Mohsen Zarebanadkouki

Root hairs enable high transpiration rates in drying soils

Do root hairs help roots take up water from the soil? Despite the well-documented role of root hairs in phosphate uptake, their role in water extraction is controversial. We grew barley (Hordeum vulgare cv Pallas) and its root-hairless mutant brb in a root pressure chamber, whereby the transpiration rate could be varied whilst monitoring the suction in the xylem. The method provides accurate measurements of the dynamic relationship between the transpiration rate and xylem suction. The relationship between the transpiration rate and xylem suction was linear in wet soils and did not differ between genotypes. When the soil dried, the xylem suction increased rapidly and non-linearly at high transpiration rates. This response was much greater with the brb mutant, implying a reduced capacity to take up water. We conclude that root hairs facilitate the uptake of water by substantially reducing the drop in matric potential at the interface between root and soil in rapidly transpiring plants. The experiments also reinforce earlier observations that there is a marked hysteresis in the suction in the xylem when the transpiration rate is rising compared with when it is falling, and possible reasons for this behavior are discussed.

Root type matters: measurement of water uptake by seminal, crown, and lateral roots in maize

We showed that crown roots have a different capacity to transport water compared with seminal roots. Acknowledging such differences between root types is crucial to understand optimal root traits.

Root hairs increase rhizosphere extension and carbon input to soil

Background and AimsnAlthough it is commonly accepted that root exudation enhances plant-microbial interactions in the rhizosphere, experimental data on the spatial distribution of exudates are scarce. Our hypothesis was that root hairs exude organic substances to enlarge the rhizosphere farther from the root surface.nnnMethodsnBarley (Hordeum vulgare Pallas - wild type) and its root-hairless mutant (brb) were grown in rhizoboxes and labelled with 14CO2. A filter paper was placed on the soil surface to capture, image and quantify root exudates.nnnKey ResultsnPlants with root hairs allocated more carbon (C) to roots (wild type: 13 %; brb: 8 % of assimilated 14C) and to rhizosheaths (wild type: 1.2 %; brb: 0.2 %), while hairless plants allocated more C to shoots (wild type: 65 %; brb: 75 %). Root hairs increased the radial rhizosphere extension three-fold, from 0.5 to 1.5 mm. Total exudation on filter paper was three times greater for wild type plants compared to the hairless mutant.nnnConclusionnRoot hairs increase exudation and spatial rhizosphere extension, which probably enhance rhizosphere interactions and nutrient cycling in larger soil volumes. Root hairs may therefore be beneficial to plants under nutrient-limiting conditions. The greater C allocation below ground in the presence of root hairs may additionally foster C sequestration.

Hydraulic conductivity of soil-grown lupine and maize unbranched roots and maize root-shoot junctions

Improving or maintaining crop productivity under conditions of long term change of soil water availability and atmosphere demand for water is one the big challenges of this century. It requires a deep understanding of crop water acquisition properties, i.e. root system architecture and root hydraulic properties among other characteristics of the soil-plant-atmosphere continuum. A root pressure probe technique was used to measure the root hydraulic conductances of seven-week old maize and lupine plants grown in sandy soil. Unbranched root segments were excised in lateral, seminal, crown and brace roots of maize, and in lateral roots of lupine. Their total hydraulic conductance was quantified under steady-state hydrostatic gradient for progressively shorter segments. Furthermore, the axial conductance of proximal root regions removed at each step of root shortening was measured as well. Analytical solutions of the water flow equations in unbranched roots developed recently and relating root total conductance profiles to axial and radial conductivities were used to retrieve the root radial hydraulic conductivity profile along each root type, and quantify its uncertainty. Interestingly, the optimized root radial conductivities and measured axial conductances displayed significant differences across root types and species. However, the measured root total conductances did not differ significantly. As compared to measurements reported in the literature, our axial and radial conductivities concentrate in the lower range of herbaceous species hydraulic properties. In a final experiment, the hydraulic conductances of root junctions to maize stem were observed to highly depend on root type. Surprisingly maize brace root junctions were an order of magnitude more conductive than the other crown and seminal roots, suggesting potential regulation mechanism for root water uptake location and a potential role of the maize brace roots for water uptake more important than reported in the literature.

Rhizosphere hydrophobicity limits root water uptake after drying and subsequent rewetting

Background and AimsRecent experiments showed that rhizosphere of several plant species turns temporarily hydrophobic after severe drying and subsequent rewetting. Whether or not such hydrophobicity limits root water uptake is not known.MethodsA set of experiments was performed to test whether rhizosphere water repellency negatively affects root water uptake. To this end, a commercial surfactant was used as a rewetting agent to facilitate the wettability of the rhizosphere of lupins (Lupinus albus) in a sandy soil. Lupin plants were grown in rhizoboxes and were subjected to a severe drying cycle. Then half of the plants were irrigated with water and half with the surfactant solution. Time-series neutron radiography technique was used to monitor water redistribution in the rhizosphere during irrigation. In a second experimental set-up, lupins were grown in a sandy soil partitioned in five vertical compartments separated by a 1-cm layer of coarse sand (acting as a capillary barrier). Water and surfactant were injected in different compartments and the rehydration of the root tissues beyond the irrigated compartments was monitored with neutron radiography for 2–3xa0h. Root rehydration rates were used to estimate the water fluxes across the root-soil interface.ResultsThe rhizosphere of lupin roots in sandy soil irrigated with water remained partly dry for at least 2–3xa0h, while it was rapidly rewetted when irrigated with surfactant. Water flow into the roots irrigated with surfactant solution was 6.5 times faster than into the roots irrigated with water.ConclusionsThese results prove that water repellency of the rhizosphere of lupins in sandy soils limited the water fluxes into the roots and root rehydration during the first two to three hours after irrigation. This might not always be negative, because it can limit water losses from roots to dry soil and therefore avoid severe root dehydration.

Impact of Pore-Scale Wettability on Rhizosphere Rewetting

Vast amounts of water flow through a thin layer of soil around the roots, the rhizosphere, where high microbial activity takes place – an important hydrological and biological hotspot. The rhizosphere was shown to turn water repellent upon drying, which has been interpreted as the effect of mucilage secreted by roots. The effects of such rhizosphere water dynamics on plant and microbial activity are unclear. Furthermore, our understanding of the biophysical mechanisms controlling the rhizosphere water repellency remains largely speculative. Our hypothesis is that the key to describe the emergence of water repellency lies within the microscopic distribution of wettability on the pore-scale. At a critical mucilage content, a sufficient fraction of pores is blocked and the rhizosphere turns water repellent. Here we tested whether a percolation approach is capable to predict the flow behavior near the critical mucilage content. The wettability of glass beads and sand mixed with chia seed mucilage was quantified by measuring the infiltration rate of water drops. Drop infiltration was simulated using a simple pore-network model in which mucilage was distributed heterogeneously throughout the pore space with a preference for small pores. The model approach proved capable to capture the percolation nature of the process, the sudden transition from wettable to water repellent and the high variability in infiltration rates near the percolation threshold. Our study highlights the importance of pore-scale distribution of mucilage in the emergent flow behavior across the rhizosphere.

Rhizodeposition under drought is controlled by root growth rate and rhizosphere water content

AimsRhizodeposition is an important energy source for soil microorganisms. It is therefore crucial to estimate the distribution of root derived carbon (C) in soil and how it changes with soil water content.MethodsWe tested how drought affects exudate distribution in the rhizosphere by coupling 14CO2 labelling of plants and phosphor imaging to estimate C allocation in roots. Rhizosphere water content was visualized by neutron radiography. A numerical model was employed to predict the exudate release and its spatiotemporal distribution along and around growing roots.ResultsDry and wet plants allocated similar amounts of 14C into roots but root elongation decreased by 48% in dry soil leading to reduced longitudinal rhizosphere extension. Rhizosphere water content was identical (31%) independent of drought, presumably because of the high water retention by mucilage. The model predicted that the increase in rhizosphere water content will enhance diffusion of exudates especially in dry soil and increase their microbial decomposition.ConclusionRoot growth and rhizosphere water content play an important role in C release by roots and in shaping the profiles of root exudates in the rhizosphere. The release of mucilage may be a plant strategy to maintain fast diffusion of exudates and high microbial activity even under water limitation.

Nitrogen fertilization raises CO2 efflux from inorganic carbon: A global assessment

Nitrogen (N) fertilization is an indispensable agricultural practice worldwide, serving the survival of half of the global population. Nitrogen transformation (e.g., nitrification) in soil as well as plant N uptake releases protons and increases soil acidification. Neutralizing this acidity in carbonate-containing soils (7.49xa0×xa0109 xa0ha; ca. 54% of the global land surface area) leads to a CO2 release corresponding to 0.21xa0kg C per kg of applied N. We here for the first time raise this problem of acidification of carbonate-containing soils and assess the global CO2 release from pedogenic and geogenic carbonates in the upper 1xa0m soil depth. Based on a global N-fertilization map and the distribution of soils containing CaCO3 , we calculated the CO2 amount released annually from the acidification of such soils to be 7.48xa0×xa01012 xa0g C/year. This level of continuous CO2 release will remain constant at least until soils are fertilized by N. Moreover, we estimated that about 273xa0×xa01012 xa0g CO2 -C are released annually in the same process of CaCO3 neutralization but involving liming of acid soils. These two CO2 sources correspond to 3% of global CO2 emissions by fossil fuel combustion or 30% of CO2 by land-use changes. Importantly, the duration of CO2 release after land-use changes usually lasts only 1-3 decades before a new C equilibrium is reached in soil. In contrast, the CO2 released by CaCO3 acidification cannot reach equilibrium, as long as N fertilizer is applied until it becomes completely neutralized. As the CaCO3 amounts in soils, if present, are nearly unlimited, their complete dissolution and CO2 release will take centuries or even millennia. This emphasizes the necessity of preventing soil acidification in N-fertilized soils as an effective strategy to inhibit millennia of CO2 efflux to the atmosphere. Hence, N fertilization should be strictly calculated based on plant-demand, and overfertilization should be avoided not only because N is a source of local and regional eutrophication, but also because of the continuous CO2 release by global acidification.

Spatiotemporal patterns of enzyme activities in the rhizosphere: effects of plant growth and root morphology

Lentil and lupine, having contrasting root morphologies, were chosen to investigate the effects of plant growth and root morphology on the spatial distribution of β-glucosidase, cellobiohydrolase, leucine aminopeptidase, and acid phosphomonoesterase activities. Lentil kept as vegetative growth and the rhizosphere extent was constant, while the enzyme activities at the root surface kept increasing. Lupine entered reproductive growth in the seventh week after planting, the rhizosphere extent was broader in the eighth week than in the first and fourth weeks. However, enzyme activity at the root surface of lupine decreased by 10–50% in comparison to the preceding vegetative stage (first and fourth weeks). Lupine lateral roots accounted for 1.5–3.5 times more rhizosphere volume per root length than taproots, with 6–14-fold higher enzyme activity per root surface area. Therefore, we conclude that plant growth and root morphology influenced enzyme activity and shape the rhizosphere as follows: the enzyme activity in the rhizosphere increased with plant growth until reproductive stage; lateral roots have much larger rhizosphere volume per unit root length and higher enzyme activity per root surface area than the taproots.