The influence of crop rotation and soil fumigation on a mycorrhizal fungal community associated with soybean

Population densities of mycorrhizal fungal propagules in a western Kentucky field highly productive for soybean were measured by bioassay throughout a soybean production season. The primary experimental variables were crop rotation (soybeans in 1985, then 2 years in corn, milo, fescue, or soybean, then soybean in 1988 on all plots when populations of propagules were determined) and soil fumigation with 67% methyl bromide/33% chloropicrin. Of the 20 species in three genera found, Glomus predominated both in terms of number of species and population densities. Most species of Glomus occurred at higher population densities in rotated plots than in continuous soybean plots. In continuous soybean plots, species of Gigaspora made up a much higher proportion of the mycorrhizal fungal community than in rotated crops. Species richness and diversity were lower, and dominance and equitability higher, in nonfumigated continuous soybean plots than in rotated plots early in the season, but the differences were not present at the end of the season. Soil fumigation killed most propagules in the upper 15 cm of soil, but after production of a crop of soybeans, populations of total propagules and most Glomus spp. recovered to prefumigation densities. However, Gigaspora margarita and Gigaspora gigantea did not recover similarly. Fumigation reduced species richness and diversity and increased dominance, but the effects were ameliorated by the end of the season. Colonization of roots was low during vegetative growth but increased rapidly after the onset of soybean reproduction. There was no evidence for mutualism during the early half of the season, perhaps due to high soil P and low dependency of soybean. Fumigation increased soybean yields. A stable mycorrhizal fungal community appeared to become established with continuous soybean production, and both crop rotation and soil fumigation disrupted the community.     

Elevated atmospheric CO2 effects on biomass production and soil carbon in conventional and conservation cropping systems

Increasing atmospheric CO₂ concentration has led to concerns about potential effects on production agriculture as well as agriculture's role in sequestering C. In the fall of 1997, a study was initiated to compare the response of two crop management systems (conventional and conservation) to elevated CO₂. The study used a split‐plot design replicated three times with two management systems as main plots and two CO₂ levels (ambient=375 μL L^?1 and elevated CO₂=683 μL L^?1) as split‐plots using open‐top chambers on a Decatur silt loam (clayey, kaolinitic, thermic Rhodic Paleudults). The conventional system was a grain sorghum (Sorghum bicolor (L.) Moench.) and soybean (Glycine max (L.) Merr.) rotation with winter fallow and spring tillage practices. In the conservation system, sorghum and soybean were rotated and three cover crops were used (crimson clover (Trifolium incarnatum L.), sunn hemp (Crotalaria juncea L.), and wheat (Triticum aestivum L.)) under no‐tillage practices. The effect of management on soil C and biomass responses over two cropping cycles (4 years) were evaluated. In the conservation system, cover crop residue (clover, sunn hemp, and wheat) was increased by elevated CO₂, but CO₂ effects on weed residue were variable in the conventional system. Elevated CO₂ had a greater effect on increasing soybean residue as compared with sorghum, and grain yield increases were greater for soybean followed by wheat and sorghum. Differences in sorghum and soybean residue production within the different management systems were small and variable. Cumulative residue inputs were increased by elevated CO₂ and conservation management. Greater inputs resulted in a substantial increase in soil C concentration at the 0–5 cm depth increment in the conservation system under CO₂‐enriched conditions. Smaller shifts in soil C were noted at greater depths (5–10 and 15–30 cm) because of management or CO₂ level. Results suggest that with conservation management in an elevated CO₂ environment, greater residue amounts could increase soil C storage as well as increase ground cover.     

Evaluation of chemical seed coat sterilants

Summary Three seed coat sterilants were evaluated for effectiveness as surface sterilants and for effects on seed germination. Several exposure times in sodium hypochlorite, hydrogen peroxide and peracetic acid were evaluated. Seeds of the following crop plants were used: wheat (Triticum aestivum L.); sorghum [Sorghum bicolor (L.) Moench]; soybean [Glycine max (L.) Merrill]. All treatments reduced germination of wheat and soybean seed, but only the most severe peracetic acid treatments substantially reduced germination of sorghum seed. Considering both sterilization effectiveness and seed germination, a treatment with 5% sodium hypochlorite for 45 min appeared to be the most satisfactory of the treatments used.     

Nitrogen mineralization and potential nitrification at different depths in acid forest soils

Yield decline of cereals grown in monoculture may be alleviated with alternative crop management strategies. Crop rotation and optimized tillage and fertilizer management can contribute to more sustainable food and fiber production in the long-term by increasing diversity, maintaining soil organic matter (SOM), and reducing adverse effects of excessive N application on water quality. We investigated the effects of crop sequence, tillage, and N fertilization on long-term grain production on an alluvial, silty clay loam soil in southcentral Texas. Crop sequences consisted of monoculture sorghum (Sorghum bicolor (L.) Moench,) wheat (Triticum aestivum L.), and soybean (Glycine max (L.) Merr), wheat/soybean double-crop, and rotation of sorghum with wheat/soybean. Grain yields tended to be lower with no tillage (NT) than with conventional tillage (CT) early in the study and became more similar after 11 years. Nitrogen fertilizer required to produce 95% to maximum sorghum yield was similar for monoculture and rotation upon initiation of the experiment and averaged 16 and 11 mg N g^-1 grain with NT and CT, respectively. After 11 years, however, the N fertilizer requirement became similar for both tillage regimes, but was greater in monoculture (17 mg N g^-1 grain) than in rotation (12 mg N g^-1 grain). Crop sequences with double-cropping resulted in greater land use efficiency because similar or lower amounts of N fertilizer were required to produce equivalent grain than with less intensive monoculture systems. These more intensive crop sequences produced more stover with higher N quality primarily due to the inclusion of soybean in the rotation. Large quantities of stover that remained on the soil surface with NT led to greater SOM content, which increased the internal cycling of nutrients in this soil. In southcentral Texas, where rainfall averages nearly 1000 mm yr^-1, more intensive cropping of sorghum, wheat, and soybean with moderate N fertilization using reduced tillage can increase grain production and potentially decrease N losses to the environment by cycling more N into the crop-SOM system.     

Green manures and crop sequences influence alfalfa root rot and pathogen inhibitory activity among soil-borne streptomycetes

A two-year trial was conducted to determine the effects of green manures and crop sequences on plant disease, streptomycete and bacterial densities, and inhibitory activity of indigenous streptomycetes against four target pathogens. Green manure treatments, buckwheat (Fagopyrum esculentum L.), canola (Brassica napus L.), sorghum-sudangrass (Sorghum bicolor) (L.) Moench × Sorghum sudanense (Piper) Stapf.), and fallow control were tested in conjunction with three crop sequences in a Phytophthora-infested soil placed in containers. Alfalfa (Medicago sativa L.), potato (Solanum tubersoum L.), or corn (Zea mays L.) was grown in the first year, and alfalfa was grown in all containers in the second year. Compared to fallow controls, alfalfa grown in sorghum-sudangrass- or buckwheat-treated soil had significantly greater stand counts and total biomass, respectively. In addition, alfalfa grown in fallow-treated soils had the greatest Phytophthora root rot as a function of stand count. Crop rotation also had a significant effect on alfalfa root rot and yield. Potato scab disease intensity was greatest on tubers grown in fallow-treated soils, while tubers grown in canola-treated soils had the highest yields (total tuber weight). Green-manure-treated soils tended to have greater streptomycete and bacterial densities than fallow-treated soils. In addition, buckwheat- or sorghum-sudangrass-treated soils had greater proportions of streptomycetes that were antagonistic against the target pathogens than fallow-treated soils. The proportion of antagonists in soil was negatively correlated with alfalfa root rot, and positively correlated with alfalfa stand counts. Inhibitory activity of the streptomycetes was also negatively correlated with potato scab and positively correlated with potato yield. These data suggest that green manures may provide a strategy for increasing pathogen inhibitory activity within the streptomycete community in soil, and, in conjunction with crop rotation, may contribute to the control of a diverse collection of soil-borne plant pathogens on multiple crop species.     

Interactive effects of methyl jasmonate and salicylic acid on floret opening in spikelets of Sorghum

Floret opening of excised spikelets in Sorghum bicolor (L.) Moench and Sorghum Sudanesis (Piper) Stapf was significantly stimulated by immersing into 2 mM methyl jasmonate (MeJA) solution. In male sterile (MS) lines of Sorghum bicolor (L.) Moench, floret opening was more sensitive to MeJA than that in maintainer (MT) lines. Salicylic acid (SA) could abolish the effect of MeJA on the opening of spikelets. These data indicate that MeJA plays a role in floret opening in sorghum, and that SA interacts with MeJA in regulating of this process.     

Associations between host plant concentrations of selected biochemical nutrients and Mexican rice borer,Eoreuma loftini,infestation

The Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), is an economic pest of sugarcane (Saccharum spp. hybrids) and other graminaceous host crops, and it attacks grassy weeds. Oviposition preference has been known to be for plants with leaves that form folds. This study is the first to associate the nutritional quality of crop and forage plant hosts with Mexican rice borer injury. Three experiments were conducted to determine the levels of selected biochemical nutrients, including free amino acids and sugars, in four grass weeds [barnyardgrass, Echinochloa crus‐galli (L.); broadleaf signalgrass, Urochloa platyphylla (Munro ex C. Wright); johnsongrass, Sorghum halepense (L.) Pers.; and Vasey's grass, Paspalum urvillei Steud.], a forage grass [sudangrass, Sorghum bicolor (L.) Moensch spp. drummondi (Nees ex Steud.) de Wet & Harlan], three crop species [maize, Zea mays L.; sorghum, Sorghum bicolor (L.) Moench; and sugarcane, Saccharum spp.], and a sorghum‐sudangrass forage hybrid (all Poaceae). Of 16 free amino acids detected among plants in the first two experiments, only high accumulations of free histidine in sudangrass and maize were associated with increased infestations by Mexican rice borer larvae. In all three experiments, high levels of fructose were associated with heightened Mexican rice borer infestation. Ramifications of these findings on the potential dispersal of this invasive pest in the USA and possible applications of fructose in baits are discussed.     

Recent advances in breeding for improving iron utilization by plants

Iron-chlorosis deficiency may occur when an iron-inefficient genotype is grown on calcareous soil. One way to correct the problem is to modify the genotype by plant breeding. Cultivars have been released for oat (Avena byzantina C. Koch), sorghum [Sorghum bicolor (L.) Moench], dry bean (Phaseoulus vulgaris L.), and soybean [Glycine max (L.) Merr.]. Progress is being made in peanut (Arachis hypogea L.), forage species such as clovers (Trifolium sp.) and bluestems (Botriochloa sp.), and pepper (Capsicum annuum L.). Screening of rootstocks is done on citrus (Citrus sp.), mango (Manguifera indica L.), and avocado (Persea americana Mill.).     

Benefit and cost analysis and phosphorus efficiency of VA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels

Sorghum [Sorghum bicolor (L.) Moench] was grown in a greenhouse in a low P (3.6 mg kg^-1) soil (Typic Argiudolls) inoculated with the vesicular-arbuscular mycorrhizal fungi (VMAF) Glomus fasciculatum and P added at 0, 12.5, 25.0, and 37.5 mg kg^-1 soil to determine the effects of VAMF-root associations on plant growth, benefit and cost analysis, and P efficiency (dry matter produced/unit P absorbed). Root colonization with VAMF and shoot growth enhancements decreased with increased soil P applications. Mycorrhizal plants were less P efficient than nonmycorrhizal plants. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants were considered the benefit derived by plants from VAMF-root associations. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants with similar P concentrations were considered the costs paid by plants for VAMF-root associations. Values of benefit and cost analysis for VAMF-root associations were highest when soil P was lowest and decreased with increasing P applications. Genotypic differences for calculated costs were pronounced, but not benefits. Benefit and cost analysis.may be helpful to evaluate host plant genotypes and VAMF species to optimize efficiencies of VAMF symbiosis in different soil environments.     

Characterization of Strigolactones, Germination Stimulants for the Root Parasitic Plants Striga and Orobanche, Produced by Maize, Millet and Sorghum

The germination stimulants for root parasitic plants Striga and Orobanche produced by sorghum (Sorghum bicolor (L.) Moench), maize (Zea mays L.), and pearl millet (Pennisetum typhoideum Rich.) were examined. Characterization of strigolactones in the root exudates from the plants grown hydroponically was conducted by comparing retention times of germination stimulants on reverse phase high performance liquid chromatography (HPLC) with those of synthetic standards, and by using HPLC linked with tandem mass spectrometry (LC/MS/MS). All the plants tested, except for a sorghum cultivar Swarna, were found to exude two major stimulants, 5-deoxy-strigol, which is known as a branching factor for arbuscular mycorrhizal (AM) fungi, and an isomer of strigol, tentatively named sorghumol. Swarna was found to exude 5-deoxy-strigol and strigol. These results imply that 5-deoxy-strigol is one of major germination stimulants of gramineous plants and that major stimulants may differ even among cultivars within the same species.     

Influence of washing on elemental analysis of leaves of fieldgrown crop plants

Summary Information is limited on soil contamination of leaves from field-grown row crops, especially with respect to aluminum (Al) analyses. The objective of this study was to determine the influence of washing leaf samples with either deionized water or detergent solution on elemental analyses for several agronomic crop plants. The crop plants sampled were corn (Zea mays L.), soybean (Glycine max L. Merr.), grain sorghum (Sorghum bicolor L. Moench), and wheat (Triticum aestivum L.). The crops were grown on a range of soil types, soil pH values, and tillage practices. Samples of upper leaves and lower leaves were collected separately. The samples were either not washed, washed with deionized water, or washed with detergent solution. After drying, grinding, and digesting, the samples were analyzed for Al, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu). For all crop plants and conditions studied, there was no effect on measured N, P, K, Ca, Mg, Mn, Zn, or Cu concentrations, but measured Al and Fe concentrations were influenced by washing. In general, washing had a greater effect on Al analyses than on Fe analyses. Soybean samples were most affected by washing, while wheat samples seemed to be least affected. The results reflected greater contamination of lower leaves than upper leaves. Decontamination procedures appear necessary prior to Al and Fe analyses of field-grown crop plants.     

Nitrate Reductase Activity: A Biochemical Criterion of Hybrid Vigour in Sorghum bicolor (L.) Moench

F₁ hybrids of Sorghum bicolor (L.) Moench and their inbred parentswere analysed for NADH-nitrate reductase activity during theearly stages of seedling growth. In all the hybrids both mid-parentaland better parental heterosis were discernible in shoots whereasin roots two hybrids out of the three tested, showed heteroticlevels. It is suggested that in sorghum nitrate reductase activityduring seedling stages can be used as a biochemical criterionfor evaluating hybrid vigour. Sorghum bicolor (L.) Moench, sorghum, hybrid vigour, nitrate reductase     

Chloroplast DNA polymorphism in fertile and male-sterile cytoplasms of sorghum (Sorghum bicolor (L.) Moench)

Summary Restriction endonuclease patterns of chloroplast DNA (cpDNA) were consistently distinguishable between fertile and male-sterile cytoplasms of sorghum [Sorghum bicolor (L.) Moench], whereas no differences in restriction patterns of cpDNA among male-sterile (A₁) lines, including six isocytoplasmic strains, were revealed in this study. It is suggested that chloroplast DNA may contribute to the male sterility of A₁ lines used currently in hybrid sorghum production.This research was supported by a research grant from Kansas Grain Sorghum Commission, Kansas Board of Agriculture. Contribution 90-293-J from the Kansas Agricultural Experiment Station     

Restriction endonuclease analysis of mitochondrial DNA from sorghum with fertile and male-sterile cytoplasms

Summary Mitochondrial DNA from four paired (fertile and male-sterile) lines and six isocytoplasmic strains of sorghum (Sorghum bicolor (L.) Moench) were fragmented by endonucleases and their electrophoretic patterns were examined. Cytoplasmic male sterile lines differed from their male-fertile counterparts consistently. Among the isocytoplasmic strains, KS 36A (S. verticilli-florum cytoplasm), KS 38A (S. conspicum cytoplasm), and KS 39A (S. niloticum cytoplasm) showed minor differences from the other strains. Results suggest that restriction endonuclease patterns are useful in detecting differences in mitochondrial genomes.This study was supported by a research grant from Kansas Grain Sorghum Commission, Kansas Board of Agriculture. Contribution 89-28-J from the Kansas Agricultural Experiment Station.     

Resistance and resilience of root fungal communities to water limitation in a temperate agroecosystem

Understanding crop resilience to environmental stress is critical in predicting the consequences of global climate change for agricultural systems worldwide, but to date studies addressing crop resiliency have focused primarily on plant physiological and molecular responses. Arbuscular mycorrhizal fungi (AMF) form mutualisms with many crop species, and these relationships are key in mitigating the effects of abiotic stress in many agricultural systems. However, to date there is little research examining whether (1) fungal community structure in agroecosystems is resistant to changing environmental conditions, specifically water limitation and (2) resilience of fungal community structure is moderated by agricultural management systems, namely the integration of trees into cropping systems. Here, we address these uncertainties through a rainfall reduction field experiment that manipulated short‐term water availability in a soybean‐based (Glycine max L. Merr.) agroforest in Southern Ontario, Canada. We employed terminal restriction fragment length polymorphism analysis to determine the molecular diversity of both general fungal and AMF communities in soybean roots under no stress, stress (rainfall shelters added), and poststress (rainfall shelters removed). We found that general fungal and AMF communities sampled from soybean roots were resistant to rainfall reduction in a monoculture, but not in an agroforest. While AMF communities were unchanged after stress removal, general fungal communities were significantly different poststress in the agroforest, indicating a capacity for resiliency. Our study indicates that generalist fungi and AMF are responsive to changes in environmental conditions and that agroecosystem management plays a key role in the resistance and resilience of fungal communities to water limitation.     

Biochemical Basis of Heterosis in Sorghum: Changes in Chlorophylls and Ascorbic Acid Turnover during Seedling Growth

Shoots of three F1 hybrids of Sorghum bicolor (L.) Moench withtheir inbred parents were analyzed for chlorophyll a and chlorophyllb contents as well as for ascorbic acid turnover during theearly stages of seedling growth. In all the hybrids both chlorophylland ascorbic acid turnover showed better-parental and/or mid-parentalheterosis. Since ascorbate participates in photosynthesis andnitrate reduction, it is suggested that a well co-ordinatedsystem incorporating photosynthetic efficiency and nitrate assimilationmay be associated with the manifestation of hybrid vigour insorghum. Sorghum bicolor (L.) Moench, hybrid vigour, chlorophyll, ascorbic acid, nitrate reductases     

Effects of species of VA-mycorrhizal fungi on growth and mineral uptake of sorghum at different temperatures

Sorghum [Sorghum bicolor (L.) Moench] plants were grown in growth chambers at 20, 25 and 30°C in a low P Typic Argiudoll (3.65 µg P g^–1 soil, pH 8.3) inoculated with Glomus fasciculatum, Glomus intraradices, and Glomus macrocarpum to determine effects of vesicular-arbuscular mycorrhizal fungi (VAMF) species on plant growth and mineral nutrient uptake. Sorghum root colonization by VAMF and plant responses to Glomus species were temperature dependent. G. macrocarpum colonized sorghum roots best and enhanced plant growth and mineral uptake considerably more than the other VAMF species, especially at 30°C. G. fasciculatum enhanced shoot growth at 20 and 25°C, and mineral uptake only at 20°C. G. intraradices depressed shoot growth and mineral uptake at 30°C. G. macrocarpum enhanced shoot P, K, and Zn at all temperatures, and Fe at 25 and 30°C above that which could be accounted for by increased biomass. Sorghum plant growth responses to colonization by VAMF species may need to be evaluated at different temperatures to optimize beneficial effects.     

Molecular genetic analysis of the soriz genome (<Emphasis Type="Italic">Sorghum oryzoidum</Emphasis>)

A molecular genetic analysis of soriz genotypes (Sorghum oryzoidum), its parental form Sorghum bicolor (L.) Moench (grain sorghum), possible parents (Sorghum sudanense (Piper.) Stapf. (Sudan grass) and Oryza sativa L. (Rice planting), as well as its closest relatives, has been carried out with the use of microsatellite loci of sorghum and rice. Based on the obtained data, the genetic distances were calculated and the examined species were clustered. It was shown that soriz did not carry rice DNA fragments, but its genome contained DNA fragments, which belonged to Sudan grass. This confirms that the origin of soriz is associated with representatives of Sorghum sudanense.     

Prospects for using conventional techniques and molecular biological tools to enhance performance of `orphan' crop plants on soils low in available phosphorus

Molecular biology, combined with Mendelian and quantitative genetics in quantitative trait locus (QTL) mapping and marker-assisted selection (MAS), provides powerful new tools to facilitate efficient genetic manipulation by plant breeders of complex traits such as drought tolerance and phosphorus (P) acquisition ability. This paper examines current opportunities for genetically manipulating the ability of crop plants to more efficiently acquire (i.e. access and take up) essential soil nutrients, using as examples P and several of the crops in the genetic improvement mandate of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) – chickpea (Cicer arietinum L.), groundnut (Arachis hypogaea L.), pearl millet [Pennisetum glaucum (L.) R. Br.], pigeonpea [Cajanus cajan(L.) Millsp.], and sorghum [Sorghum bicolor (L.) Moench]. It is concluded that for at least some of these important, but often academically and economically orphaned tropical food, forage and feed-grain crops, the genetic variation and molecular tools that we will need already exist or can be expected to become available in the very near future. With appropriate, targeted research, these tools can permit empirical exploration of the potential for marker-facilitated mapping and manipulation of major genes that can contribute to enhanced ability of these crops to acquire P from sources with limited availability. With these tools, delivery of new versions of currently popular high-yielding, high quality, disease resistant crop cultivars, having genetically improved ability to acquire P currently in soils but unavailable for crop growth, could take as little as five to seven years. Sustainable use of such improved cultivars would require their utilization as components of integrated soil fertility management systems.     

Genetic transformation of Sorghum bicolor

Great millet (Sorghum bicolor (L.) Moench) is cultivated across the world for food and fodder. It is typically grown in semiarid regions that are not suitable for cultivation of other major cereals. Sexual incompatibility and shortage of available genes in germplasm to combat biotic and abiotic stresses resulted in marginalized yields of this crop. Genetic modification of sorghum with agronomically useful genes can address this problem. Here, we tried to review and summarize the key aspects of sorghum transformation work being carried out so far by various research groups across the world. The approaches used and the obstacles in generating transgenic sorghum are also pointed out and discussed.