The overall aim of the proposed research is to understand how plant hydraulic redistribution (HR) changes root exudation of biomolecules to the rhizosphere. HR is the flux of water from wet soil to dry soil via plant roots, occurring in the absence of leaf transpiration (e.g. night). This flux of water may also entrain and transport plant metabolites into the rhizosphere (exudates). This flux of water and exudates may in turn trigger hot moments of rhizosphere microbial activity that sustain soil biogeochemical cycling during drought, and HR can thus play a key role in ecosystem resilience to drought. The specific objectives of the project are to quantify how HR alters the magnitude, composition, and location of root exudation. We aim to test the following hypotheses: (1) HR increases exudate carbon flux to the rhizosphere, in particular by increasing the exudation of primary metabolites that are entrained in the advective flux of water to the rhizosphere. (2) HR expands the spatial extent of the rhizosphere by advectively transporting exudates further than diffusive fluxes do under non-HR conditions.
(3) HR increases root exudation throughout the fine root network, in contrast to non-redistributive conditions where exudation is focused at root tips. The proposed work is comprised of two phases. In the first phase, we will refine and optimize extant methods that allow for the imaging of exudate compounds in the rhizosphere via Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Imaging (FT-ICR-MSI). In the second phase of the project, we will use these optimized procedures in the context of a greenhouse experiment to examine exudation of two focal tree species, comparing exudation from the same roots between HR and non-HR conditions. To test hypotheses 1 and 3, we will analyze exudate quantity and composition across different root morphological features (e.g. root tips, 1st order mature root, 2nd order roots), under both HR and non-HR conditions. We will compare signal intensities for select individual compounds of known metabolic function (e.g. primary vs. secondary metabolism), and we will further use ordination techniques to compare ensemble shifts in exudate composition. We will test hypothesis 2 by examining the steepness of exudate gradients perpendicular to the root under both HR and non-HR conditions, for select high-abundance exudates. The proposed work is a component of a larger DOE BER-funded research project that uses a model-experiment (ModEx) framework to examine how HR alters rhizosphere carbon cycling. The imaging and exudate characterization data obtained from the proposed EMSL work will be used in the larger project to parameterize two models that model rhizosphere carbon dynamics at different scales and levels of complexity: a single root-rhizosphere biogeochemical model and a plant-scale ecophysiological model. The proposed work will advance understanding of the environmental factors that drive hot spots and hot moments of biogeochemical activity and advance a mechanistic understanding of ecosystem resilience to drought, key goals of BER.