Root Senescence Research Articles

Enzymes of Nitrogen Metabolism and Proteases Activity in Hairy Roots of Clover and Carrots, with and without Arbuscular Mycorrhizal Fungi

ABSTRACT Hairy roots can be used for metabolic studies and also as a substrate for arbuscular mycorrhizal fungi (AMF). However, little is known about the behavior of these roots when infected with AMF. The metabolism of hairy roots grown with or without AMF, on two culture media, was investigated. An increase in protease activity was observed when the roots were cultured on medium with high nitrogen (N)-content. Glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) activities increased along the development in both carrot and clover hairy roots. With the advancement of root senescence (at 132 d) a greater degradation of root tissue occurred, which was characterized by an increase in catabolic enzymes activity (proteases and GDH deamination). The results indicate that the glutamine synthetase/GOGAT pathway, characteristic of plant tissues, ceases to act in synchronism and is replaced by the GDH/GOGAT system under conditions of stress or senescence when excess ammonium is present.

Genotype and Planting Density Effects on Rooting Traits and Yield in Cotton (Gossypium hirsutum L.)

Root density distribution of plants is a major indicator of competition between plants and determines resource capture from the soil. This experiment was conducted in 2005 at Anyang, located in the Yellow River region, Henan Province, China. Three cotton (Gossypium hirsutum L.) cultivars were chosen: hybrid Bt-cultivar CRI46, conventional Bt-cultivars CRI44 and CRI45. Six planting densities were designed, ranging from 1.5 to 12.0 plants/m2Root parameters such as surface area, diameter and length were analyzed by using the DT-SCAN image analysis method. The root length density (RLD), root average diameter and root area index (RAI), root surface area per unit land area, were studied. The results showed that RLD and RAI differed between genotypes; hybrid CRI46 had significantly higher (P < 0.05) RLD and RAI values than conventional cultivars, especially under low planting densities, less than 3.0 plants/m2The root area index (RAI) of hybrid CRI46 was 61% higher than of CRI44 and CRI45 at the flowering stage. The RLD and RAI were also significantly different (P= 0.000) between planting densities. The depth distribution of RAI showed that at increasing planting densities RAI was increasingly distributed in the soil layers below 50 cm. The RAI of hybrid CRI46 was for all planting densities, obviously higher than other cultivars during the flowering and boll stages. It was concluded that the hybrid had a strong advantage in root maintenance preventing premature senescence of roots. The root diameter of hybrid CRI46 had a genetically higher root diameter at planting densities lower than 6.0 plants/m2Good associations were found between yield and RAI in different stages. The optimum planting density ranged from 4.50 plants/m2 to 6.75 plants/m2 for conventional cultivars and around 4.0–5.0 plants/m2 for hybrids. (Managing editor: Ya-Qin Han)

Transcriptional analysis between two wheat near-isogenic lines contrasting in aluminum tolerance under aluminum stress

To understand the mechanisms of aluminum (Al) tolerance in wheat (Triticum aestivum L.), suppression subtractive hybridization (SSH) libraries were constructed from Al-stressed roots of two near-isogenic lines (NILs). A total of 1,065 putative genes from the SSH libraries was printed in a cDNA array. Relative expression levels of those genes were compared between two NILs at seven time points of Al stress from 15 min to 7 days. Fifty-seven genes were differentially expressed for at least one time point of Al treatment. Among them, 28 genes including genes for aluminum-activated malate transporter-1, ent-kaurenoic acid oxidase-1, beta-glucosidase, lectin, histidine kinase, and phospoenolpyruvate carboxylase showed more abundant transcripts in Chisholm-T and therefore may facilitate Al tolerance. In addition, a set of genes related to senescence and starvation of nitrogen, iron, and sulfur, such as copper chaperone homolog, nitrogen regulatory gene-2, yellow stripe-1, and methylthioribose kinase, was highly expressed in Chisholm-S under Al stress. The results suggest that Al tolerance may be co-regulated by multiple genes with diverse functions, and those genes abundantly expressed in Chisholm-T may play important roles in enhancing Al tolerance. The down-regulated genes in Chisholm-S may repress root growth and restrict uptake of essential nutrient elements, and lead to root senescence.

Endophyte infection accelerates morpho‐physiological responses to water deficit in tall fescue

Endophyte‐infected (E+) tall fescue plants are usually more persistent under water deficit than endophyte‐free plants (E‐). The reason for this is still unclear because of conflicting reports about growth and water status variables. A glasshouse experiment was carried out to compare morpho‐physiological responses of E+ and E‐ tall fescue plants to water deficit during a cycle of water deprivation and recovery. Under non‐limiting conditions total dry mass per container was higher in E+ plants than in E‐ plants. However, no differences were found between endophyte treatments in root:shoot ratio, water status or in reducing sugar content of leaf blades or tiller bases. Under water limiting conditions an accelerated response to water deficit was observed in E+ plants. The relatively steeper growth cessation as well as higher leaf and root senescence in E+ than in E‐ were linked with a higher accumulation of sugars within the stem base of E+ plants which also tended to show a more pronounced decrease in water potential. Under water limiting conditions relative root: shoot ratio tended to decrease in E+ while remaining unchanged in E—. Following re‐irrigation, a general recovery in growth and water status variables was observed. A simple explanation is proposed to reconcile our results with previous reports, although further research is needed.

EFFECT OF SHADING ON THE SENESCENCE OF FINE ROOTS OF FRAXINUS MANDSHURICA SEEDLINGS

Fine root production and mortality play essential roles in carbon allocation and nutrient cycling in forest ecosystems. Despite the ecological importance of fine root turnover and the tremendous research effort on this subject, our knowledge of fine root mortality is still limited because little is understood on processes of fine root senescence. The most important function of fine roots is nutrient and water acquisition from the soil. Thus, to perform this function, carbohydrates fixed in leaves must be transported belowground to build the root system and support their functions, such as root elongation, nutrient and water uptake, and support microbes in the rhizosphere. More recently, studies on root longevity and root orders have found that first-order roots at the distal end of a root system are thinner in diameter and shorter in life-span than higher order roots. Also, fine root mortality was caused mainly by exhaustion of carbohydrates, suggesting that carbon allocation to fine roots may be an important factor affecting the senescence and mortality of fine roots. The objective of this study was to answer the following two questions: how does senescence of fine roots with different orders responds to reduced C allocation and what is the sequence of senescence from first-order to higher order roots? This study was conducted in greenhouse on the campus of Northeast Forestry University during growing season from April to October in 2004 using Fraxinus mandshurica seedlings as experimental materials. Three one-year-old F. mandshurica seedlings were planted in each of 30 pots with crop soil in April 2004. At the end of June, two light treatments were carried out. Seedlings in 15 pots were grown under natural sunlight (1 000 μmol·m -2 ·s -1 ) used as controls, and the other 15 pots were completely shaded. After treatments for 30, 60 and 120 d, roots in five pots were carefully excavated, washed and separated into root tips, first-order, second-order and third-order roots. Four physiological indices (i.e. root vigor, concentrations of soluble sugars and soluble proteins, and membrane permeability) were analyzed from the root samples of different orders for assessing the degree of fine root senescence. The results showed that there was reduced C allocation to the fine roots in the shading treatment that induced reduced root vigor, decreased concentrations of soluble sugars and soluble proteins, and increased membrane permeability, compared with controls. After the 60 and 120 d of shading, root vigor reduced 50% and 95%, soluble sugar concentrations decreased about 74% and 73%, and soluble proteins reduced 5% and 30%, respectively, but membrane permeability increased one-fold after 120 d.These results suggest that reduced C allocation to fine roots caused by leaf shading has great impacts on the physiological functions of fine roots and stimulated root senescence. Root senescence differed significantly among root order: root tips exhibited senescence followed by the first- and second-order roots, and the third-order roots had the least amount of senescence. As the physiological functions of the fine roots changed, coloration of the roots also changed with the root tips being a dark brown, and the first-, second- and third-order roots becoming increasing lighter in color. These results indicat that fine roots senesce in sequence explaining why fine roots of the first-order at the distal ends of a root system have a short life-span and rapid turnover.

Phosphorus Application Affects the Soybean Root Response to Water Deficit at the Initial Flowering and Full Pod Stages

Application of phosphorus (P) fertilizer is an important factor for improving the tolerance to water deficit in many plants. A pot experiment was conducted to identify the effects of P application on soybean adaptability to water deficit at the R1 (initial flowering) and R4 (full pod) stages through the investigation of root morphological traits, plant P uptake and resultant yield in two soybean (Glycine max L. Merrill) cultivars (Dongnong 46 and Heisheng 101). The four levels of P application were 0, 7.3, 14.6 and 29.2 mg kg−2, respectively. The three water treatments were (1) 65–75% of field water capacity (FWC) as a well-watered control, (2) 30–40% of FWC at the R1 stage, and (3) 30–40% of FWC at the R4 stage. Root traits, plant uptake of P and yield were significantly reduced by water deficiency at different growth stages, especially at the R4 stage. Application of P enabled to alleviate the adverse effects of water deficit, to increase the root dry weight, root length and root surface area, and to slow root senescence after the R5 (initial pod filling) stage. The response of soybean genotypes to both water and P deficit was different. In the absence of P application, Dongnong 46 showed relatively low adaptability to water deficit at the R4 stage, whereas Heisheng 101 showed a lower reduction of root traits and yield. The beneficial effects of P application for Dongnong 46 were more pronounced than those for Heisheng 101. Based on this experiment, we suggested that P fertilizer application to soybean may be justified in low-rainfall years because of its ability to enhance the soybean adaptability to water deficit stress by improving the root morphology, P uptake and consequently yield.

The mycorrhizal community in a forest chronosequence of Sitka spruce [Picea sitchensis (Bong.) Carr.] in Northern England

Demography and fungal diversity of the belowground ectomycorrhizal community in a chronosequence of Sitka spruce [Picea sitchensis (Bong.) Carr.] in Northumberland, Northern England, were analysed; mycorrhizal root samples were taken from 6-, 12-, 30- and 40-year-old stands, and fungal fruiting bodies were collected in autumn to complement the survey. Naturally germinated seedlings less than 1 year of age (taken from the 30-year-old stand) were also examined. A total of 118,000 mycorrhizal root tips were extracted from 40 soil cores (ten per age class) and from the complete root systems of 25 seedlings and separated into active and senescent root tips according to their morphology and anatomy. Active tips were distinguished according to their mycobionts which were characterised and identified microscopically. Although almost 100% of all fine roots were mycorrhizal, EM fungal diversity throughout the chronosequence was low, consisting of a total of 16 species of which three were only found as fruiting bodies. Of the six mycobionts found most regularly below ground, Tylospora fibrillosa was the most common, colonising about 70% of all root tips and more than 90% of those of seedlings and young trees. Root density and mycorrhizal diversity increased, but percentage of vital root tips decreased with increasing tree age, levelling off in the 30- and 40-year-old stand. Among the five subdominant fungal species, Dermocybe crocea was found to have its peak of distribution in the 12-year-old stand and Russula emetica, Lactarius rufus, Hymenoscyphus ericae agg. and the unidentified Piceirhiza sulfo-incrustata in the 30- and 40-year-old stands. The possible correlations between the mycorrhizal community structure and biotic and abiotic factors are discussed.

Enhanced nitrogen mineralization in mowed or glyphosate treated cover crops compared to direct incorporation

Information on how management by mowing and herbicide alter residue quality and nitrogen (N) inputs would be valuable to improve prediction of N availability. Mowing and glyphosate application are widely used by growers to limit cover crop growth and facilitate incorporation. A mixture of cover crops, hairy vetch (Vicia villosa L.), oriental mustard (Brassica junceaL.) and cereal rye (Secale cerealeL.), was investigated as a means to improve soil quality and optimize N availability. There is limited information on how mowing or glyphosate application influence cover crop decomposition and N mineralization from these heterogeneous residues. A rye cover crop was grown in the field over the winter and transferred to containers as an intact soil profile to conduct a greenhouse study. Management treatments (mowing and glyphosate) were imposed eight days before incorporation. Plant and soil N dynamics were monitored. The experiment was repeated with the addition of a tri-mixture cover crop. Inorganic NO3− in bare soil ranged from 6 to 10 μg N g soil−1 over 39 days. Similar or lower levels of soil NO3− were observed after rye residue incorporation, from 2 to 6 μg N g−1; consistent with N-immobilization. Application of untreated, mixed cover crop residues generally was associated with higher levels of soil inorganic NO3−, from 3 to 11 μg N g−1. For both rye and mixed residues, management by mowing or glyphosate enhanced N mineralization by 10 to 100%, compared to untreated residues. At the same time, application of mowing or glyphosate 8 days before cover crop incorporation seemed to reduce the amount of residues by about half compared to untreated controls. Belowground biomass was reduced more than aboveground, although recovery of senescent roots may have been incomplete. Management by glyphosate or mowing enhanced soil inorganic N availability in the short-term while simultaneously reducing carbon and N inputs.

Microfungal endophytes in roots

A wide range of ascomycetous microfungi inhabits roots without forming the anatomical features typical of mycorrhizas or causing overt signs of pathogenesis. The most-studied taxa have darkly pigmented hyphal walls and are referred to as "dark septate endophytes" (DSE). We provide a dichotomous key and annotated descriptions for a cross-section of the most common dark septate endophytes. The term DSE is sometimes used to imply taxonomic and physiological similarity even though a diverse range of root endophytic taxa form pigmented hyphae. Among these, Phialocephala fortinii Wang &amp; Wilcox is a well-known representative; it is widespread, easily observed in roots, and readily grown in culture and with plants. Nevertheless, the basis of its symbiotic relationship with plants remains ambiguous. It may be a weak pathogen, a saprotroph on senescent root tissues, or a mutualist. More detailed studies of interactions between identified taxa of microfungal endophytes and host plants are necessary to elucidate the functional basis of these symbioses; it may be necessary to look beyond the paradigms of traditional mycorrhizal and pathogenic associations to understand the ecological roles of these fungi. Reports of cryptic speciation in Phialocephala fortinii emphasize the need for accurate identification of isolates of microfungal endophytes used in experiments.Key words: dark septate endophytes (DSE), Phialocephala fortinii, mycorrhiza, fungus–host interactions, fungi.

Influence of elevated CO2and nitrogen nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon and dissolved CH4

Rice (Oryza sativa) was grown in six sunlit, semi-closed growth chambers for two seasons at 350 µ LL −1 (ambient) and 650 µ LL −1 (elevated) CO2 and different levels of nitrogen (N) supplement. The objective of this research was to study the influence of CO2 enrichment and N nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon (DOC) and dissolved CH4. Elevated CO2 concentration ([CO2]) demonstrated a wide range of enhancement to both above- and below-ground plant biomass, in particular to stems and roots (for roots when N was not limiting) in the mid-season (80 days after transplanting) and stems/ears at the final harvest, depending on season and the level of N supplement. Elevated [CO2] significantly increased microbial biomass carbon in the surface 5 cm soil when N( 90 kg ha −1 ) was in sufficient supply. Low N supplement (30 kg ha −1 ) limited the enhancement of root growth by elevated [CO2], leading consequently to diminished response of soil microbial biomass carbon to CO2 enrichment. The concentration of dissolved CH4 (as well as soil DOC, but to al esser degree) was observed to be positively related to elevated [CO2], especially at high rate of N application (120 kg ha −1 ) or at 10 cm depth (versus 5 cm depth) in the later half of the growing season (at 80 kg N ha −1 ). Root senescence in the late season complicated the assessment of the effect of elevated [CO2] on root growth and soil organic carbon turnover and thus caution should be taken when interpreting respective high CO2 results.

Cloning and characterization of a gene encoding cysteine proteases from senescent leaves of Gossypium hirsutum

A gene encoding a cysteine proteinase was isolated from senescent leave of cotton (Gossypium hirsutum) cv liaomian No. 9 by utilizing rapid amplification of cDNA ends polymerase chain reaction (RACE-PCR), and a set of consensus oligonucleotide primers was designed to anneal the conserved sequences of plant cysteine protease genes. The cDNA, which designated Ghcysp gene, contained 1368 bp terminating in a poly(A)+ trail, and included a putative 5′(98 bp) and a 3′(235 bp) non-coding region. The opening reading frame (ORF) encodes polypeptide 344 amino acids with the predicted molecular mass of 37.88 kD and theoretical pI of 4.80. A comparison of the deduced amino acid sequence with the sequence in the GenBank database has shown considerable sequence similarity to a novel family of plant cysteine proteases. This putative cotton Ghcysp protein shows from 67% to 82% identity to the other plants. All of them share catalytic triad of residues, which are highly conserved in three regions. Hydropaths analysis of the amino acid sequence shows that the Ghcysp is a potential membrane protein and localizes to the vacuole, which has a transmembrane helix between resides 7–25. A characteristic feature of Ghcysp is the presence of a putative vacuole-targeting signal peptide of 19-amino acid residues at the N-terminal region. The expression of Ghcysp gene was determined using northern blot analysis. The Ghcysp mRNA levels are high in development senescent leaf but below the limit of detection in senescent root, hypocotyl, faded flower, 6 d post anthesis ovule, and young leaf.

A Mathematical Model of Phytoremediation for Petroleum-Contaminated Soil: Model Development

We present a simple model for root length density that combines the generally accepted spatial (exponential decrease with depth) and temporal (sinusoidal) variability of root length. Parameters in this model for root length density can be determined from assumed or measured information regarding the annual biomass turnover, maximum standing biomass, and maximum depth of root penetration. The root length density model, coupled with information regarding the average root lifespan, gives specific root growth and senescence functions that are the forcing functions for the phytoremediation model. We present a screening level mathematical model for phytoremediation that accounts for the growth and senescence of roots in the system. This is an important factor for recalcitrant, immobile compounds found in weathered crude oil contaminated soils. The phytoremediation model is based on variable volume compartments that have individual first-order degradation rate constants; as the roots move through the soil, the soil cycles through the rhizosphere zone, decaying root zone and bulk soil zone. Thus, although the oil is immobile, as the roots penetrate through the soil the oil is brought into contact with the rhizosphere.

In situ ageing of fine beech roots ( Fagus sylvatica) assessed by transmission electron microscopy and electron energy loss spectroscopy: description of microsites and evolution of polyphenolic substances

Root biomass is quantitatively and qualitatively important in most ecosystems, but its contribution to the pool of organic matter in the soil is not clear. This work was designed to specify root ageing on an ultrastructural scale by transmission electron microscopy combined with microanalysis by electron energy loss spectroscopy. This approach is very suitable for studying the soil/plant interface, and for semi-quantitative analysis of the evolution of polyphenolic substances during root evolution. Three root segments were studied according to a gradient of root senescence: the apical and basal segments of the mycorrhiza and the mycorrhiza-carrier root. Each segment contained a certain proportion of senescent cells, some of which were of fungal origin, and this proportion increased as the root aged. In the three segments, the soil/plant interfaces were differentiated, and the micro-organisms observed in situ were described. Senescent root cells contained many polyphenolic substances and our results showed that these substances were, according to the root segment, differently associated with Ca, N and Si. When all these ultrastructural data are correlated with more global data, they can be usefully applied to root cell physiology, microbiology and pedology. This approach makes it possible to specify the evolution of organic matter in situ in soils whatever its origin.

Lack of evidence for programmed root senescence in common bean (Phaseolus vulgaris) grown at different levels of phosphorus supply

Summary The influence of phosphorus (P) availability on root lifespan in common bean (Phaseolus vulgaris cv. Carioca) is reported here, as well as patterns of root survivorship in relation to vegetative and reproductive development. Plants were grown in a glasshouse in a sand‐culture system with varying P availability, and in a high‐P field soil with greater biotic activity. Root dynamics were assessed using a minirhizotron system. Phosphorus limitation, which reduced plant growth by 90%, did not diminish root survivorship. At nonlimiting and limiting P, root survivorship was not synchronized closely with shoot development and senescence; a substantial portion of the roots were present and seemingly alive even beyond reproductive maturity. In field‐grown beans and in beans under severe P limitation in sand culture, survivorship fell approx. 50%, but only after pod fill was nearly complete. Common bean does not exhibit programmed root senescence during and immediately following pod fill. Roots died after the shoot had undergone other senescence processes, such as leaf drop and pod dry‐down. Under field conditions, soil organisms are likely to modulate the effects of phosphorus deficiency and pod fill on root death.

In vitro propagation and life cycle of the arbuscular mycorrhizal fungus Glomus etunicatum

Progress in understanding the biology of arbuscular mycorrhizal fungi is hampered by the limited number of species that can be successfully propagated and studied in vitro. We report the establishment of monoxenic cultures of Glomus etunicatum in association with excised Ri T-DNA transformed carrot roots. The fungus can be propagated in vitro using monoxenically formed resting spores and/or colonized root fragments. Modified White's medium buffered with 10 mM MES (pH 6) or MOPSO (pH 6.5) was most optimal for the host root growth as well as for G. etunicatum spore germination and mycorrhiza formation. The number of resting spores formed in vitro correlated positively with the length of roots occupied by arbuscular mycorrhizal structures, including arbuscules and vesicles. Spores first appeared in dual cultures within two weeks of root inoculation. Sporulation was asynchronous and continued until root senescence. Under applied culture conditions, spores achieved mature appearance within 5–7 d after their initiation. Approximately 6% of monoxenic spores were aborted at different stages of their development. Although G. etunicatum spores formed in vitro exhibited general morphological and anatomical similarity to soil-borne inoculum, they were significantly smaller and had thicker spore walls than their soil-borne counterparts. Caution should, therefore, be exercised in utilizing the in vitro system as a model of growth and development of glomalean fungi in soil.

Temporal changes in spatial pattern of fine‐root mass and nutrient concentrations in Indian bamboo savanna

Abstract. Temporal variations in the spatial distribution of fine‐root mass and nutrient concentrations were studied in recently harvested and mature bamboo savanna sites in the dry tropical Vindhyan region in India. The soil block method and root‐free‐soil cages were used to investigate fine‐root dynamics. The mean annual fine‐root biomass was 596 and 690 g/m2 in harvested and mature sites, respectively. The fine‐root net production calculated by different methods ranged from 486 to 749 g m‐2 yr‐1 in the harvested site and 485 to 875 g m‐2 yr1 in the mature site. All fine‐root mass fractions decreased with increase in distance from the base of bamboo clumps, and the herb root mass showed the reverse trend. Bamboo fine roots were better developed in the 10 ‐ 20 cm soil depth and those of herbs in the upper 10 cm. The ingrowth of fine roots in root‐free‐soil cages showed maximum biomass accumulation during the rainy season (64.2 ‐ 69.9 g m‐2 mo‐1) and minimum in the summer (4.5 ‐ 7.5 g m‐2 mo‐1). The fine‐root nutrient concentrations were strongly related to their diameter. The fine‐root nutrient concentrations varied considerably in different seasons. The highest nutrient concentrations in all categories were recorded in summer followed by winter and rainy seasons. Nutrient concentrations in live roots were always greater than those found in dead roots in different diameter classes. We suggest the occurrence of nutrient retranslocation from senescent roots to surviving roots in bamboo savanna. Fine roots in the bamboo savanna increased as a function of N‐mineralization and nitrification rates. This tendency further increased after the harvest of bamboo, suggesting the crucial role of fine roots in the bamboo savanna after the harvesting of bamboo culms.

Tissue magnesium and calcium affect arbuscular mycorrhiza development and fungal reproduction

Applications of high levels of MgSO4 resulted in reduced root colonization and sporulation by Glomus sp. (INVAM isolate FL329) with sweet potato and onion in aeroponic and sand culture, respectively. Onion shoot-Mg concentrations were elevated when a nutrient solution containing 2.6 or 11.7 mM MgSO4 was applied. Magnesium application depressed tissue-Ca levels. With lower Ca in the tissue, colonization was reduced from > 30% of root length to < 10%, and sporulation from > 1200 to ca. 200 spores per plant, 10 weeks after transplantation and the start of nutrient application. These effects on colonization and sporulation were independent of changes in tissue-P concentration. High Mg/low Ca tissue concentrations induced premature root senescence, which may have disrupted the mycorrhizal association. Our results confirm the importance of Ca for the maintenance of a functioning mycorrhiza.

Survival of propagules of arbuscular mycorrhizal fungi in soils in eastern Australia used to grow cotton

SUMMARYSoil‐borne spores and hyphae of arbuscular mycorrhizal fungi are important propagules in cracking clay soils of northern NSW, Australia. In these soils, senescent roots were uncommon. Although c. 4–200 spores g−1 soil were found, less than 6% established arbuscular mycorrhizas in trap plants, and this percentage declined over 24 months. Using tetrazolium red as a vital stain, 16–21 % of spores from field soils were found to be viable in fresh soil and 6–7 % after 24 months of storage. Using fluorescein diacetate, the length of stained hyphae of c. 0–5 m g−1 soil was shown to be halved over 32 wk. The density of viable propagules of arbuscular mycorrhizal fungi in soil declined over time and was reduced by severe disturbance. The fungi that survived to 12 months included a species thought to form dormant spores, while those initiating infection after 24 months, did not.

Organic matter dynamics in maize agroecosystems as affected by earthworm manipulations and fertility source

We quantified the mass of five size classes (>6, 2–6, 0.25–2, 0.053–0.25 and <0.053 mm) of soil organic matter (SOM) in maize ( Zea mays) agroecosystems in which earthworm populations were manipulated. Soil samples were collected on eight dates from May 1993 to April 1994. The soil was dispersed and then sieved to separate the organic matter into the five size classes. The organic matter content of the whole, unsieved soil and the material retained on sieves was determined by mass loss on ignition. The unmanipulated and addition earthworm treatments were combined as a class since they were not different for any SOM class. The reduced EW treatment had significantly higher total, 0.25–2 and <0.053 mm SOM ( P < 0.05). No significant overall earthworm effects were noted for the 0.053–0.25, 2–6 and >6 mm SOM. The reduced EW treatment had higher 0.053–0.25 mm SOM only in May just prior to spring amendment addition. Fertility treatment effects were not significant overall except for an increased amount in the >6 mm SOM for the manure treatment. There were significant date effects for all SOM size classes with increases at the times of spring amendment and residue incorporation and fall root senescence followed by rapid declines.

Comparison of Root Death in Common Bean (Phaseolus vulgaris) grown in Sand Culture and Field Conditions

In annual plant species, root death has been assumed to be closely correlated with shoot senescence. However, in a preliminary study with common bean grown in sand culture beyond physiological maturity (114 days), no root death occurred. We investigated whether the incidence of bean root death was higher under field conditions than in sand culture. Root death was defined as root disappearance. The sand culture consisted of silica sand and P-loaded alumina. Plants in this system were fertigated twice daily with complete nutrients supplied in adequate amounts. The field planting was on a Hagerstown silt loam in central Pennsylvania. Roots were observed using minirhizotrons every 1 to 3 weeks after planting. In sand culture, incidence of root death was monitored on a population of 170 roots from three plants between 25 and 88 days (shoot senescence) after planting Root death was 10%. In the field study, 55% of the 53 roots examined died between 32 and 93 days (shoot senescence) after planting. Biological factors present in the field and not present in sand culture appeared to contribute to root death. The persistence of roots in sand culture suggests a lack of programmed root senescence in contrast to shoot senescence. This has interesting implications for resource allocation during reproduction and in face of belowground herbivores and pathogens.