Mucilage, a gelatinous substance comprising mostly polysaccharides, is exuded by maize nodal and underground root tips. Although mucilage provides several benefits for rhizosphere functions, studies on the variation in mucilage amounts and its polysaccharide composition between genotypes are still lacking. In this study, eight maize (Zea mays L.) genotypes from different globally distributed agroecological zones were grown under identical abiotic conditions in a randomized field experiment. Mucilage exudation amount, neutral sugars and uronic acids were quantified. Galactose (∼39–42%), fucose (∼22–30%), mannose (∼11–14%), and arabinose (∼8–11%) were the major neutral sugars in nodal root mucilage. Xylose (∼1–4%), and glucose (∼1–4%) occurred only in minor proportions. Glucuronic acid (∼3–5%) was the only uronic acid detected. The polysaccharide composition differed significantly between maize genotypes. Mucilage exudation was 135 and 125% higher in the Indian (900 M Gold) and Kenyan (DH 02) genotypes than in the central European genotypes, respectively. Mucilage exudation was positively associated with the vapor pressure deficit of the genotypes’ agroecological zone. The results indicate that selection for environments with high vapor pressure deficit may favor higher mucilage exudation, possibly because mucilage can delay the onset of hydraulic failure during periods of high vapor pressure deficit. Genotypes from semi-arid climates might offer sources of genetic material for beneficial mucilage traits.
Temporally and spatially confined, the rhizosphere is crossed by immense volumes of water (Bengough, 2012) while plant roots and microorganisms enhance biogeochemical fluxes, which turn this thin layer of soil around the roots into a distinct example of a hotspot in soil (Kuzyakov & Blagodatskaya, 2015). By the release of a multitude of compounds (Walker et al., 2003), soil organisms engineer the physical properties of their soil surroundings (Benard et al., 2019;Flemming et al., 2016;Naveed et al., 2019). Prominent among these substances are highly polymeric substances, such as EPS (extracellular polymeric substances) and mucilage, which alter the physical properties of the soil solution (
<p>Bacteria alter the physical properties of soil hotspots by secreting extracellular polymeric substances EPS. Despite the biogeochemical importance of these alterations is well accepted, the physical mechanisms by which EPS shapes the properties of the soil solution and its interactions with the soil matrix are not well understood.</p><p>Here we show that upon drying in porous media EPS forms one-dimensional filaments and two-dimensional interconnected structures spanning across multiple pores. Unlike water, primarily shaped by surface tension, EPS remains connected upon drying thanks to its high extensional viscosity. The integrity of one-dimensional structures is explained by the interplay of viscosity and surface tension forces (characterized by the Ohnesorge number), while the formation of two-dimensional structures requires consideration of the interaction of EPS with the solid surfaces and external drivers, such as the drying rate. During drying, the viscosity of EPS increases and, at a critical point, when the friction between polymers and solid surfaces overcomes the water adsorption of the polymers, the concentration of the polymer solution at the liquid-gas interface increases asymptotically and the polymers can no longer follow the retreating gas-water interface. At this critical point the polymers do not move any longer and are deposited as two-dimensional surfaces, such as hollow cylinders or interconnected surfaces. EPS viscosity, specific soil surface and drying rates are the key parameters determining the transition from one- to two-dimensional structures.</p><p>The high viscosity of EPS maintains the connectivity of the liquid phase during drying in soil hotspots, such as bacterial colonies, the rhizosphere and biological soil crusts.</p>
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