title: Biophysical factors controlling root exudation and rhizosphere extension
Creators: name: Holz, Maire
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Boden Pflanze Interaktion
subject: Rhizosphärenausdehnung
subject: Trockenstress
subject: Mucilage
subject: 580

description: The rhizosphere, the region of soil near the roots, plays a crucial role in water and nutrient uptake by plants because root exudates enhance microbial activity and nutrient availability to plants. The degree to which root exudates can interact positively with plants depends on their spatiotemporal distribution around the roots. The objective of this work was therefore to understand what factors control the spatiotemporal distribution of root exudates in soil. We investigated how root traits such as root hairs and mucilage secretion and rhizosphere water related properties mediate the distribution of root exudates in soil. The first experiment focused on the effect of soil drying on rhizosphere extension. We coupled 14CO2 labelling and phosphor imaging of plants with neutron radiography to image 14C allocation and rhizosphere water content (WC). Exudate distribution around growing roots was predicted by a numerical model. Plants grown in dry and wet soil allocated similar amounts of 14C into roots but root elongation decreased by 48% in dry soil, reducing longitudinal rhizosphere extension. Rhizosphere WC was identical in dry and wet soils, presumably because of the high water retention by mucilage. The increase in rhizosphere WC enhanced root exudation in dry soil and enlarged the radial rhizosphere extension. The release of mucilage may be beneficial to plants because it maintains fast diffusion of exudates and high rhizospere extension under water limitation. Alongside low molecular carbon compounds, roots release enzymes to mineralize nutrients. The objective of the second experiment was therefore to test how the local soil WC affects phosphatase activity in soil. Barley plants were grown in rhizoboxes and subjected to a drying cycle, while soil water content (WC) and phosphatase activity were monitored by neutron radiography and soil zymography.. Phosphatase activity and soil WC were strongly correlated in rhizosphere and bulk soil. The power-law relation between soil WC and enzyme activity confirmed our hypothesis that enzyme activity is controlled by diffusion. Phosphatase activity in the rhizosphere, having a high WC compared to the bulk soil, was significantly larger than phosphatase activity in the bulk soil. This can be explained by mucilage released into the rhizosphere which retain water upon soil drying. The first two studies highlight the effect of mucilage on root exudate distribution. However, so far no method is available to measure the spatial distribution of mucilage in soil. The aim of the third experiment was therefore to test whether the C-H signal caused by fatty acids in mucilage can be measured with infrared (IR) spectroscopy to determine the spatial distribution of mucilage in soil. Measurements were carried out along transects of 1.5 mm perpendicular to root channels of plants grown in rhizoboxes. We measured distinct profiles of C-H and the amounts of mucilage we measured were comparable with results obtained in previous studies. IR spectroscopy is therefore a promising tool to measure the spatial distribution of mucilage in soil. The aim of the fourth study was to test whether root hairs exude C to enlarge rhizosphere extension. Barley wild type (WT) and its root-hairless mutant (brb) were grown in rhizoboxes and labelled with 14CO2. Root exudates were captured on filter paper and quantified by phosphor imaging. WT plants allocated more carbon (C) below ground while the hairless mutant allocated more C to shoots. Root hairs increased the radial rhizosphere extension 3-fold, from 0.5 to 1.5 mm. Total exudation was 3 times greater for WT plants compared to the hairless mutant. The increase in rhizosphere extension may enhance the positive effect of root exudates to plants. While 14C imaging of root exudates on filter paper was successfully applied in the fourth study, this approach is restricted to moist soil conditions. In the last study we therefore tested whether 14C imaging on the soil-root surface can be applied to quantify root exudates. The attenuation coefficient of 14C in soil was calculated and the expected 14C profiles were calculated. The profiles were strongly affected by: a) the 14C activity in the root, b) the root radius, c) the position of the root, d) the amount of root exudates and e) by the presence of air gaps between soil and imaging screen. Inaccurate measurements of any of these parameters would cause artefacts in the estimation of root exudates distribution in the rhizosphere using phosphor imaging. By combining complementary imaging methods and numerical modelling we showed that root hair production and mucilage release increase rhizosphere extension and total exudation. Mucilage movement was restricted to shorter distances from the root surface (0.6 mm) compared to overall exudates (1.2 mm). Nevertheless, mucilage’s water retention strongly affects the diffusion of low molecular root exudates and enzymes. Root hairs increase total exudation and rhizosphere extension, which probably enhances rhizosphere interactions in larger soil volumes. Breeding for long and dense root hairs as well as increased mucilage secretion may be suitable strategies for future agriculture where nutrients are expected to become scarce