Spatial Distribution of Mucilage in the Rhizosphere Measured With Infrared Spectroscopy

Abstract

Mucilage is receiving increasing attention because of its putative effects on plant growth, but so far no method is available to measure its spatial distribution in the rhizosphere. We tested whether the C-H signal related to mucilage fatty acids is detectable by infrared spectroscopy and if this method can be used to determine the spatial distribution of mucilage in the rhizosphere. Maize plants were grown in rhizoboxes filled with soil free of organic matter. Infrared measurements were carried out along transects perpendicular as well as axially to the root channels. The perpendicular gradients of the C-H proportions showed a decrease of C-H with increasing distance: 0.8 mm apart from the root center the C-H signals achieved a level near zero. The measured concentrations of mucilage were comparable with results obtained in previous studies, which encourages the use of infrared spectroscopy to quantitatively image mucilage in the rhizosphere.

Simulating rhizodeposition patterns around growing and exuding root systems

“…The simulated radial extent of citrate and mucilage rhizopheres in which the rhizodeposit concentration was below the threshold value, but still detectable, was 4 – 9 mm and 2 – 5 mm , respectively. For mucilage, these values are in the same range as the experimental findings by Holz et al (2018a) and the calculated values by Zickenrott et al (2016), who reported rhizosphere extents between 0.6 mm and 2 mm . For citrate, the radial rhizosphere extents are in the same order of magnitude as the results for rhizodeposited 14 C by Kuzyakov et al (2003), who measured a zone of maximum carbon exudate concentration within a distance of 1 – 2 mm from the root surface and a zone of less significant amounts of carbon exudate concentration within a distance of 3 – 10 mm from the root surface.…”

“…For citrate, the maximum simulated radial hotspot extent of 1 mm and the detectable concentration extent of 4 − 9 mm were of the same order of magnitude as the results for rhizodeposited 14 C from Kuzyakov et al (2003) who measured a zone of maximum carbon exudate concentration at a distance of 1 − 2 mm from the root surface and a zone of less significant amounts of carbon exudate concentration at a distance of − mm from the root surface. It must be noted that the experimental conditions and model assumptions in the studies by Holz et al (2018a), Zickenrott et al (2016) and Kuzyakov et al (2003) were not the same as in our modelling setup. They differed with regard to plant species, plant age, water content and pot geometry and may therefore only be regarded as an indicative of the order of magnitude.…”

“…There are only limited possibilities to directly measure the spatio-temporal distribution patterns of rhizodeposits around a root system. Holz et al (2018a) used infrared spectroscopy to determine the spatial distribution of mucilage in the rhizosphere. This method allowed them to visualize the axial and radial gradients of mucilage concentration around a single root at a given point in time; information on the temporally dynamic distribution of mucilage is, however, lacking.…”

Spectroscopic characterization of mucilage (Chia seed) and polygalacturonic acid

“…Thus, homogenized mucilage samples may not reflect the properties of Chia seed mucilage in natural conditions. The observed spatial heterogeneity in OM composition (in terms of relative C–H group content) corresponds with that observed by micro‐FTIR across root channels of maize seedlings ( Holz et al., ). As assumed by Carvalhais et al.…”

“…Thus, homogenized mucilage samples may not reflect the properties of Chia seed mucilage in natural conditions. The observed spatial heterogeneity in OM composition (in terms of relative C-H group content) corresponds with that observed by micro-FTIR across root channels of maize seedlings (Holz et al, 2018). As assumed by Carvalhais et al (2011) and Ahmed et al (2015), mucilage composition may be affected by growing conditions of the plants such that the composition of the excreted mucilage may vary with time causing heterogeneity in OM composition in space.…”