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Climate change scenarios forecast increasing droughts in large areas globally with significant effects on food production. Nutrient availability is an imperative factor for plant growth and it is greatly modulated by water availability. Nitrogen (N) availability extensively constrains plant growth in most terrestrial ecosystems especially in sub-Saharan Africa, where soils are unfertile and often degraded. How rhizosphere traits at the plant soil-interface affect N uptake in response to drought in N poor tropical soils remains elusive. We used 15N, and 13C pulse labelling to trace and quantify N transport from a root-restricted compartment by AMF across an air gap to the host plants coupled with quantifying the allocation of carbon to below-ground pools. Three sorghum genotypes were grown under optimal and water deficit conditions. By tracer analysis in the plant tissues, we assessed that drought enlarged uptake and delivery 15N by arbuscular mycorrhizal fungi (AMF) from the root restricted compartment across the air gap to the host plant. In addition, drought induced enhanced below-ground incorporation of recently assimilated carbon (C) into the microbial biomass pool both in rhizo-hyphosphere and hyphosphere. Enzyme assays revealed that whereas potential enzymatic reaction (Vmax) of chitinase was reduced under drought, that of leucine amino peptidase (LAP) was upregulated by water scarcity suggesting that N input from protein mineralization was relatively enhanced to that of chitin following moisture limitation. Michaelis-Menten constant (Km) of LAP strongly increased by drought compared to that of chitinase which displayed genotype-specific shifts in rhizosphere enzyme systems. We conclude that in addition to AMF symbiosis, enzyme regulation and enhanced belowground C allocation are key strategies to enhance nitrogen uptake under adverse conditions of resource limitations.

Various rhizosphere traits have been explored as plant adaptations to modulate the soil-root interface to acquire resources and to enhance plant water status under stress conditions. Mucilage exudation has been suggested to enable water uptake during soil drying. This hypothesis was tested using artificial root analogy due to technical limitations and the lack of suitable plant materials. Here, we tested whether mucilage exudation facilitates water uptake in intact cowpeas (Vigna unguiculata L.) plants growing in loamy soil during drying. We used a root pressure chamber system to measure the gradients in water potential at the root surface as well as the relationship between transpiration rate and leaf xylem water potential in two genotypes with contrasting mucilage production. Higher mucilage exudation attenuated the drop in matric potential at the root surface. In contrast, the gradients in water potential were much steeper in cowpea with less mucilage production. The attenuation of matric potential at the root surface resulted in a linear relationship between transpiration rate and leaf xylem water potential. We conclude that mucilage exudation maintains the hydraulic continuity between roots and soil and decelerates water potential dissipation near the root surface during soil drying. Our findings provide the first in vivo evidence on the role of mucilage on root water uptake.

Stomatal closure allows plants to promptly respond to water shortage. Although the coordination between stomatal regulation, leaf and xylem hydraulics has been extensively investigated, the impact of below-ground hydraulics on stomatal regulation remains unknown.We used a novel root pressure chamber to measure, during soil drying, the relation between transpiration rate (E) and leaf xylem water pressure (ψleaf-x) in tomato shoots grafted onto two contrasting rootstocks, a long and a short one. In parallel, we also measured the E(ψleaf-x) relation without pressurization. A soil–plant hydraulic model was used to reproduce the measurements. We hypothesize that (1) stomata close when the E(ψleaf-x) relation becomes non-linear and (2) non-linearity occurs at higher soil water contents and lower transpiration rates in short-rooted plants.The E(ψleaf-x) relation was linear in wet conditions and became non-linear as the soil dried. Changing below-ground traits (i.e. root system) significantly affected the E(ψleaf-x) relation during soil drying. Plants with shorter root systems required larger gradients in soil water pressure to sustain the same transpiration rate and exhibited an earlier non-linearity and stomatal closure.We conclude that, during soil drying, stomatal regulation is controlled by below-ground hydraulics in a predictable way. The model suggests that the loss of hydraulic conductivity occurred in soil. These results prove that stomatal regulation is intimately tied to root and soil hydraulic conductances.

Limited water supply is one of the largest impediments to food production worldwide in the light of climate change and increasing food demand. Stomatal regulation allows plants to promptly react to water stress and regulate water use. Although the coordination between stomatal closure and aboveground hydraulics has extensively been studied, our understanding of the impact of belowground hydraulics on stomatal regulation remains, as yet, incomplete. The overall objective of this study was to investigate the impact of belowground hydraulic conductivity as affected by differences in expressions of root and rhizosphere traits on the water use regulation of different maize genotypes.We have utilized a novel phenotyping facility to investigate the response of a selection of 48 maize (Zea mays L.) genotypes exhibiting different root and rhizosphere traits to soil drying. We measured the relation between leaf water potential, soil water potential, soil water content and transpiration rate, as well as root and rhizosphere traits (e.g. root length, rhizosheath mass) between genotypes. Our hypothesis is that stomatal response to soil drying is related to a loss in soil hydraulic conductivity and that key root and rhizosphere hydraulic traits affect such relation.We found that the genotypes differed in their responsiveness to drought and that such differences were related to belowground hydraulic traits. The critical soil water content at which plants started to decrease transpiration was related to a combination of plant- and rhizosphere traits (namely plant hydraulic conductance, maximum transpiration rate, root length and rhizosheath mass). Genotypes with a higher maximum transpiration rate and a higher plant hydraulic conductance and a smaller root system closed stomata in wetter soil conditions, meaning earlier in the drying process. This finding is explained by a soil-plant hydraulic model that assumes that stomata start to close when the soil hydraulic conductance of the soil-plant continuum starts to decline. Those findings stress the importance of belowground hydraulic properties on stomatal regulation and thereby drought responsiveness.

Recent studies have identified soil drying as a dominant driver of transpiration reduction at the global scale. Although Arbuscular Mycorrhiza Fungi (AMF) are assumed to play a pivotal role in plant response to soil drying, studies investigating the impact of AMF on plant water status and soil-plant hydraulic conductance are lacking. Thus, the main objective of this study was to investigate the influence of AMF on soil-plant conductance and plant water status of tomato under drought. We hypothesized that AMF limit the drop in matric potential across the rhizosphere, especially in drying soil. The underlying mechanism is that AMF extend the effective root radius and hence reduce the water fluxes at the root-soil interface. The follow-up hypothesis is that AMF enhance soil-plant hydraulic conductance and plant water status during soil drying. To test these hypotheses, we measured the relation between transpiration, soil and leaf water potential of tomato with reduced mycorrhiza colonization (RMC) and the corresponding wild-type (WT). We inoculated the soil of the WT with Rhizophagus irregularis spores to potentially upsurge symbiosis initiation. During soil drying, leaf water potential of the WT did not drop below -0.8 MPa during the first six days after withholding irrigation, while leaf water potential of RMC dropped below -1 MPa already after four days. Furthermore, AMF enhanced the soil-plant hydraulic conductance of the WT during soil drying. In contrast, soil-plant hydraulic conductance of the RMC declined more abruptly as soil dried. We conclude that AMF maintained the hydraulic continuity between root and soil in drying soils, hereby reducing the drop in matric potential at the root-soil interface and enhancing soil-plant hydraulic conductance of tomato under edaphic stress. Future studies will investigate the role of AMF on soil-plant hydraulic conductance and plant water status among diverse plant species growing in contrasting soil textures.

Purpose Although the coordination between stomatal closure and aboveground hydraulics has extensively been studied, our understanding of the impact of belowground hydraulics on stomatal regulation remains incomplete. Here, we investigated whether and how the water use of maize (Zea mays L.) varied under hydraulically contrasting soil textures. Our hypothesis is that a textural-specific drop in the hydraulic conductivity is associated with a distinct decrease in transpiration during soil drying. Methods Maize plants were grown in contrasting soil textures (sand, sandy loam, loam) and exposed to soil drying. We measured the relationships between transpiration rate, soil water content as well as soil and leaf water potential. We used a soil-plant hydraulic model to reproduce the experimental observations and infer the hydraulic conductance of the soil-plant system during soil drying. Results We observed the impact of soil texture on plant response to soil drying in various relationships. The soil water potentials at which transpiration decreased were more than one order of magnitude more negative in loam than in sand. The soil-plant conductance decreased not only abruptly but also at less negative soil water potentials in sand than in sandy loam or loam. Stomata closed at less negative leaf water potentials in sand than in loam. The model predictions matched well the experimental observations. Conclusion The results elucidated that the critical soil water content and potential at which plants close stomata depends on the soil texture. These findings support our plea to include soil properties for understanding and predicting stomatal regulation during soil drying., Plant and Soil, 478, ISSN:0032-079X, ISSN:1573-5036

Hotspots in agricultural soils, which include rhizosphere, detritusphere and drilosphere, are characterized by strongly different dynamics than those of natural ecosystems. This involves hotspot properties such as element cycling intensity, microbial activation, lifetime or spatial extension. Evidence from studies around the globe suggests that key hotspot characteristics intensify or increase under strongly limiting cropping conditions e.g. low-input agriculture: i) nutrient mining is more intensive around roots in infertile soils, ii) root exudates decompose more slowly under water limitation, and iii) the rhizo-hyphosphere forms a more spatially extended hyphal network under P deficiency. These examples suggest that smart management of hotspots might be a sustainable strategy to overcome soil limitations, not only for crop production on marginal soils but also as a strategy to save resources for future agriculture.Here, we will present a set of studies applying management strategies, which actively modify hotspot intensity, lifetime or spatial extension with the aim to manipulate biogeochemical cycles of the respective agroecosystem. Most traditional, tillage enlarges the topsoil detritusphere or moves it to lower soil depths. Rather novel but increasingly studied approaches seek to modify rhizosphere properties: applying genotypes with i) specific root traits such as an optimized root morphology (e.g. modified root hairs or deeper fine root system) or ii) modified root exudate compositions and resulting rhizosphere microbiomes. Such approaches need to be applied site- and agroecosystem-specifically to optimize resource utilization. Moreover, as agroecosystems are under long-term controls, hotspot management strategies are not limited to one growing season but can stretch over years of cultivation. The generation of specific biopores – the root channels - created by e.g. tap-rooted or deep-rooting cover crops is a management practise inducing a rhizosphere-detritusphere-rhizosphere transition over time. ‘Re-activated’ hotspots feature unique biogeochemical conditions for young roots as well as microbial communities. Such ‘highways to subsoil’ foster rhizosphere establishment in subsoils, where i) hotspots remain moist and thus active under drought and ii) where gradients from hotspots to bulk soils are for magnitudes higher compared to topsoils. All these aspects present a unique, however largely unexploited potential for future agriculture, yet.By a novel set of methodological approaches and their combinations, comprising multi-isotope applications, in-situ imaging techniques, biomarkers and microbial activity measures with high spatial resolution, we will provide new insights into the potential of hotspot management in agroecosystems. We will discuss implications for crop production under resource limitation up to the potential for a sustainable development of future agricultural production systems especially in the face of projected climate change.

The projected global warming risks due to high emissions of greenhouse gases, mainly from anthropogenic activities, increases the need for an agricultural practice with high carbon sink capacity and low water requirements without compromising on environment and productivity. On one hand, it’s well accepted that soil moisture directly affects microbial activity, whereas on the other hand, drought stress was recently postulated to increase root exudates, which in turn will accelerate soil organic matter mineralization “priming effects”. Thus, the objective of this study was to investigate the interplay between soil moisture (well-watered and drought stressed) and maize (Zea mays L.) root exudates on soil CO2 efflux. The experiment consisted of three treatments, which are well-watered, drought stressed maize plus a control (without plants) lysimeters (1 m3), Soil CO2 efflux, soil temperature and moisture content were measured weekly during the growing season (April to September) and monthly in the fallow period. Under well-watered conditions, the annual average of CO2 efflux was 0.12 g CO2-C m-2 hr-1, which was 24.5 and 20% significantly higher than under drought stressed and the control, respectively. Moreover, well-watered treatment had significantly greater primed carbon than drought stressed maize. Soil temperature in deeper soil layers (25, 50 and 75 cm) correlated positively (with the CO2 efflux, while soil moisture correlated negatively at the 5 cm and 25 cm. Overall, these results suggested that the root exudates decreased under drought conditions, which decreasing soil respiration. Drought tolerance varieties could be an option to decrease soil respiration and maintain productivity.