. While seed mass and electrical conductivity were not significantly affected by irrigation system, seedling dry mass and germination percentage were reduced under deficit irrigation. Partial root-zone irrigation reduced seed quality, Persian clover needs to experience no water deficit during seed formation and maturation period to produce high quality and quantity seeds.

Potential role of D-myo-inositol-3-phosphate synthase and 14-3-3 genes in the crosstalk between Zea mays and Rhizophagus intraradices under drought stress.

Arbuscular mycorrhizal (AM) symbiosis is known to stimulate plant drought tolerance. However, the mechanisms underlying the synergistic responses of the symbiotic partners to drought stress are largely unknown. A split-root experiment was designed to investigate the molecular interactions between a host plant and an AM fungus (AMF) under drought stress. In the two-compartment cultivation system, an entire or only a half root system of a maize plant was inoculated with an AMF, Rhizophagus intraradices, in the presence of localized or systemic drought treatment. Plant physiological parameters including growth, water status, and phosphorus concentration, and the expression of drought tolerance-related genes in both roots and R. intraradices were recorded. Although mycorrhizal inoculation in either one or both compartments systemically decreased abscisic acid (ABA) content in the whole root system subjected to systemic or local drought stress, we observed local and/or systemic AM effects on root physiological traits and the expression of functional genes in both roots and R. intraradices. Interestingly, the simultaneous increase in the expression of plant genes encoding D-myo-inositol-3-phosphate synthase (IPS) and 14-3-3-like protein GF14 (14-3GF), which were responsible for ABA signal transduction, was found to be involved in the activation of 14-3-3 protein and aquaporins (GintAQPF1 and GintAQPF2) in R. intraradices. These findings suggest that coexpression of IPS and 14-3GF is responsible for the crosstalk between maize and R. intraradices under drought stress, and potentially induces the synergistic actions of the symbiotic partners in enhancing plant drought tolerance.

Severity and duration of osmotic stress on partial root system: effects on root hydraulic conductance and root growth

Effects of severity and duration of osmotic stress on partial root system were investigated on root hydraulic conductance (L p ) and growth in each sub-root system. The maize seedlings were raised in nutrient solution with the roots divided equally into two containers. At 12 days after transplanting, half of root system was subjected to osmotic stress of −0.2, −0.4 and −0.6 MPa using PEG 6000 added in the solution, and no PEG 6000 solution was taken as control (CK). L p and root growth in each sub-root system were measured at 0, 0.25, 0.5, 1, 3, 5, 7 and 9 days after treatment (DAT). Results show that compared to CK, −0.2 MPa treatment enhanced the hydraulic conductance per root area (L pr ) in non-stressed sub-root system by 12 % at 0.5 DAT. In −0.4 MPa treatment, L pr in non-stressed sub-root system was significantly increased at 9 DAT. But in −0.6 MPa treatment, significant decrease of 12–40 % was observed in non-stressed sub-root system during 0.5–3 DAT if compared to CK, indicating that the threshold of osmotic stress for the compensatory effect of water uptake in non-stressed sub-root system was between −0.2 and −0.4 MPa. In addition, the root area growth rate in non-stressed sub-root system was significantly higher than that in CK during 0.25–0.5, 1–3 and 5–7 DAT for −0.2, −0.4 and −0.6 MPa treatments, respectively. Similar trend also was observed for root length growth rate, indicating that osmotic stress could still stimulate the compensatory effect of root growth other than water uptake under partial root system. Moreover, the occurrence of such compensatory effect delayed with the increasing osmotic stress. Meanwhile, −0.2 MPa treatment had no significant effect on maize shoot growth but −0.4 and −0.6 MPa treatments inhibited shoot biomass accumulation, which resulted in higher root/shoot ratio in −0.4 and −0.6 MPa treatments. Thus root plasticity in hydraulic conductance and root growth in different root zones varied largely depending on the severity and duration of osmotic stress under partial root system.

Localized and non-localized effects of arbuscular mycorrhizal symbiosis on accumulation of osmolytes and aquaporins and on antioxidant systems in maize plants subjected to total or partial root drying.

The arbuscular mycorrhizal (AM) symbiosis alters host plant physiology under drought stress, but no information is available on whether or not the AM affects respond to drought locally or systemically. A split-root system was used to obtain AM plants with total or only half root system colonized as well as to induce physiological drought affecting the whole plant or non-physiological drought affecting only the half root system. We analysed the local and/or systemic nature of the AM effects on accumulation of osmoregulatory compounds and aquaporins and on antioxidant systems. Maize plants accumulated proline both, locally in roots affected by drought and systemically when the drought affected the whole root system, being the last effect ampler in AM plants. PIPs (plasma membrane intrinsic proteins) aquaporins were also differently regulated by drought in AM and non-AM root compartments. When the drought affected only the AM root compartment, the rise of lipid peroxidation was restricted to such compartment. On the contrary, when the drought affected the non-AM root fraction, the rise of lipid peroxidation was similar in both root compartments. Thus, the benefits of the AM symbiosis not only rely in a lower oxidative stress in the host plant, but it also restricts locally such oxidative stress.

Local and systemic N signaling are involved in Medicago truncatula preference for the most efficient Sinorhizobium symbiotic partners

• Responses of the Medicago truncatula-Sinorhizobium interaction to variation in N₂-fixation of the bacterial partner were investigated. • Split-root systems were used to discriminate between local responses, at the site of interaction with bacteria, and systemic responses related to the whole plant N status. • The lack of N acquisition by a half-root system nodulated with a nonfixing rhizobium triggers a compensatory response enabling the other half-root system nodulated with N₂-fixing partners to compensate the local N limitation. This response is mediated by a stimulation of nodule development (number and size) and involves a systemic signaling mechanism related to the plant N demand. In roots co-infected with poorly and highly efficient strains, partner choice for nodule formation was not modulated by the plant N status. However, the plant N demand induced preferential expansion of nodules formed with the most efficient partners when the symbiotic organs were functional. The response of nodule expansion was associated with the stimulation of symbiotic plant cell multiplication and of bacteroid differentiation. • A general model where local and systemic N signaling mechanisms modulate interactions between Medicago truncatula and its Sinorhizobium partners is proposed.

Avaliação de acessos de Psidium spp. quanto à resistência a Meloidogyne enterolobii

Quarenta e sete acessos de Psidium spp. foram avaliados em casa de vegetação quanto à resistência a M. enterolobii, visando ao desenvolvimento de porta-enxertos e cultivares resistentes. Cinco a sete mudas de cada acesso foram produzidas por propagação vegetativa ou a partir de sementes e, no estádio de quatro pares de folhas, foram submetidas à inoculação com 500 ovos desse nematoide. Cento e trinta e cinco dias após, extraiu-se metade do sistema radicular das plantas (cortado ao longo de seu eixo longitudinal) para extração de ovos. As mudas foram replantadas com as raízes restantes e mantidas vivas em casa de vegetação. As contagens de ovos foram feitas em três alíquotas de 1 mL/planta, e multiplicadas por dois para obtenção da população final (Pf). As Pf's de todas as alíquotas foram submetidas a ANOVA, a qual indicou diferenças significativas (p<0,01) entre acesssos e entre plantas de mesmo acesso. A classificação dos acessos quanto à resistência foi feita pelo fator de reprodução (FR=Pf/500). Todas as plantas do araçazeiro (P. cattleyanum) (acessos 115 e 116) foram resistentes (FR<1) a M. enterolobii, enquanto que em outros araçás e goiabas houve um número variável de indivíduos com FR abaixo ou pouco acima de 1. Estas plantas serão multiplicadas vegetativamente e reavaliadas quanto à resistência a M. enterolobii.

Nitrogen Fixation Control under Drought Stress. Localized or Systemic?

Legume-Rhizobium nitrogen fixation is dramatically affected under drought and other environmental constraints. However, it has yet to be established as to whether such regulation of nitrogen fixation is only exerted at the whole-plant level (e.g. by a systemic nitrogen feedback mechanism) or can also occur at a local nodule level. To address this question, nodulated pea (Pisum sativum) plants were grown in a split-root system, which allowed for half of the root system to be irrigated at field capacity, while the other half was water deprived, thus provoking changes in the nodule water potential. Nitrogen fixation only declined in the water-deprived, half-root system and this result was correlated with modifications in the activities of key nodule's enzymes such as sucrose synthase and isocitrate dehydrogenase and in nodular malate content. Furthermore, the decline in nodule water potential resulted in a cell redox imbalance. The results also indicate that systemic nitrogen feedback signaling was not operating in these water-stressed plants, since nitrogen fixation activity was maintained at control values in the watered half of the split-root plants. Thus, the use of a partially droughted split-root system provides evidence that nitrogen fixation activity under drought stress is mainly controlled at the local level rather than by a systemic nitrogen signal.

Alternate Application of Osmotic and Nitrogen Stresses to Partial Root System: Effects on Root Growth and Nitrogen Use Efficiency

ABSTRACT Effects of alternately applying osmotic and nitrogen (N) stresses to partial root zone were studied on maize seedlings. The maize seedlings were grown in Hoagland's nutrient solution with the roots divided equally into two containers. Half of the root system was subjected to an osmotic stress (W) using PEG 6000 added in the solution at −0.2 MPa and/or nitrogen stress (N) with the N-free Hoagland's solution. After 6 days, four of such treatments, C (control), W, N, and WN were shifted to the other halves of the root systems. Results showed that when compared to the control, total root dry weight and the root/shoot ratio increased markedly for all the stress treatments. There were no statistically significant differences in shoot biomass and N accumulation of the whole plant among all the treatments. Nitrogen use efficiency for the N and WN was significantly enhanced when compared to the C and W treatments, and the shoot N concentrations were less for N and WN than for C and W. The root vitality of the non-treated halves under the W, N, or WN treatments increased by 21.1–65.6% when compared to the other halves of the same root system or by 19.7–68.9% when compared to the control roots. Such a compensation of root function was evident during our days 1, 3, and 5 observations. After each alternation, rates of biomass accumulation of all previously stressed half root systems were significantly higher than the other halves or the controls. Our results suggested that significant compensation mechanisms of growth and functions in the root system occurred both when N and osmotic stresses were localized in part of the root system and during the subsequent recovery from either of such stresses.

Host sanctions and the legume-rhizobium mutualism.

Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.

Accumulation of ABA in maize roots in response to root severing.

When isolated maize (Zea mays L.) routs were incubated tW 3 h in o fully hydrated state in air, ABA content was found to increase by around 100% in nil the different root tissues examined - i.e. mature primary root sections. young and growing primary root sections, secondary roots and primary root tips - from both well watered plants and previously droughted plants. This ABA increase was not turgor-related, as usually is the case in draughted plants, and was not quantitatively related to the injury in the cutting surface. A time- sequence of ABA accumulation showed a peak accumulation at about 2-3 h after initial incubation. Oxygen was essential for this increase in ABA accumulation because- when roots were incubated in pure nitrogen, no such increase was found. Evidence suggests that this increase was derived from xanthophylls. the common precursors for stress-induced ABA. Maize leaves did not show such an accumulation when incubated under the same conditions. When halt the root system was severed, ABA content in the remaining half root system increased by Kill 100-200% in 3 h. We suggest that there is a systemic effect on the ABA biosynthesis in the root system following the shock of root severing, and that roots may supply substantial amounts of ABA to the shoots even at time when there is not apparent dehydration.

Response of Split-Root Sour Orange Seedlings to NaCl and Polyethylene Glycol Stresses

Soil water cotent and salinity levels are seldom uniform in the field, particularly with the use of micro-irrigation systems that may water only a portion of the root zone. For studying non-uniform salinity, a split-root experiment was designed to evaluate growth and water relations when half of the root system of sour orange (Citrus aurantium) seedlings was stressed with sodium chloride (NaCl) or polyethylene glycol (PEG). This study also determined if non-stressed portions of the root system compensated for the decrease in water uptake by the stressed portions. One or both halves of the root system were treated for four months with nutrient solution adjusted with NaCl or PEG to osmotic potentials of −0.10, −0.20, or −0.35 MPa. Shoot dry weight was reduced by only 9% when half of the root system was irrigated with saline solution at −0.10 MPa, but with both halves of the root system at −0.10 MPa, shoot and root dry weights were reduced as much as 45%. Similarly, leaf water and osmotic potentials were also more disturbed under uniform salinity than under non-uniform salinity conditions. Plant growth, leaf water potential, osmotic potential, stomatal conductance, and evapotranspiration decreased with increasing NaCl and PEG concentrations in the nutrient solution. Turgor potential and leaf thickness increased in response to NaCl treatments. Microscopic examination showed that the increase in leaf thickness was due to the development of larger cells in the spongy mesophyll. Shoot growth did not correlate with the average osmotic potential of the two root halves. Seedlings with one stressed half-root system had shoot dry weight and leaf water potential values closer to those of the non-stressed control than to those with the completely stressed root system.

Effect of Localized Nitrogen Availability to Soybean Half-Root Systems on Photosynthate Partitioning to Roots and Nodules

Soybean (Glycine max [L.] Merr. cv Davis) was grown in a split-root growth system designed to maintain control of the root atmosphere. Two experiments were conducted to examine how 80% Ar:20% O(2) (Ar:O(2)) and air (Air) atmospheres affected N assimilation (NH(4)NO(3) and N(2) fixation) and the partitioning of photosynthate to roots and nodules. Application of NH(4)NO(3) to nonnodulated half-root systems enhanced root growth and root respiration at the site of application. A second experiment applied Ar:O(2) or air to the two sides of nodulated soybean half-root systems for 11 days in the following combinations: (a) Air to both sides (Air/Air); (b) Air to one side, Ar:O(2) to the other (Air/Ar:O(2)), and (c) Ar:O(2) to both sides (Ar:O(2)/Ar:O(2)). Results indicated that dry matter and current photosynthate ((14)C) were selectively partitioned to nodules and roots where N(2) was available. Both root and nodule growth on the Air side of Air/Ar:O(2) plants was significantly greater than the Ar:O(2) side. The relative partitioning of carbon and current photosynthate between roots and nodules on a half-root system was also affected by N(2) availability. The Ar:O(2) sides partitioned relatively more current photosynthate to roots (57%) than nodules (43%), while N(2)-fixing root systems partitioned 36 and 64% of the carbon to roots and nodules, respectively. The Ar:O(2) atmosphere decreased root and nodule respiration by 80% and nitrogenase activity by 85% compared to half-root systems in Air while specific nitrogenase activity of nodules in Ar:O(2) was 50% of nodules supplied Air. Results indicated that nitrogen assimilation, whether from N(2) fixation or inorganic sources, had a localized effect on root development. Nodule development accounted for the major decrease in total photosynthate partitioning to non-N(2)-fixing nodules. Soybean compensates for ineffective nodulation by controlling the flux of carbon to ineffective nodules and their associated roots.

ACCUMULATION OF PHOSPHORUS, DRY MATTER AND BETAINE DURING NaCl STRESS OF SPLIT-ROOT CITRUS SEEDLINGS COLONIZED WITH VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI ON ZERO, ONE OR TWO HALVES.

Sodium chloride tolerance and phosphorus content were examined in split-root Carrizo citrange seedlings [Poncirus trifoliata (L.) Raf. ×Citrus sinensis (L.) Osbeck] colonized with a vesicular-arbuscular mycorrhizal fungus (Glomus intraradices Schenck & Smith) on zero, one or two root halves. Plants were treated with NaCl at 0, 25, 50 or 100 mM, and the degree of stress was measured as reduction of dry matter accumulation and rise in level of leaf proline-betaine (stachydrine). Shoot and root dry weight production during this period decreased with increasing levels of salt. Absolute reductions were similar for plants inoculated on one vs two half-root systems, but percentage decreases were less in the latter due to greater overall growth in all treatments. Betaine levels in leaf tissues were positively related to soil salt levels for each mycorrhizal treatment. Significant differences in betaine levels were also detected in plants with and without mycorrhizal fungi, and mean levels tended to be higher for those colonized on one vs two halves of their root system. In contrast, a half-root system and its fungal symbiont supplied enough phosphorus to allow concentrations of leaf P to equal those of fully infected root systems, yet the two groups did not show equal growth under control conditions or percentage reductions with NaCl stress.

The Use of a Split-root Technique to Study Zn Movement and Activity in a Nodulating Cowpea Root System

Abstract Equal half-root systems of cowpea seedlings were achieved with a split-root technique. By use of a high humidity chamber and growth pouches, it was determined that the survival rate of seedlings with bisected root systems was greater than 95%, and the development of the half root systems was uniform and adequate. There were no marked differences in growth and development of plants when treatments were applied to different halves of the root system. However, there was evidence of Zn translocation from noninoculated half-root systems which received Zn, to inoculated half-roots not receiving Zn, which resulted in increased nodulation and N2 fixation. This result supports reports of a direct relationship of Zn nutrition to nodulation and N2 fixation.

Effect of Salinity on Nodule Formation by Soybean

A split-root growth system was employed to evaluate the effect of NaCl on nodule formation by soybean (Glycine max L. Merr. cv Davis). By applying the salinity stress and rhizobial inoculum to only one-half the root system, the effects of salinity on shoot growth were eliminated in the nodulation process. Rhizobium colonization of inoculated root surfaces was not affected by the salt treatments (0.0, 26.6, 53.2, and 79.9 millimolar NaCl). While shoot dry weight remained unaffected by the treatments, total shoot N declined from 1.26 grams N per pot at 0.0 millimolar NaCl to 0.44 grams N per pot at 79.9 millimolar NaCl. The concentration of N in the shoot decreased from 3.75% N (0.0 millimolar NaCl) to 1.26% N at 79.9 millimolar NaCl. The decrease in shoot N was attributed to a sharp reduction in nodule number and dry weight. Nodule number and weight were reduced by approximately 50% at 26.6 millimolar NaCl, and by more than 90% at 53.2 and 79.9 millimolar NaCl. Nodule development, as evidenced by the average weight of a nodule, was not as greatly affected by salt as was nodule number. Total nitrogenase activity (C(2)H(2) reduction) decreased proportionally in relation to nodule number and dry weight. Specific nitrogenase activity, however, was less affected by salinity and was not depressed significantly until 79.9 millimolar NaCl. In a second experiment, isolates of Rhizobium japonicum from nodules formed at 79.9 millimolar NaCl did not increase nodulation of roots under salt stress compared to nodule isolates from normal media (0.0 millimolar NaCl). Salt was applied (53.2 millimolar NaCl) to half root systems at 0, 4, 12, and 96 hours from inoculation in a third experiment. By delaying the application of salt for 12 hours, an increase in nodule number, nodule weight, and shoot N was observed. Nodule formation in the 12- and 96-hour treatments was, however, lower than the control. The early steps in nodule initiation are, therefore, extremely sensitive to even low concentrations of NaCl. The sensitivity is not related to rhizobial survival and is probably due to the salt sensitivity of root infection sites.

Effect of Salinity on the Functional Components of the Soybean‐Rhizobium japonicum Symbiosis1

A split‐root solution culture system was used to partition the effects of salinity on the functional components of the soybean [Glycine max (L.) Merr] ‐ Rhizobium japonicum symbiosis. Nodules were confined to half of the root system. After nodules were well developed (40 days from planting), 120 mM NaCl was applied to the half‐root systems in the following combinations: 1)unstressed, no salt to either half‐root system; 2) partially stressed shoot and unstressed nodules, salt to only the nonnodulated halfroot system; 3) partially stressed shoot and stressed nodules, salt to only the nodulated half‐root system; and 4) stressed shoot and nodules, salt to both half‐root systems. Leaf osmotic potentials (ψs) for the four treatments were: 1) −1.02; 2) −1.12; 3) −1.23; and 4) −1.83 MPa. Nodule ψs for the treatments were: 1) −0.70; 2) −0.72; 3) −1.12; and 4) −1.21 MPa. While total and specific nitrogenase activity as well as shoot N content were suppressed in treatment 3, the greatest decrease in activity and shoot N was observed when the plant was stressed by having both half‐root systems in salt (treatment 4). The rate of leaf expansion in treatment 4 was half the rate observed in the other treatments. We conclude that reduced N2 fixation by nodulated soybeans growing in saline environments was more a result of the effect of salt on leaf ψs and expansion, than the direct action of salt on the functional processes of the nitrogenase system.