Daniel W. Israel

Nitrogen Transfer Between Plants: A 15N Natural Abundance Study with Crop and Weed Species

An increasing amount of evidence indicates that N can be transferred between plants. Nonetheless, a number of fundamental questions remain. A series of experiments was initiated in the field to examine N transfer between N2-fixing soybean (Glycine max [L.] Merr.) varieties and a non-nodulating soybean, and between N2-fixing peanut (Arachis hypogaea L.) or soybean and neighboring weed species. The experiments were conducted in soils with low N fertilities and used differences in N accumulation and/or 15N natural abundance to estimate N transfer. Mixtures of N2-fixing and non-nod soybean indicated that substantial inter-plant N transfer occurred. Amounts were variable, ranging from negligible levels to 48% of the N found in the non-nod at maturity. Transfer did not appear to strongly penalize the N2-fixing donor plants. But, in cases where high amounts of N were transferred, N content of donors was noticeably lowered. Differences were evident in the amount of N transferred from different N2-fixing donor genotypes. Results of experiments with N2-fixing crops and the weed species prickly sida (Sida spinosa L.) and sicklepod (Senna obtusifolia [L.] Irwin & Barneby) also indicated substantial N transfer occurred over a 60-day period, with amounts accounting for 30–80% of the N present in the weeds. Transfer of N, however, was generally very low in weed species that are known to be non-hosts for arbuscular mycorrhizae (yellow nutsedge, Cyperus esculentus L. and Palmer amaranth, Amaranthus palmeri [S.] Watson). The results are consistent with the view that N transfer occurs primarily through mycorrhizal hyphal networks, and they reveal that N transfer may be a contributing factor to weed problems in N2-fixing crops in low N fertility conditions.

Altered aluminum inhibition of soybean root elongation in the presence of magnesium

Variations in genotype rankings among screenings for Al tolerance in hydroponics may be related to differences in the composition of the solutions. In the present study, we investigated the involvement of Mg ions in modifying Al rhizotoxicity in soybeans. Root elongation was strongly inhibited by Al in a simple, 800 μM CaSO4 solution, but elongation increased noticeably when the solutions also contained Mg. Amelioration of Al rhizotoxicity was not associated with an increase in ionic strength of treatment solutions because Al3+ activities were kept constant. Concentration series experiments indicated that the Mg effect occurred in the μM range, while Ca amelioration of Al toxicity occurred at mM concentrations. The positive effect of Mg on root elongation was greatest for Al-sensitive genotypes and minimized genotypic differences for Al-tolerance. The Mg protection against Al rhizotoxicity apparently does not occur with all species, because it was not observed in Atlas and Scout 66 wheat varieties. The ability of Mg to ameliorate Al toxicity in soybean at μM levels suggests the involvement of distinct physiological factors.

Alterations in Soybean Leaf Development and Photosynthesis in a Co2- Enriched Atmosphere

This study was conducted to characterize changes in the canopy photosynthetic leaf area of developing soybean (Glycine max [L.] Merr. cv Lee) exposed to a CO2-enriched atmosphere. Young, vegetative plants were exposed to 350 or 700 μL L-1 CO2 for 15 d. Plant dry mass and total leaf area were greater in the CO2-enriched environment. Emergence and expansion rates of main stem leaves increased at high CO2, but the areas of individual leaves at full expansion were affected very little (5%-10% greater than controls). More rapid leaf expansion rates occurred in the light and dark. Under CO2-enriched conditions, the net CO2 exchange rates of all leaves on the main stem were higher before and after full expansion. Stomatal conductance was lower in high CO2 only after leaves approached full expansion. Leaf development on the lateral branches also was increased at high CO2, accounting for 40% of the total increase in leaf area by the end of the experiment. We conclude that more rapid rates of leaf development under CO2 enrichment likely resulted from increased photosynthesis rates and that both direct and indirect effects were involved.

Enhanced nodulation of leguminous plant roots by mixed cultures ofAzotobacter vinelandii and rhizobium

SummaryAzotobacter vinelandii strains caused the formation of increased numbers of root nodules onGlycine max, Vigna unguiculata andTrifolium repens by their respective rhizobial symbionts. Increased nodulation due to inoculation withA. vinelandii also occurred in field grownG. max. Mutant strains ofA. vinelandii unable to fix nitrogen caused nodulation increases comparable to those caused by nitrogen-fixing strains. This indicates that nitrogen fixation byA. vinelandii was not responsible for the enhanced nodulation. The effect ofA. vinelandii on nodulation was greatest when cells from the mid-exponential phase of growth were applied as inoculants. Non viable cell preparations ofAzotobacter vinelandii were also found to cause an increase in the number of root nodules formed onGlycine max Rhizobium japonicum under greenhouse conditions. The nodulation enhancement activity was influenced by the method chosen to kill theA. vinelandii cells. Heat treatment and treatment with lethal levels of streptomycin destroyed the activity, whereas the activity was unaffected by ultraviolet-light treatment of the cells. Cell-free extracts ofA. vinelandii were found to enhance nodulation. On the other hand, culture supernatants ofA. vinelandii had no effect on nodulation. A split-root experiment suggested that the agent(s) responsible for the increased nodulation was not translocatable throughout the plant. The results suggest a non-excretable protein, produced byA. vinelandii, as a possible mechanism for nodulation enhancement.

Phosphorus‐deficiency effects on response of symbiotic N2 fixation and carbohydrate status in soybean to atmospheric CO2 enrichment

Abstract The impact of phosphorus (P) deficiency on response of symbiotic N2 fixation and carbohydrate accumulation in soybean (Glycine max [L.] Merr.) to atmospheric CO2 enrichment was examined. Plants inoculated with Bradyrhizobium japonicum MN 110 were grown in growth chambers with controlled atmospheres of 400 and 800 μL CO2 L‐1 and supplied either 1.0 mM‐P (P‐sufficient) or 0.05 mM‐P (P‐deficient) nitrogen (N)‐free nutrient solution. When plants were supplied with sufficient P, CO2 enrichment significantly increased whole plant dry mass (83%), nodule mass (67%), total nitrogenase activity (58%), and N (35%) and P (47%) accumulation at 35 days after transplanting (DAT). Under sufficient P supply, CO2 enrichment significantly increased starch concentrations in nodules compared to the normal atmospheric CO2 treatment. Under normal CO2 levels (400 μL L‐1) nonstructural carbohydrate concentration (starch plus soluble sugar) was significantly higher in leaves of P‐deficient plants than in leaves of P‐suffi...

Effects of defoliation on seed protein concentration in normal and high protein lines of soybean

Two high (NC106, NC111) and two normal (NC103, NC107) seed protein concentration lines, derived from two different recurrent selection populations of soybean (Glycine max L. Merr.) were subjected to partial defoliation at beginning seed fill (R5) under outdoor pot culture and field conditions. The aim of this study was to test the hypothesis that capacity to store N in vegetative organs and/or to mobilize that N to reproductive organs is associated with the high seed protein concentration trait. Symbiotic N2 fixation was the sole source of N in the pot experiment and the major source of N (met > 50% of the N requirement) in the low N soil used in the field experiment. Seed protein concentration and seed yield at maturity in both experiments and N accumulation and mobilization between R5 and maturity in the pot experiment were measured. The four genotypes did not differ significantly with respect to the amount of N accumulated before beginning seed fill (R5). Removal of up to two leaflets per trifoliolate leaf at R5 significantly decreased the seed protein concentration of NC107/111 but had no effect on this trait in NC103/106. Defoliation treatments significantly decreased seed yield, whole plant N accumulation (N2-fixation) during reproductive growth and vegetative N mobilization of all genotypes. Differences in harvest indices between the high and low protein lines accounted for approximately 35% of the differences in protein concentration. The two normal protein lines mobilized more vegetative N to the seed (average. 5.26 g plant−1) than the two high protein lines (average. 4.28 g plant−1). The two high seed protein lines (NC106, NC111) exhibited significantly different relative dependencies of reproductive N accumulation on vegetative N mobilization, 45% vs. 29%, in the control treatment. Whereas, NC103 with normal and NC106 with high seed protein concentration exhibited similar relative dependencies of reproductive N accumulation on vegetative N mobilization, (47% vs. 45%). Collectively, these results indicate that N stored in shoot organs before R5 and greater absolute and relative contribution of vegetative N mobilization to the reproductive N requirement are not responsible for the high seed protein concentration trait.

Nitrogen Release from Coated Urea Fertilizers in Different Soils

The nitrogen (N) release from coated urea fertilizers (Arborite and ESN), traditional N fertilizers [urea, (NH2)2CO], and urea ammonium nitrate (UAN) [(NH2)2CO, NH4NO3] in three North Carolina (USA) soils was measured throughout a 12-week laboratory incubation. Treatments were N source and moisture level (60% and 80% of field capacity). In both the Candor and Cecil soils, 40% of the Arborite N had released by day 2 after addition to the soil. Maximum Arborite N release was achieved by week 6 for both soils. The ESN N release began between weeks 1 and 2, and maximum release was attained by week 6 for the Candor and by week 8 for the Cecil soil. The ESN reached 70% release on Portsmouth by week 8 and Arborite had an immediate release. Field studies of these coated ureas would be needed to determine if they are economically viable over more traditional N fertilizers in North Carolina.

Comparative Effects of Two Forage Species on Rhizosphere Acidification and Solubilization of Phosphate Rocks of Different Reactivity

ABSTRACT Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H+) and the removal of the dissolved products [calcium (Ca2 +) and dihydrogen phosphate (H2PO4 −)]. Plant roots may excrete H+ or OH− in quantities that are stoichiometrically equal to excess cation or anion uptake in order to maintain internal electroneutrality. Extrusion of H+ or OH− may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a phosphorus (P)-deficient Ultisol with low Ca content. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility with chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had a higher content of calcite than the high solubility PRs, which led to increased soil pH values (> 7.0) and exchangeable Ca, and relatively little change in bicarbonate-extractable soil P. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake. Stylosanthes had an excess cation uptake, defined by a large Ca uptake and its dependence on N2 fixation, which induced a significant H+ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with nitrate (NO3 −) comprising 92% of total anion uptake. Nitrate and sulfate (SO4 2 −) reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic hydroxide (OH−), which could have increased cytoplasmic pH and induced synthesis of organic acids and OH− extrusion from roots.

Soybean root growth in relation to ionic composition in magnesium-amended acid subsoils: Implications on root citrate ameliorating aluminum constraints

Abstract Hydroponic studies with soybean (Glycine max [L.] Merr.) have shown that µmol L−1 additions of Mg2+ were as effective in ameliorating Al rhizotoxicity as additions of Ca2+in the mmol L−1 concentration range. The objectives of this study were to assess the ameliorative effects of Mg on soybean root growth in acidic subsoils and to relate the soil solution ionic compositions to soybean root growth. Roots of soybean cultivar Plant Introduction 416937 extending from a limed surface soil compartment grew for 28 days into a subsurface compartment containing acid subsoils from the Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic) series. The three Mg treatments consisted of native equilibrium soil solution concentrations in each soil (50 or 100 µmol L−1) and MgCl2 additions to achieve 150 and 300 µmol L−1 Mg (Mg150 and Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO3 treatment limed to achieve a soil pH value of 6. Subsoil root growth responses to the Mg treatments were less than for the lime treatments. Root length relative to the limed treatments for all subsoils (RRL) was poorly related to the activity of the soil solution Al species (Al3+ and Al-hydroxyl species) and Mg2+. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity, including (Ca2+), (Al3+)/(Ca2+) and (Al3+)/([Ca2+] + [Mg2+]), suggesting that Ca could be a primary factor ameliorating Al and H+ rhizotoxicity in these subsoils. Increased tolerance to Al rhizotoxicity of soybean by micromolar Mg additions to hydroponic solutions, inducing citrate secretion from roots to externally complex toxic Al, may be less important in acid subsoils with low native Ca levels.

Phenotypic and genotypic diversity of similar serotypes of soybean bradyrhizobia from two soil populations

Abstract The physiological and genetic diversity within two major serotypic groups of bradyrhizobial isolates obtained from soybean (Glycine max L. Merr.) plants grown on a Dothan and a Cape Fear soil was examined. All isolates serotyped as 31 94 had large colonies with smooth borders and high resistances to erythromycin, streptomycin and spectinomycin with minimal inhibitory concentration values (MIC) ranging from 200 to 400 μg ml−1. Pulsed-field gel-electrophoresis (PFGE) separation of DNA fragments generated with the rarely cutting restriction endonuclease, Xba I, revealed six genotypes among 28 different 31 94 isolates. Four of these genotypes were common to both soils and only 21% of the isolates were classified as having high N2-fixation capacity. Leaf chlorosis was induced by 46% of the 31 94 isolates. Among the 122 124 isolates, MIC values were lower than for 31 94 isolates ranging from 122 124 isolates produced small colonies (50%) and large colonies with rough borders (50%) when plated on YEM. The genetic diversity of serotype 122 124 isolates differed with soil type as PFGE patterns revealed nine genotypes among the 16 isolates from the mineral organic (Cape Fear) soil and only three genotypes among the 14 isolates from the sandy mineral (Dothan) soil. Only two of the 12 genotypes were common to both soils. Sixty percent of the 122 124 isolates were classified as having high N2-fixation capacity and none induced foliar chlorosis. Pulsed-field gel-electrophoresis pattern was the only trait that generated groups of isolates that were similar with respect to other measured traits.