Interaction of arbuscular mycorrhizal fungi and rhizobia: Effects on flax yield in spoil‐bank clay

This study focused on the application of native strains of arbuscular mycorrhizal fungi (AMF) and Sinorhizobium in effective crop production during reclamation of coal‐mine spoil banks. Two greenhouse experiments were conducted in spoil‐bank clay with a low dose of organic amendment to determine whether the microbial inoculation improves growth and utility qualities of two cultivars of Linum usitatissimum L. (oil and fiber flax). Inoculation with two native AMF isolates (Glomus mosseae, G. intraradices, and their mixture) significantly increased growth and shoot phosphorus (P) concentration of both flax cultivars. Inoculated fiber flax plants produced fivefold more fibers than the uninoculated ones. In oil flax, mycorrhizal inoculation significantly but quantitatively to a minor degree decreased the concentration of nonsaturated fatty acids in the seed oil. A mixture of five native Sinorhizobium sp. strains supported growth and P uptake of oil flax only in the absence of AMF. However, these beneficial effects of the bacteria were significantly lower as compared to AMF. No synergic action of Sinorhizobium strains and AMF was observed, and their interactions were often even antagonistic. Inoculation with AMF significantly decreased population density of Sinorhizobium in the soil. These results suggest that a careful selection of suitable bacterial strains is necessary to provide effective AMF combinations and maximize flax‐growth support.     

Bottom-up down from the top: Honeydew as a carbon source for soil organisms

Honeydew produced by aphids is a well-studied food source for mutualistic ants. Although considerable amounts drop onto the soil surface there are hardly any studies dealing with the potential effects of this additional C source for soil organisms, particularly those living at the soil surface. We studied this question by simulating honeydew and investigating plots under Populus canescens formerly infested with aphids on two ruderal sites differing in soil organic matter content and vegetation cover. The simulated honeydew input increased microbial biomass with some delay in week 10. Both natural and simulated honeydew raised the activity densities of an epigeic Collembola taxon, the Bourletiellidae, but not of the dominant Hemisotoma thermophila. The honeydew effect varied over time and with the site, and was more pronounced at the nutrient-poor site for Collembola, but not for microbial biomass. Ant consumption reduced the amount of honeydew reaching the soil surface by 50%. The activity density of H. thermophila was negatively related to ants and spiders, hinting at a top-down control. Honeydew clearly acts as a bottom-up force for soil organisms, which is however only part of the complex network connecting the food webs of primary producers and decomposers.     

Xylem traits in European beech (Fagus sylvatica L.) display a large plasticity in response to canopy release

Key message

The position of trees in the canopy impacts xylem structure and its inter-annual variation. After canopy release, the increase in the hydraulic conductivity of growth rings was driven by an increase in radial growth in large trees, and by both an increase in radial growth and changes in xylem structure in saplings.

Context

Forest canopies are frequently subjected to disturbances that allow understory trees to access the upper canopy. The effect of canopy release on xylem anatomy has been assessed in juvenile trees and saplings, while the potential acclimation of larger trees remains poorly documented.

Aims

We estimated the potential hydraulic conductivity of growth rings in large understory trees compared to overstory trees, and evaluated the responses to canopy release in large trees and in saplings.

Methods

We recorded radial growth, wood density, and vessel structure in beech trees according to their position within the canopy and their size. Xylem traits were followed during 6 years after canopy release for large trees, and during 2 years for saplings. Vessel diameter and frequency as well as ring area were used to compute the potential annual ring hydraulic conductivity.

Results

Large understory trees displayed lower radial growth increments and lower potential annual ring hydraulic conductivity than overstory trees. After canopy release, potential annual ring hydraulic conductivity increased in large trees, due exclusively to increased radial growth without any change in specific hydraulic conductivity. It increased in saplings due to both increased radial growth and increased specific conductivity.

Conclusion

Tree size impacted xylem structure and resulted in plasticity of the potential hydraulic conductivity of the annual tree ring following canopy release.

     

Influence of Growing Medium and Seed Size on Germination and Seedling Growth of Pinus gerardiana Wall.

Pinus gerardiana Wall. is an important nut-producing pine having restricted distribution in the world. It has been observed that natural regeneration in the species is extremely poor or entirely lacking. The species has erratic and infrequent seed years, dormancy-related problems, and slow growth of seedling, which reduces its regeneration process in natural habitats. Therefore, we investigated the effect of growing medium and seed size on germination and seedling growth of the Pinus gerardiana. The seeds were categorized into two sizes, viz, small (<2.35 cm) and large (>2.35 cm) and five growing medium treatments were used, viz, Soil:Sand:FYM; Soil:Sand:Moss:FYM; Soil:Moss:Vermicompost; Soil:Sand:Vermicompost; and Soil:Sand:Moss:Vermicompost, for assessing their impact on germination and seedling growth. It was observed that, among different growing medium treatments, higher germination and seedling growth parameters were recorded, when Soil:Sand:Moss:Vermicompost was used for the study. Between different seed sizes, higher germination and seedling growth were recorded when seed size was large.     

Heavy metal pollutant tolerance of Azolla Pinnata

The effects of Hg, As, Pb, Cu, Cd, and Cr (1, 2, and 5 mg L^?1 each) on Azolla pinnata R. Br. were analyzed. The treatments (2 and 5 mg L^?1) of the heavy metal pollutants decreased Hill activity, chlorophyll, protein and dry wt, and increased tissue permeability over control values. The effects were most pronounced with the treatment of 5 mg L^?1. The harmful effects of the metals were, in general, found by the treatments in the order: Cd 〉 Hg 〉 Cu 〉 As 〉 Pb 〉 Cr. There was no significant change in these parameters at 1 mg L^?1 of the metals over control. Thus Azolla pinnata shows tolerance to the heavy metals tested up to 1 mg L^?1 each.     

Specific polyphenol oxidase activity of red clover (Trifolium pratense) and its relation with forage quality in field experiments

Abstract

In feeding studies, red clover (RC) influenced positively the N utilization by ruminants. A relationship between polyphenol oxidase (PPO) activity and forage quality has not been established. Our objective was to investigate seasonal, site, genotype, and management effects on specific PPO activity in RC in three experiments under field conditions, and relate the activity to forage-quality parameters. In Experiment 1, six RC genotypes at two study sites were submitted to a 3-cut system. Specific PPO activity, forage quality, and vegetation stage were determined. PPO activity varied between harvest and study sites, with genotypes differing up to 3-fold in PPO activity within harvests. The specific PPO activity, forage quality, and vegetation stage in RC subjected to 5-cut system and grazing (Experiment 2) were determined. Additionally, in Experiment 3, cutting frequency in RC swards including mechanical stress (rolling) was investigated. The induction of PPO activity in RC by grazing or mechanical stress (Experiments 2 and 3) increased the activity up to 2.5-fold compared with RC at similar vegetation stage submitted to the 5-cut system. Mechanical stress induced by grazing or rolling, and seasonal differences, seem to have a larger influence on specific PPO activity than does the genotype effect observed in Experiment 1. For forage quality, an increased specific PPO activity explained 29–46% of the reduction in protein fraction ‘A’ content (non-protein N) in the cutting systems in Experiments 2 and 3. Other CP fractions achieved a lower relation. Furthermore, the precipitation-to-temperature ratio preceeding a harvest explained 63% of the variation in the specific PPO activity. In conclusion, the PPO activity in RC is induced by grazing and rolling. Whereas weather conditions preceeding a harvest showed a large influence, genotype influence had only minor relevance. These results may have implications for regional harvest management towards efficient N utilization by ruminants.     

Response of the pigeonpea-Rhizobium symbiosis to salinity stress: variation among Rhizobium strains in symbiotic ability

Summary There were significant differences among pigeonpea [Cajanus cajan (L.) Millsp] Rhizobium sp. strains (IC 3506, IC 3484, IC 3195, and IC 3087) in their ability to nodulate and fix N₂ under saline conditions. Pigeonpea plants inoculated with IC 3087 and IC 3506 were less affected in growth by salinity levels of 6 and 8 dS m^-1 than plants inoculated with the other strains. For IC 3506, IC 3484, and IC 3195, there was a decrease in the number of nodules with increasing salinity, while the average nodule dry weight and the specific nitrogenase activity remained unaffected. However, in IC 3087, the number of nodules increased slightly with increasing salinity. Leaf-P concentrations increased with salinity in the inoculated plants irrespective of the Rhizobium sp. strain, and leaf-N concentrations decreased with increasing salinity in IC 3484 and IC 3195 only. Shoot-Na and-Cl levels were further increased in these salt-sensitive strains only at 8 dS m^-1. Therefore there may be scope for selecting pigeonpea Rhizobium sp. symbioses better adapted to saline conditions. The Rhizobium sp. strains best able to form effective symbioses at high salinity levels are not necessarily derived from saline soils.Submitted as JA No. 919 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)