Radial Oxygen Loss from Plant Roots—Methods

Function and induction of the root barrier to radial O2 loss. A commentary on 'Exogenous abscisic acid induces the formation of a suberized barrier to radial oxygen loss in adventitious roots of barley (Hordeum vulgare)'

This article comments on:

Katsuhiro Shiono and Haruka Matsuura, Exogenous abscisic acid induces the formation of a suberized barrier to radial oxygen loss in adventitious roots of barley (Hordeum vulgare), Annals of Botany, Volume 133, Issue 7, 6 June 2024, Pages 931–940 https://doi.org/10.1093/aob/mcae010

Energy harvesting from plants using hybrid microbial fuel cells; potential applications and future exploitation

Microbial Fuel Cells (MFC) can be fuelled using biomass derived from dead plant material and can operate on plant produced chemicals such as sugars, carbohydrates, polysaccharides and cellulose, as well as being "fed" on a regular diet of primary biomass from plants or algae. An even closer relationship can exist if algae (e.g., prokaryotic microalgae or eukaryotic and unicellular algae) can colonise the open to air cathode chambers of MFCs driving photosynthesis, producing a high redox gradient due to the oxygenic phase of collective algal cells. The hybrid system is symbiotic; the conditions within the cathodic chamber favour the growth of microalgae whilst the increased redox and production of oxygen by the algae, favour a more powerful cathode giving a higher maximum voltage and power to the photo-microbial fuel cell, which can ultimately be harvested for a range of end-user applications. MFCs can utilise a wide range of plant derived materials including detritus, plant composts, rhizodeposits, root exudates, dead or dying macro- or microalgae, via Soil-based Microbial Fuel Cells, Sediment Microbial Fuel Cells, Plant-based microbial fuel cells, floating artificial islands and constructed artificial wetlands. This review provides a perspective on this aspect of the technology as yet another attribute of the benevolent Bioelectrochemical Systems.

Spatial O2 Profile in Coix lacryma-jobi and Sorghum bicolor along the Gas Diffusion Pathway under Waterlogging Conditions

While internal aeration in plants is critical for adaptation to waterlogging, there is a gap in understanding the differences in oxygen diffusion gradients from shoots to roots between hypoxia-tolerant and -sensitive species. This study aims to elucidate the differences in tissue oxygen concentration at various locations on the shoot and root between a hypoxia-tolerant species and a -sensitive species using a microneedle sensor that allows for spatial oxygen profiling. Job's tears, a hypoxia-tolerant species, and sorghum, a hypoxia-susceptible species, were tested. Plants aged 10 days were acclimated to a hypoxic agar solution for 12 days. Oxygen was profiled near the root tip, root base, root shoot junction, stem, and leaf. An anatomical analysis was also performed on the roots used for the O2 profile. The oxygen partial pressure (pO2) values at the root base and tip of sorghum were significantly lower than that of the root of Job's tears. At the base of the root of Job's tears, pO2 rapidly decreased from the root cortex to the surface, indicating a function to inhibit oxygen leakage. No significant differences in pO2 between the species were identified in the shoot part. The root cortex to stele ratio was significantly higher from the root tip to the base in Job's tears compared to sorghum. The pO2 gradient began to differ greatly at the root shoot junction and root base longitudinally, and between the cortex and stele radially, between Job's tears and sorghum. Differences in the root oxygen retention capacity and the cortex to stele ratio are considered to be related to differences in pO2.

Integrating Microbial Fuel Cell and Hydroponic Technologies Using a Ceramic Membrane Separator to Develop an Energy-Water-Food Supply System

In this study, a microbial fuel cell was integrated into a hydroponic system (MFC-Hyp) using a ceramic membrane as a separator. The MFC-Hyp is a passive system that allows the transport of nutrients from wastewater in the microbial fuel cell (MFC) to water in the hydroponic vessel (Hyp) through a ceramic membrane separator, with no external energy input. The performance of this system was examined using potato-process wastewater as a source of energy and nutrients (K, P, N) and garlic chives (Allium tuberosum) as a hydroponic plant. The results showed that based on dry weight, the leaves of Allium tuberosum grew 142% more in the MFC-Hyp than those of the plant in the Hyp without the MFC, in a 49-day run. The mass fluxes of K, P, and NO3--N from the MFC to the Hyp through the ceramic membrane were 4.18 ± 0.70, 3.78 ± 1.90, and 2.04 ± 0.98 µg s-1m-2, respectively. It was apparent that the diffusion of nutrients from wastewater in the MFC enhanced the plant growth in the Hyp. The MFC-Hyp in the presence of A. tuberosum produced the maximum power density of 130.2 ± 45.4 mW m-2. The findings of this study suggest that the MFC-Hyp system has great potential to be a "carbon-neutral" technology that could be transformed into an important part of a diversified worldwide energy-water-food supply system.

A meta-analysis of plant tissue O2 dynamics

Adequate tissue O2 supply is crucial for plant function. We aimed to identify the environmental conditions and plant characteristics that affect plant tissue O2 status. We extracted data and performed meta-analysis on >1500 published tissue O2 measurements from 112 species. Tissue O2 status ranged from anoxic conditions in roots to >53kPa in submerged, photosynthesising shoots. Using information-theoretic model selection, we identified 'submergence', 'light', 'tissue type' as well as 'light×submergence' interaction as significant drivers of tissue O2 status. Median O2 status were especially low (Solanum tuberosum ) tubers and root nodules. Mean shoot and root O2 were ~25% higher in light than in dark when shoots had atmospheric contact. However, light showed a significant interaction with submergence on plant O2 , with a submergence-induced 44% increase in light, compared with a 42% decline in dark, relative to plants with atmospheric contact. During submergence, ambient water column O2 and shoot tissue O2 correlated stronger in darkness than in light conditions. Although use of miniaturised Clark-type O2 electrodes has enhanced understanding of plant O2 dynamics, application of non-invasive methods in plants is still lacking behind its widespread use in mammalian tissues.

Mitigation of Greenhouse Gas Emissions from Rice via Manipulation of Key Root Traits

Rice production worldwide represents a major anthropogenic source of greenhouse gas emissions. Nitrogen fertilization and irrigation practices have been fundamental to achieve optimal rice yields, but these agricultural practices together with by-products from plants and microorganisms, facilitate the production, accumulation and venting of vast amounts of CO₂, CH₄ and N₂O. We propose that the development of elite rice varieties should target root traits enabling an effective internal O₂ diffusion, via enlarged aerenchyma channels. Moreover, gas tight barriers impeding radial O₂ loss in basal parts of the roots will increase O₂ diffusion to the root apex where molecular O₂ diffuses into the rhizosphere. These developments result in plants with roots penetrating deeper into the flooded anoxic soils, producing higher volumes of oxic conditions in the interface between roots and rhizosphere. Molecular O₂ in these zones promotes CH₄ oxidation into CO₂ by methanotrophs and nitrification (conversion of NH₄⁺ into NO₃^-), reducing greenhouse gas production and at the same time improving plant nutrition. Moreover, roots with tight barriers to radial O₂ loss will have restricted diffusional entry of CH₄ produced in the anoxic parts of the rhizosphere and therefore plant-mediated diffusion will be reduced. In this review, we describe how the exploitation of these key root traits in rice can potentially reduce greenhouse gas emissions from paddy fields.

Sediment arsenic remediation by submerged macrophytes via root-released O2 and microbe-mediated arsenic biotransformation

Arsenic (As)-contaminated water restoration is extremely challenging because As remobilization from sediments can result in episodic or long-term release of As to the overlying water. In this study, by combining high-resolution imaging techniques with microbial community profiling, we examined the feasibility of utilizing the rhizoremediation of submerged macrophytes (Potamogeton crispus) to decrease As bioavailability and regulate its biotransformation in sediments. Results showed that P. crispus considerably decreased the rhizospheric labile As flux to lower than 4 pg cm^-2 s^-1 from larger than 7 pg cm^-2 s^-1, suggesting its effectiveness in promoting As retention in sediments. Iron plaques induced by radial oxygen loss from roots decreased the mobility of As by sequestering it. Additionally, Mn-oxides may act as an oxidizer for the oxidation of As(III) to As(V) in the rhizosphere, which can further increase the As adsorption owing to the strong binding affinity between As(V) and Fe-oxides. Furthermore, microbially mediated As oxidation and methylation were intensified in the microoxic rhizosphere, which decreased the mobility and toxicity of As by changing its speciation. Our study demonstrated that root-driven abiotic and biotic transformation contribute to As retention in sediments, which lays a foundation for applying macrophytes to the remediation of As-contaminated sediments.

High-resolution imaging of O2 dynamics and metal solubilization in the rhizosphere of the hyperaccumulator Leersia hexandra Swartz

The mobilization of trace metals in the rhizosphere can be affected by the redox potential, which is closely related to the O₂ dynamics. This study examined the distributions of O₂ and trace metals in the rhizosphere of the subaquatic hyperaccumulator Leersia hexandra Swartz under chromium (Cr) stress using planar optodes and the diffusive gradients in thin films technique coupled with laser ablation inductively coupled plasma mass spectrometry. The O₂ concentrations and oxidized areas in the rhizosphere significantly increased with increases in the light intensity, air humidity, and atmospheric CO₂ concentrations (p < 0.05). The O₂ concentration first increased with increasing ambient temperatures, then decreased when the temperature increased from 25 to 32 ℃. The O₂ concentration in the rhizosphere was significantly decreased under Cr stress (p < 0.05), with a prolonged response time to the altered ambient temperature. Cr stress led to decreased mobilities of As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sb, V, W, and Zn in the rhizosphere, which were negatively correlated with the concentrations of O₂. These results provide new insights into the role of changes in the O₂ concentration induced by the roots of hyperaccumulator plants in controlling the mobility of trace metals in soils.

Responses of key root traits in the genus Oryza to soil flooding mimicked by stagnant, deoxygenated nutrient solution

Excess water can induce flooding stress resulting in yield loss, even in wetland crops such as rice (Oryza). However, traits from species of wild Oryza have already been used to improve tolerance to abiotic stress in cultivated rice. This study aimed to establish root responses to sudden soil flooding among eight wild relatives of rice with different habitat preferences benchmarked against three genotypes of O. sativa. Plants were raised hydroponically, mimicking drained or flooded soils, to assess the plasticity of adventitious roots. Traits included were apparent permeance (PA) to O2 of the outer part of the roots, radial water loss, tissue porosity, apoplastic barriers in the exodermis, and root anatomical traits. These were analysed using a plasticity index and hierarchical clustering based on principal component analysis. For example, O. brachyantha, a wetland species, possessed very low tissue porosity compared with other wetland species, whereas dryland species O. latifolia and O. granulata exhibited significantly lower plasticity compared with wetland species and clustered in their own group. Most species clustered according to growing conditions based on PA, radial water loss, root porosity, and key anatomical traits, indicating strong anatomical and physiological responses to sudden soil flooding.

On the Use of Polymer-Based Composites for the Creation of Optical Sensors: A Review

Polymers are widely used in many areas, but often their individual properties are not sufficient for use in certain applications. One of the solutions is the creation of polymer-based composites and nanocomposites. In such materials, in order to improve their properties, nanoscale particles (at least in one dimension) are dispersed in the polymer matrix. These properties include increased mechanical strength and durability, the ability to create a developed inner surface, adjustable thermal and electrical conductivity, and many others. The materials created can have a wide range of applications, such as biomimetic materials and technologies, smart materials, renewable energy sources, packaging, etc. This article reviews the usage of composites as a matrix for the optical sensors and biosensors. It highlights several methods that have been used to enhance performance and properties by optimizing the filler. It shows the main methods of combining indicator dyes with the material of the sensor matrix. Furthermore, the role of co-fillers or a hybrid filler in a polymer composite system is discussed, revealing the great potential and prospect of such matrixes in the field of fine properties tuning for advanced applications.