Fusarium oxysporum and post-emergence herbicide for the control of the parasitic plant Striga hermonthica in maize

Two trials were conducted to evaluate the efficacy of a granular mycoherbicide formulation based on Fusarium oxysporum and post-emergence herbicide for the control of the parasitic plant, Striga hermonthica in the Nigerian Savanna. Four fungal treatments were used: F. oxysporum followed by 2,4-D, F. oxysporum followed by supplementary hoe weeding, F. oxysporum followed by Triclopyr and a control (No F. oxysporum but hoe-weeded). The experiments were laid out in a split plot design with three replications in the two locations. The two varieties (Across 97 TZL and farmer's local variety) formed the main plot treatments, while the Striga fungal treatments formed the sub-plot treatments. At the Lafia location, the emergence of Striga was delayed by 7 days (46 days) as compared to the Makurdi location, which germinated earlier 39 days after sowing. Maize variety Across 97 TZL similarly delayed the time to Striga emergence when compared to the farmer's local variety. However, the different Striga control methods did not have any significant effect on the time of Striga emergence. Generally, number of maize plants infected with Striga was highest with the farmer's local variety throughout the period of observation, while in the Striga control treatments, hoe-weeded check recorded the highest; the minimum was obtained with plots treated with F. oxysporum followed by postemergence application of Triclopyr at the rate of 0.36 kg a.i./ha at 6 weeks after sowing. Highest maize grain yields were obtained at Lafia with Across 97 TZL and plots treated with F. oxysporum followed by either post-emergence 2,4-D or Triclopyr at 0.36 kg a.i./ha each. The results demonstrate the high potentiality of using F. oxysporum as a spot application at planting followed by post-emergence herbicide (2,4-D or Triclopyr) application at 6 weeks after sowing for the control of the parasitic plant S. hermonthica in the Nigerian Savanna.     

Variability in host plant resistance of Sorghum to Striga Hermonthica infestation in West Africa

Fourteen elite sorghum lines were evaluated for their resistance to Striga hermonthica at three locations in Nigeria and Mali. Results showed that many of the lines especially MALISOR 84-1, SAMSORG 41, 97-SB-F5DT-64 (Keninkédié) and the check SRN 39 remained resistant to Striga in all locations with low emerged Striga counts, while SAMSORG 14 had the highest Striga infestation in all locations. Considerable variation in reaction to Striga infestation was observed on Séguètana, 97-SB-F5DT-63 (Wasa), 97-SB-F5DT-65, CMDT 38, CMDT 39 and CMDT 45 which were susceptible to Striga at Samaru, Nigeria but were resistant to Striga at both locations in Mali. Based on low Striga resistance and high grain yield, lines MALISOR 84-1, SAMSORG 41, 97-SB-F5DT-64, 97-SB-F5DT-65, CMDT 39 and SAMSORT 14 have been nominated for wider evaluation across more West African countries.     

Effect of intercropping maize and soybeans on Striga hermonthica parasitism and yield of maize

Striga hermonthica a major biotic constraint to cereal production can be controlled by trap crops. Soybean cultivars vary in ability to stimulate suicidal germination of the weed. An experiment was conducted to select soybean (Glycine max) varieties with the ability to stimulate germination of S. hermonthica seeds. Experiments were conducted with strigol Nijmegen 1® (GR 24), a synthetic stimulant, as a check. In the pot and field experiments, maize (variety WH507) was intercropped with soybeans. Variation occurred among soybean varieties in inducing germination of S. hermonthica. The relative germination induction by soybean varieties ranged from 8% to 66% compared to 70% for synthetic stimulant check. Varieties TGX1448-2E, Tgm 1576, TGX1876-4E and Tgm 1039 had the highest relative germination. Soybean varieties TGX 1831-32E, Tgm944, Tgm 1419 and Namsoy4m had high stimulation but low attachment. Intercropping maize with soybeans in the field led to a low S. hermonthica count and high maize yield.     

The parasitic angiosperm Striga hermonthica can reduce photosynthesis of its sorghum and maize hosts in the field

Two cultivars of sorghum (CK60 and Ochuti) and one cultivarof maize (H511) were grown in field plots in western Kenya inthe presence or absence of the parasitic angiosperm Striga hermonthica,with or without a single addition of nitrogen fertilizer (150kg N ha^–1) using a factorial design. A progressive declinein rates of photosynthesis of Striga-infected plants were observedfor the sorghum cultivar CK60 from 30 d after planting (DAP)and for maize from 40 DAP, until measurements ended 63 DAP.At this time photosynthetic rates were 46% and 31% lower inthe Striga-infected sorghum and maize cultivars, respectively,compared to uninfected control plants. No decline in photosynthesiswas observed in the second sorghum cultivar studied, Ochuti,a local land race reported to show some tolerance to the parasite.The trends in photosynthesis reflected stunting of the cereals,as determined by the height of the youngest emerged ligule,however, only the grain yield of the sorghum cultivar CK60 wassignificantly reduced by the presence of the parasite. The nitrogenapplication influenced neither the growth nor the photosyntheticparameters measured, and possible explanations for the absenceof responses are discussed. It is concluded that S. hermonthicacan reduce photosynthetic rates of field-grown sorghum and maize,and suggest that an ability to maintain high rates of photosynthesiswhilst infected may be an important correlate of tolerance tothe parasite. Key words: Parasitic angiosperm, photosynthesis, nitrogen, tropical weeds, tropical agriculture     

Rational design of Striga hermonthica-specific seed germination inhibitors

The obligate hemiparasitic weed Striga hermonthica grows on cereal roots and presents a severe threat to global food security by causing enormous yield losses, particularly in sub-Saharan Africa. The rapidly increasing Striga seed bank in infested soils provides a major obstacle in controlling this weed. Striga seeds require host-derived strigolactones (SLs) for germination, and corresponding antagonists could be used as germination inhibitors. Recently, we demonstrated that the common detergent Triton X-100 is a specific inhibitor of Striga seed germination by binding noncovalently to its receptor, S. hermonthica HYPO-SENSITIVE TO LIGHT 7 (ShHTL7), without blocking the rice (Oryza sativa) SL receptor DWARF14 (OsD14). Moreover, triazole ureas, the potent covalently binding antagonists of rice SL perception with much higher activity toward OsD14, showed inhibition of Striga but were less specific. Considering that Triton X-100 is not suitable for field application and by combining structural elements of Triton and triazole urea, we developed two hybrid compounds, KK023-N1 and KK023-N2, as potential Striga-specific germination inhibitors. Both compounds blocked the hydrolysis activity of ShHTL7 but did not affect that of OsD14. Binding of KK023-N1 diminished ShHTL7 interaction with S. hermonthica MORE AXILLARY BRANCHING 2, a major component in SL signal transduction, and increased ShHTL7 thermal specificity. Docking studies indicate that KK023-N1 binding is not covalent but is caused by hydrophobic interactions. Finally, in vitro and greenhouse tests revealed specific inhibition of Striga seed germination, which led to a 38% reduction in Striga infestation in pot experiments. These findings reveal that KK023-N1 is a potential candidate for combating Striga and a promising basis for rational design and development of further Striga-specific herbicides.

Designed strigolactone antagonists inhibit Striga seed germination.     

Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi

Root colonization by arbuscular mycorrhizal (AM) fungi reduces stimulation of seed germination of the plant parasite Striga (Orobanchaceae). This reduction can affect not only host plants for Striga, resulting in a lower parasite incidence, but also false hosts or trap crops, which induce suicidal Striga seed germination, thereby diminishing their effectiveness. In order to better understand these AM-induced effects, we tested the influence of root colonization by different AM fungi on the seed-germination activity of root exudates of the Striga hermonthica nonhost plants cowpea and cotton on S. hermonthica. We also tested the effect of AM fungi on the seed-germination activity of the Striga gesnerioides host plant cowpea on S. gesnerioides. Moreover, we studied whether mycorrhization affects the transport of seed-germination activity to above-ground plant parts. Mycorrhization not only resulted in a lower seed germination of S. gesnerioides in the presence of root exudates of the S. gesnerioides host cowpea but also seed germination of S. hermonthica was also lower in the presence of root exudates of the S. hermonthica nonhosts cowpea and cotton. Downregulation of the Striga seed-germination activity occurs not only in root exudates upon root colonization by different AM fungi but also in the compounds produced by stems. The lowered seed-germination activity does not appear to depend on the presence of seed germination inhibitors in the root exudates of mycorrhizal plants. The implication for Striga control in the field is discussed.     

Field assessment of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> based mycoherbicide for control of <Emphasis Type="Italic">Striga hermonthica</Emphasis> in Nigeria

Fusarium oxysporum (isolate PSM 197) based mycoherbicide was evaluated for its efficacy under field conditions in trials conducted during 1999--2001 cropping seasons in the Nigerian savanna. In the 1999 cropping season, spot application of 5--10 g of mycoherbicide was found to give effective control of Striga hermonthica. Results of on-farm trials at Barhim and Dutsen-Ma areas showed the application of the mycoherbicide to significantly (p= 0.05) increase stand count at both 3 weeks and at harvest, reduced Striga shoot count and increased crop yield in both improved and local sorghum varieties, as compared with the same varieties not treated with the mycoherbicide. Results establish the efficacy of F. oxysporum as a mycoherbicide and the need for further development of the mycoherbicide into formulated granules or seed treatment for use in control of S. hermonthica under field conditions.     

Uptake and Partitioning of Nitrogen by Maize Infected withStriga hermonthica

The uptake and partitioning of nitrogen (N) by maize infectedwith the parasitic angiosperm,Striga hermonthicawas investigatedin sand culture in a glasshouse. The purpose was to determinethe effect ofStrigaon N uptake and partitioning in maize. Maizewas grown at 22, 66 and 133 mg N per plant and sampled fivetimes. There was no significantStrigaxN interaction in any measuredresponse. Leaf dry matter ofStriga-infected maize, averagedover all N treatments, was 92% that of uninfected maize at thefour-leaf stage but by the 18-leaf stage it had decreased to58%. Similarly, stem dry matter of infected maize which was91% that of uninfected maize at the four-leaf stage was only42% at the 18-leaf stage. Root dry matter was similar for infectedand uninfected maize. N concentration in the leaf, stem androot declined asymptotically from the first to the last samplingdate for both infected and uninfected maize. The asymptoticvalue of N concentration inStriga-infected maize was 16% greaterin the leaf, 55% in the stem, and 21% in the root than in uninfectedmaize. The concentration of N inStrigawas higher than in maizeat the 16- and 18-leaf stages. Uptake of N was similar for infectedand uninfected plants at the four–eight leaf stage butat the eight–12 leaf stage, N uptake by infected maizewas 52% that of uninfected maize. However, the proportion oftotal plant nitrogen partitioned to the root was greater (P<0.001)forStriga-infected maize. These results showed that the extentto whichS. hermonthicareduced maize growth and N uptake, butincreased the proportion of N partitioned to the roots, didnot depend on the rate of N fertilizer applied.Copyright 1998Annals of Botany Company Maize; nitrogen; partitioning;Striga hermonthica; uptake.     

Fusarium oxysporum f. sp. strigae strain Foxy 2 did not achieve biological control of Striga hermonthica parasitizing maize in Western Kenya

The production of maize, a major staple food crop in sub-Saharan Africa is being constrained by the parasitic weed Striga hermonthica. The fungus Fusarium oxysporum f. sp. strigae (Foxy 2) that causes fusarium wilt of Striga in Ghana, West Africa, is being considered for biological control of the weed in Western Kenya. The present study investigated the efficacy of F. oxysporum f. sp. strigae (Foxy 2) for S. hermonthica management in Western Kenya. Research was conducted in post-entry quarantine (PEQ) facilities at Alupe, Busia, Homabay, Kibos and Siaya field stations for two seasons. Each PEQ was a split-plot, with 4 main blocks each having 6 treatment subplots. The treatments included seeds of two S. hermonthica-susceptible maize varieties, either coated with Foxy 2 using gum Arabic, gum Arabic alone, or left untreated. Data was collected over seven sampling periods on S. hermonthica population per plant, percentage of those that were wilting, and the severity of wilting. Maize plant growth parameters assessed included duration to 50% anthesis and 50% silking, plant height, number of leaves, stover and cob weights, and maize yield per hectare. Statistical analysis was done using SAS 9.1 software. Data on S. hermonthica population were analyzed by χ²-test using Proc Genmod (Poisson); while the other parameters were analyzed by Proc Mixed using study location, season and blocks as random effects, and the sampling periods as repeated effects. All the assessed parameters were similar between plants grown from seeds inoculated with F. oxysporum f. sp. strigae (Foxy 2), those coated with gum Arabic, and the ones without any coating. These parameters were also not different between the maize varieties. There are varying reasons for the disparities between results on F. oxysporum f. sp. strigae (Foxy 2) obtained in this Kenyan study, and those from researches outside this country. In conclusion, F. oxysporum f. sp. strigae strain Foxy 2 is predominantly safe on maize growth, but its efficacy in controlling S. hermonthica was not evident on the tested Kenyan soils.     

Towards effective resistance to Striga in African maize

The fascinating biology of Striga parasitism is manifest through a series of signal exchanges between the parasite and its host. As an obligate root hemi-parasite, Striga development is cued to exudates and solutes of host roots but with negative ramifications on host plant health. Striga control in crops, via a variety of biotechnological approaches, needs to be based on increased understanding of this intricate biology. Maize has become the major cereal crop of Africa. However, this New World transplant has shown a paucity of Striga resistance characters relative to native sorghum. In this paper, we review growing evidence for maize genetic defenses against early pre-emergent phases of the Striga life cycle, when the tolls of parasitism are first manifest. Resistance characters first described in maize wild relatives have now been captured in Zea mays. The possible stacking of new and complementary sources of resistance in improved maize varieties targeted for Striga prone areas is discussed. An integrated approach combining genetic with other control measures is advocated with a more realistic view of the resource challenges prevalent in African agriculture.Key words: Striga, parasitic weed, maize, sorghum, resistance, integrated control     

Variation of Seed-coat Ornamentation in Striga hermonthica (Scrophulariaceae)

The surface features of the seed of the parasitic floweringplant Striga hermonthica were examined with the scanning electronmicroscope. The details of ornamentation were constant on seedsfrom one plant but varied within and between populations andare probably due to out-breeding. The variation was not relatedto geographical origin or to host-preference. Testa patternssimilar to those reported for other Striga species were foundin the samples. Striga hermonthica, Scrophulariaceae: Rhinanthoideae, hemi-parasite, testa, seed-coat, scanning electron microscope, genetic variation     

Root nitrogen concentration of sorghum above 2% produces least Striga hermonthica seed stimulation

A series of pot and laboratory experiments was carried out to assess the effects of N status of sorghum roots and timing of N application (as NH₄NO₃) on the germination of Striga hermonthica seeds. Root N concentrations varied from 10 to 26 mg N g^?1. The cut root and the root exudates technique used in assaying S. hermonthica seed germination gave similar results. However, the cut root technique was easier to handle and more discriminating at low germination levels. S. hermonthica seed germination per unit sorghum root mass followed a broken‐stick model. It decreased with increasing root N concentrations, reaching lowest levels at a root N concentration of 19.5 mg N g^?1, after which no further reduction occurred. It was not possible to reduce S. hermonthica seed germination to a zero level. Timing of N application influenced the time a higher N concentration is reached, not the S. hermonthica seed germination. Both timing and rate of N application are important in maintaining root N concentrations above 19.5 mg N g^?1, thereby potentially reducing S. hermonthica germination in the field. Translation of results to reductions in infection levels and yield losses is hampered by density‐dependent relations after the S. hermonthica germination stage.     

Winter wheat roots grow twice as deep as spring wheat roots, is this important for N uptake and N leaching losses?

Nitrogen (N)-deficiency and lack of phosphorus (P) availability are major constraints to maize yields in Western Kenya. In a two-season field study in the lake Victoria basin, we tested the capacity of white lupin (Lupinus albus (L.), cv. Ultra), as a nitrogen-fixing crop with a highly efficient P-acquisition capacity, to increase maize yields when used as a companion or cover crop, or as a source of organic matter. Each experiment was performed on three different fields (Vertisols) differing in N/P availability, previous cropping history and in levels of infestation by the parasitic weed Striga hermonthica (Del.) Benth. Our results show that white lupin led to significantly higher yields of maize when used as a cover crop. When lupin was grown as a companion crop, it also slightly enhanced the yield of the co-cultivated maize. When lupin shoots were incorporated to the soil, the positive effect of lupin on maize growth was field-dependent and only occurred in the field most heavily infested with S. hermonthica. Despite the beneficial impact on maize yield, no clear effect of lupin on soil N and P availability or on maize N/P uptake were observed. In contrast, lupin significantly inhibited infestation of maize by S. hermonthica: when lupin was grown together with maize in pots inoculated with S. hermonthica, the emergence of the weed was strongly reduced compared to the pots with maize only. This work opens a new range of questions for further research on white lupin and its potential beneficial impact as a S. hermonthica-inhibiting crop.     

Striga hermonthica decreases photosynthesis in Zea mays through effects on leaf cell structure

Maize seedlings were grown in pots either with or without preconditionedseeds of the parasitic weed, Striga hermonthica. After between4 and 8 weeks, net photosynthesis in the leaves of maize plantsinfected with Striga decreased compared to leaves of uninfectedcontrol plants. The activities of four enzymes of photosyntheticmetabolism were, however, little affected by infection. A pulse-chaseexperiment using ¹⁴CO₂ showed that C₄ acids were the main earlyproducts of assimilation even when the rate of photosynthesiswas much decreased by infection, but more radio-activity appearedin glycine and serine than in leaves of healthy maize plants.Leaves of infected maize required longer to reach a steady rateof photosynthesis upon enclosure in a leaf chamber than leavesof uninfected plants after similar treatment. Electron microscopy of transverse sections of the leaves ofinfected maize indicated that the cell walls in the bundle sheathand vascular tissue were less robust than in leaves of healthyplants. The results suggest that infection with Striga causesan increase in the permeability of cell walls in the bundlesheath, leakage of CO₂ from the bundle sheath cells and decreasedeffectiveness of C₄ photosynthesis in host leaves. Key words: Zea mays, Striga hermonthica, photosynthesis, photorespiration, enzyme activity     

Colonization by Arbuscular Mycorrhizal Fungi of Sorghum Leads to Reduced Germination and Subsequent Attachment and Emergence of Striga hermonthica

Two sorghum cultivars: the Striga-tolerant S-35 and the Striga-sensitive CK60-B were grown with or without arbuscular mycorrhizal (AM) fungi, and with or without phosphorus addition. At 24 and 45 days after sowing (DAS) of sorghum, root exudates were collected and tested for effects on germination of preconditioned Striga hermonthica seeds. Root exudates from AM sorghum plants induced lower germination of S. hermonthica seeds than exudates from non-mycorrhizal sorghum. The magnitude of this effect depended on the cultivar and harvest time. A significantly (88–97%) lower germination of S. hermonthica seeds upon exposure to root exudates from AM S-35 plants was observed at both harvest times whereas for AM inoculated CK60-B plants a significantly (41%) lower germination was observed only at 45 DAS. The number of S. hermonthica seedlings attached to and emerged on both sorghum cultivars were also lower in mycorrhizal than in non-mycorrhizal plants. Again, this reduction was more pronounced with S-35 than with CK60-B plants. There was no effect of phosphorus addition on Striga seed germination, attachment or emergence. We hypothesize that the negative effect of mycorrhizal colonization on Striga germination and on subsequent attachment and emergence is mediated through the production of signaling molecules (strigolactones) for AM fungi and parasitic plants.Key Words: arbuscular mycorrhiza, root exudate, sorghum, striga, strigolactones, germination     

Antitranspirant-induced Heat Stress in the Parasitic Plant Striga hermonthica--A Novel Method of Control

Leaf temperatures (T₁) of the parasitic plant Striga hermonthicaare substantially below those of the air (T_a), [T_a–T₁]reaching 7 ?C at T_a = 40 ?C. This results from high rates oftranspiration and the consequent evaporative cooling of theleaf. Application of an antitranspirant, which mechanicallyimpedes foliar loss of water vapour, reduced transpiration andstomatal conductance by 40% and 57%, respectively, and reduced[T_a– T₁] to 2 ?C at T_a = 40 ?C. The temperature sensitivityof photosynthesis in the host-parasite association differed,the optima (T_opt) being 37.2 and 40.1 ?C for S. hermonthicaand sorghum, respectively. Once Topt had been exceeded in S.hermonthica net photosynthesis declined rapidly, reaching thelethal limit (Tmax) at 42.6 ?C. S. hermonthica is particularlysensitive to high temperatures and antitranspirant-induced overheatingleads to blackening and shrivelling of the leaf after as littleas 4 h at T_a = 40 ?C. Application of an antitranspirant underfield conditions in the Sudan at T_a = 40 ?C resulted in 28%and 67% reductions in transpiration and stomatal conductance,together with a 5 ?C increase in T₁, and subsequent leaf death.In addition to these short-term physiological responses, antitranspirantspraying of the arasite increased the grain and straw yieldof the crop by factors of 3.4 and 2.6, respectively. Antitranspirantsmay have potential use as a method of controlling Striga inthe field. Key words: Striga hermonthica, sorghum, photosynthesis, transpiration, high temperature stress, anti-transpirant     

Growth and carbon allocation in Pennisetum typhoides infected with the parasitic angiosperm Striga hermonthica

Abstract Growth and gas exchange measurements are used in conjunction with a carbon balance model to describe the millet (Pennisetum typhoides)–witchweed (Striga hermonthica) host—parasite association. Striga hermonthica reduces the growth of millet by 28% and radically alters the architecture of infected plants. Whilst grain yield and stem dry weight are reduced (by 80 and 53%, respectively), leaf and root growth are stimulated (by 41 and 86%, respectively). The difference in production between infected and uninfected millet plants can be accounted for by two processes: first, export of carbon to the parasite (accounting for 16% of the dry weight not gained); and second, parasite-induced reductions in host photosynthesis (accounting for 84% of the dry weight not gained). Striga hermonthica is dependent on carbon exported from the host, since the plant has low rates of photosynthesis coupled with high rates of respiration. The carbon balance model suggests that in mature S. hermonthica plants parasitic on millet, 85% of the carbon is host-derived. Carbon fluxes are also estimated using δ¹³C measurements, since S. hermonthica is a C₃ plant parasitizing a C₄ host. In conjunction with gas exchange measurements, these suggest that in root, stem and leaf of S. hermonthica, 87, 70 and 49% of carbon is hostderived, respectively.     

Genetic variation in strigolactone production and tillering in rice and its effect on Striga hermonthica infection

Tillering in cereals is a complex process in the regulation of which also signals from the roots in the form of strigolactones play an important role. The strigolactones are signalling molecules that are secreted into the rhizosphere where they act as germination stimulants for root parasitic plants and hyphal branching factors for arbuscular mycorrhizal fungi. On the other hand, they are also transported from the roots to the shoot where they inhibit tillering or branching. In the present study, the genetic variation in strigolactone production and tillering phenotype was studied in twenty rice varieties collected from all over the world and correlated with S. hermonthica infection. Rice cultivars like IAC 165, IAC 1246, Gangweondo and Kinko produced high amounts of the strigolactones orobanchol, 2′-epi-5-deoxystrigol and three methoxy-5-deoxystrigol isomers and displayed low amounts of tillers. These varieties induced high S. hermonthica germination, attachment, emergence as well as dry biomass. In contrast, rice cultivars such as Super Basmati, TN 1, Anakila and Agee displayed high tillering in combination with low production of the aforementioned strigolactones. These varieties induced only low S. hermonthica germination, attachment, emergence and dry biomass. Statistical analysis across all the varieties confirmed a positive correlation between strigolactone production and S. hermonthica infection and a negative relationship with tillering. These results show that genetic variation in tillering capacity is the result of genetic variation in strigolactone production and hence could be a helpful tool in selecting rice cultivars that are less susceptible to S. hermonthica infection.     

Nitrogen Fixation of Faba Bean (Vicia faba L.) Interacting with a Non-legume in Two Contrasting Intercropping Systems

A field experiment was carried out to quantify biological nitrogen fixation (BNF) using the ¹⁵N isotope natural abundance method in maize (Zea mays L.)/faba bean (Vicia faba L.) and wheat (Triticum aestivum L.)/faba bean intercropping systems. Faba bean was yielding more in the maize/faba bean intercropping, but not in the wheat/faba bean intercropping. Biomass, grain yield and N acquisition of faba bean were significantly increased when intercropped with maize, and decreased significantly with wheat, irrespective of N-fertilizer application, indicating that the legume could gain or lose productivity in an intercropping situation. There was yield advantage of maize/faba bean intercropping, but no in wheat/faba bean intercropping. The grain yield of the faba bean intercropped with maize was greater than that of faba bean monoculture due to increases of the stems per plant and the pods per stem of faba bean. N fertilization inhibited N fixation of faba bean in maize/faba bean and wheat/faba bean intercropping and faba bean monoculture. The responses of different cropping systems to N-fertilizer application, however, were not identical, with competitive intercropping (wheat/faba bean) being more sensitive than facilitative intercropping (maize/faba bean). Intercropping increased the percentage of N derived from air (%Ndfa) of the wheat/faba bean system, but not that of the maize/faba bean system when no N fertilizer was applied. When receiving 120 kg N/ha, however, intercropping did not significantly increase %Ndfa either in the wheat/faba bean system or in the maize/faba bean system in comparison with faba bean in monoculture. The amount of shoot N derived from air (Ndfa), however, increased significantly when intercropped with maize, irrespective of N-fertilizer application. Ndfa decreased when intercropped with wheat, albeit not significantly at 120 kg N/ha. Ndfa was correlated more closely with dry matter yield, grain yield and competitive ratio, than with %Ndfa. This indicates that that total dry matter yield (sink strength), not %Ndfa, was more critical for the legume to increase Ndfa. The results suggested that N fixation could be improved by yield maximization in an intercropping system.