Ineffective Nodulation Research Articles

Genetic Linkage Tests for the Rj4 Gene in Soybean

The Rj4 allele in soybean, Glycine max (L.) Merr., conditions an ineffective nodulation response primarily with bradyrhizobial strains classified as Bradyrhizobium elkanii, which are less efficient in N2 fixation with soybean, thus protecting the plant from many of these strains. The diminishing frequency of the RJ4 allele with agronomic selection poses the question of possible linkage with genes of negative agronomic selective value. Little is known of the linkage relationships of the Rj4 locus. This report analyzes genetic linkage tests of the Kj4 locus with 12 other loci: F, Frl, Fr2,1, LI, Ln, P, Wl, Y9, Y10, Y12, and YJ7. Segregating F2 and F3, progeny of several crosses were used to generate data for analysis. The Rj4 assays were conducted by planting seeds in sterile vermiculite and inoculating with broth cultures of strain USDA 61 from the Beltsville Rhizobium Culture Collection and examining the roots after 2 to 4 wk culture in the greenhouse. Other traits were scored either in the field or in the greenhouse. Chisquare was used to test for linkage, and the product method or method of maximum likelihood was used to estimate recombination. Only the Y17 locus showed evidence of genetic linkage with Rj4. The estimated recombination of Y17 and Rj4 was 46 ± 2%.

Mapping Phytophthora Resistance Loci in Soybean with Restriction Fragment Length Polymorphism Markers

Phytophthora root rot, caused by Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin, is one of the most serious diseases of soybean [Glycine max (L.) Merr.]. Six loci with alleles giving race‐specific resistance of soybean to phytophthora have been reported. The first objective of this study was to map the phytophthora resistance (Rps) loci using restriction fragment length polymorphism (RFLP) markers. The second objective was to map the Rj2 locus for ineffective nodulation with RFLP markers, because of the linkage of Rj2 to Rps2. The mapping was conducted using a series of ‘Williams’ near‐isogenic lines (NILs) with each having one or two phytophthora resistance alleles. The NILs were screened with 141 mapped RFLP markers. At least one polymorphism was found between each NIL and the recurrent parent. Linkage tests among the polymorphic RFLP markers, Rps loci, and the RJ2 locus were conducted using F2 populations. Linkage was found between RFLP markers and Rpsl, Kps2, Rps3, Rps4, Rps5, and RJ2. Linkage was also found between Rps2 and Rj2.

Genetic Linkage of the Phytophthora Resistance Gene Rps2 and the Nodulation Response Gene Rj2 in Soybean

Genome mapping of the agronomically important genes in soybean, Glycine max (L.) Merr., is as yet in a primitive stage. In this study we tested the Rps2 locus controlling resistance to phytophthora root rot (induced by Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin) for genetic linkage to the Rj2 locus controlling ineffective nodulation with certain strains of Bradyrhizobium japonicum. The line BARC‐4 (Rj2) carrying the Rj2 and Rps2 alleles was hybridized with ‘Clark 63’ carrying the rj2 and rps2 alleles. The F2 plants were characterized for the Rj2 vs. rj2 phenotypes by inoculating seeds in growth‐tray culture with a strain (USDA 7) defining for the Rj2 locus. The F3 progeny derived from F2 plants assayed for the Rj2 gene were screened to determine the genotype at the Rps2 locus. The F3 seedlings were evaluated in hydroponic culture inoculated with Race 4 of P. megasperma. Analysis of data indicated a significant (P < 0.0001) chi‐square for linkage. The maximum likelihood estimate for linkage indicated a very tight linkage with 2.7 ±1 genetic map units separating these loci. The Rps2‐Rj2 linkage is designated Linkage Group 19.

Inheritance of Ineffective Nodulation in Cowpea

Nodulation is a critical step in the development of the legume‐Rhizobium symbosis. A cowpea [Vigna unguiculata (L.) Walp.] plant of the varietal mix ‘Iron & Clay’ with small, white, ineffective nodules was observed during a greenhouse experiment and designated IC‐l. Experiments were conducted to determine the genetic control of ineffective nodulation in cowpea. The IC‐1 S1 plants were ineffectively nodulated and no nitrogenase activity was detected by acetylene‐reduction assay. No segregation for symbiotic effectiveness was noted in the S2 and S3 generations of IC‐1. Crosses were made between IC‐1 S1 and normal, effectively nodulating Iron & Clay cowpea, resulting in four F2 families that segregated into effective and ineffective nodulation classes. Based on x2 analysis, ineffective nodulation in cowpea is determined by a simply inherited recessive gene, designated cpi. The interaction of cpi and eight cowpea Bradyrhizobium strains was investigated, and no strain effects were noted. Non‐strain‐specific ineffective nodulation of cowpea is conditioned by a single recessive gene pair (cpi cpi).

Residual Effects of Rhizobium and Preplant N Fertilizer on Newly Established Alfalfa1

AbstractIn areas where ineffective nodulation may be a problem, ‘starter’ nitrogen fertilization is often suggested as a useful management tool to sustain ineffectively nodulated seedling alfalfa (Medicago sativa L.) until an effective population of Rhizobium can develop. Although ineffective nodulation has been reported from several locations around the world, there have been no studies examing this possibility. A field experiment was conducted during 1982 and 1983 on a Woodburn silt loam soil (Aquultic Argixeroll) which contained a symbiotically ineffective native population of Rhizobium meliloti Dang. This study examined the residual and interactive effects of preplant N fertilizer (0, 45, 90 kg N ha−1) and inoculation on the N nutrition and yield of seedling alfalfa during the first year of growth. The sustaining effects of the fertilizer N were transient and resulted in decreased N2 fixation until 35 days following harvest. It was concluded that during the establishment year, managerial emphasis should be placed on inoculation and not on N fertilization to obtain a consistent, economical source of N for herbage protein and dry matter production.

Registration of a Non‐Nodulation and Three Ineffective Nodulation Alfalfa Germplasms

Crop ScienceVolume 28, Issue 4 cropsci1988.0011183X002800040042x p. 721-722 Registration of Germplasms Registration of a Non-Nodulation and Three Ineffective Nodulation Alfalfa Germplasms D. K. Barnes, Corresponding Author D. K. Barnes n/[email protected] USDA-ARS now Urbana Laboratories, P.O. Box 1393, St. Joseph, MO, 64502Corresponding author.Search for more papers by this authorC. P. Vance, C. P. Vance USDA-ARS now Urbana Laboratories, P.O. Box 1393, St. Joseph, MO, 64502Search for more papers by this authorG. H. Heichel, G. H. Heichel USDA-ARS now Urbana Laboratories, P.O. Box 1393, St. Joseph, MO, 64502Search for more papers by this authorM. A. Peterson, M. A. Peterson Dep. of Agronomy and Plant Genetics, Univ. of Minnesota now W-L Research, 8701 Highway 14, Evansville, WI, 53536Search for more papers by this authorW. R. Ellis, W. R. Ellis USDA-ARS, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 411 Borlaug Hall, 1991 Buford Circle, St. Paul, MN, 55108Search for more papers by this author D. K. Barnes, Corresponding Author D. K. Barnes n/[email protected] USDA-ARS now Urbana Laboratories, P.O. Box 1393, St. Joseph, MO, 64502Corresponding author.Search for more papers by this authorC. P. Vance, C. P. Vance USDA-ARS now Urbana Laboratories, P.O. Box 1393, St. Joseph, MO, 64502Search for more papers by this authorG. H. Heichel, G. H. Heichel USDA-ARS now Urbana Laboratories, P.O. Box 1393, St. Joseph, MO, 64502Search for more papers by this authorM. A. Peterson, M. A. Peterson Dep. of Agronomy and Plant Genetics, Univ. of Minnesota now W-L Research, 8701 Highway 14, Evansville, WI, 53536Search for more papers by this authorW. R. Ellis, W. R. Ellis USDA-ARS, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 411 Borlaug Hall, 1991 Buford Circle, St. Paul, MN, 55108Search for more papers by this author First published: 01 July 1988 https://doi.org/10.2135/cropsci1988.0011183X002800040042xCitations: 9AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article.Citing Literature Volume28, Issue4July–August 1988Pages 721-722 RelatedInformation

Role of the Nodulation Restrictive Allele Rj4, in Soybean Evolution

Summary The symbiosis of legumes with rhizobia has been subject to the selection pressures of 200 million years of evolution. Evidence from soybean of ecogeographic distribution of nodulation compatibilities with specific Rhizobium strains indicates that the coevolution of host and microsymbiont has played a role in the evolution of compatibilities. Some strains of bradyrhizobia that nodulate soybean also produce a substance called rhizobitoxine that interferes with chlorophyll synthesis. The Rj 4 allele in soybean conditions an ineffective nodulation response with specific strains of bradyrhizobia while the rj 4 allele permits normal nodulation with these strains. Two near isogenic soybean lines (99.95 % genetically identical) differing specifically in the Rj 4 vs. 774 alleles have been synthesized and used to test an array of chlorosis-inducing and non-chlorosis-inducing bradyrhizobia for response with the Rj 4 allele. The Rj 4 allele was found to interdict nodulation with 33.3 % of the chlorosis inducing rhizobia and only 2.7 % of the nonchlorosis inducing rhizobia. Thus, the Rj 4 allele may have a positive value to the host plant in areas with significant populations of chlorosis-inducing rhizobia by protecting the plant from nodulation with some of these rhizobia. The frequency of ineffective nodulation with the chlorosis-inducing strain USDA61 shows a progressive loss with domestication from the wild species Glycine soja (63 %) to the Asiatic plant introductions of G. max (26 %), and with selection for agronomic types in North America as demonstrated by the preliminary test lines (13.7 %) and the more highly selected uniform test lines (8.5 %). This may be an example of the loss of rhizobial specificity in a plant species occurring in historic time.

Rhizobia in tropical legumes: Ineffective nodulation of Arachis hypogaea L. By fast-growing strains

Members of the legume tribe Aeschynomeneae are thought to be nodulated by a different process to that which occurs in most other legumes. Rather than invade root-hairs, infective rhizobia directly enter the inter-ccllular spaces of the roots where they are engulfed by the cortical cells. Once within the cells, the rhizobia divideand the bacteroid-containing cells themselves undergo mitosis. Division of both bacleroids and their host-cells is characteristic of this nodulation process. As a prelude to studying the molecular genetics of the rhizobial component in such symbioses, we sought a fast-growing, plasmid-bearing Rhizobium that would nodulate a group of plants which normally favour slow-growing Bradyrhizobium sp. A collection of 27 fast-growing tropical rhizobia was used to screen the nodulation capacity of Arachis hypogaea cv. NC2. Seventy percent of the isolates were able to ineffectively nodulate groundnuts growing in “Leonard” jars. Four of these were chosen to inoculate six different A. hypogaea cultivars. Only one strain, Rhizobium sp. NGR234, was able to nodulate all six varieties. Nodules produced by Rhizobium sp. NGR234 on the different cultivars were compared with nodules that developed following infection with Bradyrhizobium sp. Apart from the fact that bacteroids induced by Rhizobium sp. NGR234 were polymorphic while those developing after infection by Bradyrhizobium sp. were spherical, the organisation of nodules produced by both groups of bacteria on all varieties of plants was similar. NGR234 multiplication in the host cytoplasm and the absence of infection threads suggest that fast-growing rhizobia also enter the roots via the direct inter-cellular route.

Dry Matter, Nitrogen Distribution, and Dinitrogen Fixation in Contrasting Alfalfa Symbioses1

Improving dinitrogen (N2) fixation of alfalfa (Medicago sativa L.) requires combining effective rhizobial strains and host genotypes capable of enhanced N2 fixation. Glasshouse experiments on pot‐grown alfalfa determined how changes in the rhizobial and host genomes associated with N2 fixation affect dry matter and N yields and distribution at vegetative, flower bud, and 30% flowering developmental stages. We compared dry matter and N yield and distribution in a host‐ and a rhizobial‐conditioned ineffective (Fix+) and in a related effective (Fix+) symbiotic combination, and investigated the effect of combined N on N2 fixation and on the distribution of symbolically fixed N2 in two unrelated Fix+ alfalfa germplasms. Dry matter and N yield and response to added N (0, 40, 80 kg/ha) of the host‐conditioned and the rhizobial‐conditioned Fix− combinations were similar. Without combined N, both Fix− combinations distributed relatively less dry matter and N to aerial organs than did the Fix+ combination. Neither the type of Fix− combination nor traits in the Fix+ germplasms supportive of increased N2 fixation altered the relative distribution of dry matter among plant organs; however, N concentration in individual organs varied with supplemental N in the Fix− but not the Fix+ combinations. Dry matter and N yields of the two unrelated Fix+ combinations differed only without combined N. Supplemental N was more inhibitory to N2 fixation by Fix+ germplasm selected for enhanced nitrogenase activity compared with unselected Fix+ germplasm. Either source of ineffective nodulation in Fix− combinations would be appropriate for developing non‐fixing controls for physiological investigations of N2 fixation and for use in programs to evaluate and enhance N2 fixation in alfalfa.

A Comparison of Rhizobial Strain Compatibilities of Glycine max and its Progenitor Species Glycine soja1

Glycine soja Seib. and Zucc. is considered the wild progenitor species from which the domesticated soybean [G. max (L.) Merr.] was developed. Glycine soja hybridizes readily with G. max and is a source of agronomically valuable genes for introgressing into G. max. Such introgression could result in concomitant introgression of genes controlling rhizobial symbiosis. Because little is known of the nodulation response of G. soja with U.S. rhizobial strains, we tested Asiatic G. soja plant introductions for nodulation response with bradyrhizobial strains USDA 7 and USDA 61 isolated from the USA. For comparison, Asian plant introductions of G. max were also tested. Seed of 537 lines of G. soja and 691 lines of G. max originating from China, Japan, Korea, and USSR in Maturity Groups OO thru ✕ were evaluated for nodulation response. Five seeds of each line were decontaminated in 50% ethyl alcohol, planted in vermiculite in growth trays, inoculated with 4‐ to 7‐day‐old broth cultures of rhizobia, and grown in the greenhouse for 4 to 5 weeks. then, roots were extracted and nodulation response was determined by visual examination. Both G. soja and G. max were polymorphic for effective vs. ineffective nodulation with both strains of bradyrhizobia. This suggests that polymorphism existed in G. soja prior to domestication of G. max and that G. max was domesticated from a polymorphic population of G. soja. The frequency of ineffective nodulation in G. soja with both strain USDA 7 (10.9%) and USDA 61 (63.4%) was higher than in G. max (2.4%, 26.4%, respectively). Therefore, use of G. soja in breeding programs could increase the probability of ineffective nodulation with these bacteria in derived lines. The marked difference in the nodulation incompatibilities between G. soja and G. max from Korea suggest that the G. max introductions were not domesticated from the G. soja lines from Korea and that there has not been a substantial introgression of genes controlling nodulation from Korean G. soja into Korean G. max.

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.

Nutritional restraints on subterranean clover grown on acid soils used for crop-pasture rotation

Lime, N and molybdenum fertilizers were used to establish how soil acidity limits the growth of subterranean clover (Trifolium subterraneum), in pot culture, on acid soils taken from north-east Victoria. The results indicate that two nutritional factors associated with the growth of subterranean clover are affected by excess acidity. These are manganese toxicity and ineffective nodulation of the clover. The plants grown on one unlimed soil (Rutherglen) had manganese concentrations and symptoms consistent with manganese toxicity. Liming the second soil (Lilliput) overcame a N deficiency by way of increased nodule formation. A third nutritional factor commonly associated with excess acidity, and found on the Lilliput soil, molybdenum deficiency, was not alleviated by the application of lime. Instead, the application of molybdenum fertilizer gave increases in clover dry matter production at all lime rates. In the absence of molybdenum fertilizer, the molybdenum concentration of the herbage was extremely low, irrespective of liming. These results are discussed in relation to already established field responses of subterranean clover to lime, and the management of the clover-ley system on the strongly acid soils.

Photosynthate and Dry Matter Distribution in Effectively and Ineffectively Nodulated Alfalfa1

Ineffectively nodulated alfalfa (Medicago sativa L.) grown with combined N [In(+N)] was compared with symbiotically‐grown, effectively‐nodulated alfalfa (Sar) to determine whether ineffective nodulation had correlated effects on photosynthate partitioning into dry matter. Ineffectively nodulated alfalfa grown with [In( + N)] and without [In( − N)] combined N were compared to determine the effects of N stress on photosynthate partitioning. Either the topmost or lowest fully‐expanded, intact green leaf on the main stem was labeled with 14CO2 in pulse‐chase experiments. After a 24 h chase period, the plants were divided into source leaf, main stem, unexpanded leaves and shoot apex on the main stem, other fully‐expanded leaves on the main stem, shoots growing from axillary buds on the main stem, shoots growing from crown buds, crown, nodules, and root. The 14C partitioning was measured as relative specific activity (RSA) and percent of total plant recovered radioactivity (%TPR). The dry mass, %TPR, and RSA of aerial organs were similar in In( + N) and Sar. However, the dry mass and %TPR of the belowground organs (roots + crown + nodules) were 46% and 25% less, respectively, in In(+N) than in Sar. This differential partitioning of dry matter and photosynthate to roots and crowns between the populations indicates that the single recessive gene for ineffective nodulation may have correlated effects on growth or development. The allocation of photosynthate from the topmost and lowest fully‐expanded leaf was significantly (P<0.05) different to all organs except nodules and roots. The topmost fullyexpanded leaf was the primary supplier of photosynthate to the unexpanded leaves and shoot apex on the main stem, the main stem, and fully‐expanded leaves on the main stem. The lowest fully‐expanded leaf was the primary supplier to the shoots growing from axillary buds on the main stem, the shoots growing from crown buds, and the crown. The dry mass, RSA, and %TPR of the main stem, shoots growing from axillary buds on the main stem, fully‐expanded leaves on the main stem, and shoots growing from the crown were less in the In(−N) than in the In(+N). The converse was true for the unexpanded leaves and shoot apex on the main stem, crown, roots, and nodules. The carbon exchange rate of the topmost fully‐expanded leaf was reduced 45%, but the export of photosynthate significantly increased, under N stress. The dry mass, %TPR, and RSA of the nodules were greater by 19‐, 36‐, and 6‐fold, respectively, in In(−N) than in In( + N). These results indicate that the nitrate inhibition of nodulation and photosynthate partitioning to alfalfa nodules is unrelated to rhizobial effectiveness or to the functioning of the bacteroids.

Characterization of Rhizobia from Ineffective Alfalfa Nodules: Ability to Nodulate Bean Plants [ Phaseolus vulgaris (L.) Savi.

This study was initiated to characterize Rhizobium isolates obtained from root nodules of ineffectively nodulated, field-grown alfalfa (Medicago sativa L.) plants. The purpose was to determine if these isolates possessed characteristics which would explain either their ineffectiveness in N(2) fixation or their apparent ability to tolerate the moderately acid soil conditions from which they originated. Isolates were characterized by analysis of growth rate, 39 degrees C tolerance, acid production on conventional media, and symbiotic performance. All isolates were ineffective in N(2) fixation on alfalfa, and they contained one or more anomalous characteristics. These included either slow growth rate, lack of 39 degrees C tolerance, or lack of acid production on conventional media. Infectiveness tests on a broad range of legumes revealed that the isolates formed root nodules on M. sativa, Medicago lupulina L., and Phaseolus vulgaris (L.) Savi. (common bean). These results provide evidence that, in some situations, ineffective nodulation of M. sativa in the field may be due to the presence of promiscuous, native Rhizobium species.

Nutritional characteristics of kura clover (Trifolium ambiguum Bieb.) compared with other forage legumes

Kura or Caucasian clover (Trifolium ambiguum Bieb.) is a perennial, rhizomatous legume indigenous to Caucasian Russia and adjacent regions (Kannenberg & Elliott,1962; Bryant, 1974). Those agronomic characteristics that lend it potential significance, namely an extensive root-rhizome system, winter-hardiness, resistance to numerous and serious clover pests, and the production of nectar-rich flowers, were enumerated by Parker & Allen (1952). These authors also pointed out that usage of kura clover was hampered by ineffective nodulation. However, in recent years commercial and effective inoculant preparations have become available. Consequently, it has been suggested that kura clover could have potential as a pasture legume and for soil conservation purposes (Bryant, 1974; Spencer et al.1975). Pertinent literature on kura clover is scanty. The present study evaluated the nutritional characteristics of kura clover relative to otherforage legumes.

Inheritance of Ineffective Nodulation in Crimson Clover1

A single plant of crimson clover (Trifolium incarnation L.) appearing to have ineffective nodules was identified under nil‐N conditions from a 1000‐pIant population of the cultivar Chief. The plant (designated CH‐I) had tiny, white, beaded nodules and only 2 cm of topgrowth. With the addition of N this plant was grown to maturity and self‐pollinated. S1 plants were inoculated with 11 different Rhizobium trifolii strains in replicated experiments, all resulting in an ineffective symbiosis. No segregation for effectiveness occurred in the S1 or S2 generations of the CH‐I line. Crosses were made between normal crimson clover lines and S1 lines of CH‐I. F1 populations were nodulated effectively with R. trifolii strain 162K13. The Fz generation segregated for symbiotic effectiveness when nodulated with strain 162K13. F2 populations from two crosses showed a good fit to a 3:1 ratio of normal to ineffective nodulation. Results indicate that non‐strain‐specific ineffective nodulation of crimson clover is controlled by a single recessive gene pair (rt1 rt2) with possible modifiers. Plants with this trait should be useful as non‐Nj fixing controls in 15N isotope dilution experiments and to study individual steps in N2 fixation.

Canopy Structure and Photosynthesis of Alfalfa Genotypes Differing in Nodule Effectiveness1

Photosynthesis is linked to N2 fixation and to other plant metabolic processes, but little is known of how N2‐fixation capability affects the development of the leaf canopy and leaf photosynthetic rates. Field experiments were undertaken to determine the effect of differences in N2‐fixation capability mediated by plant genes on canopy structure and apparent photosynthesis of alfalfa (Medicago saliva L.). An alfalfa genotype lacking the capability of establishing an effective symbiosis with Rhizobium was compared to normal alfalfa. Entries were grown at two rates of supplemental N (0 and 100 kg ha−1 as NH4NO5) to provide genetic‐ and culturally‐controlled differences in N2 fixation. Hostmediated genetic differences in N2‐fixation capability (ineffective vs. effective nodules) elicited pronounced changes in rates of apparent photosynthesis, canopy structure, and specific, leaf weight. Compared with alfalfa having effective nodules (Eff), alfalfa with ineffective nodules (In) averaged 17% greater specific leaf weight, but 73% less leaf area over both N regimes and three growth‐harvest cycles. For unshaded leaves at the top of the canopies, rate of apparent photosynthesis per leaf area of In was similar to that of Eff on vegetative plants 2 weeks before harvest and in canopies at 5% flower. Leaf canopies of In contained a more vertically uniform leaf area than Eff canopies unless they were supplied with 100 kg ha−1 combined N. In vegetative In plants 2 weeks before harvest, supplemental N at a rate of 100 kg ha−1 enhanced leaf area by 50%, but tended to reduce rate of apparent photosynthesis per leaf area. In Eff plants at this same growth stage, supplemental N also reduced apparent photosynthesis, but leaf area was not affected. In canopies at 5% flower, supplemental N did not change rate of apparent photosynthesis of either In or Eff. These results show that N stress induced by host‐mediated ineffective nodulation greatly reduced leaf area and photosynthetic rate per ground area, but that effective nodules were not a prerequisite for maintaining normal photosynthetic rates per leaf area even under severely limiting soil N supply.

Compensation against Ineffective Nodulation in Soybean1

Two cultivars of soybean [Glycine max (L.) Merr.] were inoculated with nine ratios of ineffective (SM‐5) and effective (CC709) strains Rhizobium japonicum. Plants were grown in the greenhouse and harvested after 33 days. Nitrogenase activity, total shoot N, nodule number, nodule mass, and the proportions of nodules formed by each strain were determined. Nodules were identified serologically by the fluorescent antibody technique. Significant increases in total N in the shoot occurred as both the proportion and number of nodules formed by effective strain CC709 increased. Shoot N was highly correlated with nitrogenase activity and nodule mass. The relationship between percent effective nodules and shoot N was nonlinear so that 95% of maximum N accumulation was obtained when only 75% of the nodules (on a nodule number basis) were effective. The average size an effective nodule was at least 2.5 times larger than an ineffective nodule and their average weight increased from 2.38 to 4.88 mg/ nodule as the proportion of effective nodules declined from 100 to 18.8%. The proportion of total nodule tissue that was effective was greater than the corresponding percent effective nodules. This compensatory mechanism tended to keep the amount of effective nodule tissue constant up to a point even though the proportion of effective nodules declined. There was a deleterious effect on N2 fixation only when infection by the ineffective strain was great enough to reduce total nodule mass.

Rhizobium — Host Specificities in Phaseolus coccineus L. and Phaseolus vulgaris L.1

The introduction of desirable agronomic characteristics into Phaseolus vulgaris L. is being attempted by interspecific hybridization with P. coccineus, but the effects on root nodulation are unknown. Accordingly, cross‐inoculation relationships between P. vulgaris ‘Red Kidney’ and P. coccineus ‘Scarlet Runner’ were tested in the greenhouse with a group of 16 Rhizobium strains effective on Red Kidney and a group of 12 strains effective on Scarlet Runner. Both groups had diverse origins, including commercial sources and isolations from bean fields and other sites in California.Only six strains, all isolates from Scarlet Runner, were effective on both hosts. Fifty percent of Scarlet Runner isolates were as effective on Red Kidney as the more effective of Red Kidney's homologous strains. Thirty percent of the California isolates tested on Red Kidney were more effective than two commercial inocula.Strain rejection in Red Kidney was expressed in 75% of all cases as failure to form nodules. In contrast, strain rejection by Scarlet Runner was expressed in 64 to 68% of all cases as ineffective nodulation (nodule malfunction). If Scarlet Runner is representative of P. coccineus in this respect, interspecific hybridization is likely to increase rather than reduce the tendency of P. vulgaris to nodulate with ineffective rhizobia.

Inheritance of Ineffective Nodulation and Non‐nodulation Traits in Alfalfa1

Although the roots of alfalfa (Medicago sativa L.) usually form pink nodules capable of fixing atmospheric N2 when inoculated with Rhizobium meliloti Dang., one clone incapable of forming nodules (non‐nod) and five clones which produced ineffective nodules were recently discovered. These unusual nodulation traits are useful for research on root infection, nodule formation, and N2 fixation. The objectives of this study were to determine the inheritance of the non‐nodulating trait and of the ineffective nodulation in each of the five clones, to compare the genetic controls for ineffective nodulation in the five clones, and to determine the genetic relationship between ineffective nodules and non‐nodulation.The non‐nod clone and the five ineffective clones were self pollinated, crossed in a diallel mating design, and crossed to two normal effective nodulating clones. F2 and backcross seed was produced on non‐nod ✕ normal and ineffective ✕ normal crosses. F2 seed was also produced on non‐nod ✕ ineffective crosses and ineffective ✕ ineffective crosses. Progenies were evaluated under nil‐nitrate greenhouse sand culture. Six weeks after planting, non‐nod plants were small, chlorotic, and had no nodules. Plants with ineffective nodules were small, chlorotic, and had white nodules. Plants with effective nodules were vigorous, dark green, and had pink nodules. Four of the five clones had a different basis for inheritance of ineffectiveness. Three were conditioned by a single tetrasomically inherited recessive gene (in1, in2, or in3), the nulliplex genotype in each instance producing ineffective nodules. The fourth was conditioned by two recessive genes (in4 and in5), the nulliplex condition at both loci being required for production of ineffective nodules. The non‐nod trait was conditioned by two tetrasomically inherited recessive genes (nn1, and nn2). The nulliplex condition at both loci resulted in non‐nodulation. The non‐nod trait and the four different types of ineffective nodules appeared to segregate independently of one another.