A novel design for sampling root exudates: Does root exudation differ depending on root tissue type and along the root axis?

Roots release soluble organic compounds, known as root exudates, into the soil that influence carbon and nutrient cycling. Understanding the quantity, quality and spatial dynamics of root exudates is crucial to gain deeper insights into plant-soil-microbe interactions. Despite advances, knowledge gaps remain regarding exudate dynamics and composition in soil systems, as many studies mainly relied on hydroponic methods, which may not accurately replicate natural conditions of soil. This study investigated the spatial variability and composition of root exudates by assessing the difference between localized root segment sampling and whole root system (WRS) sampling as well as the contribution of root hairs to exudation. Two genotypes of Zea mays, wildtype B73 (WT) and root-hairless mutant (rth3), were grown in soil-filled rhizoboxes under controlled conditions in a growth chamber. Root exudates were collected by custom-designed exudation traps targeting different positions along the root axis and root tissue types, and were compared to WRS exudation rates obtained with a soil-hydroponic-hybrid approach. Exudates were analysed spectrophotometrically for total dissolved organic carbon, soluble carbohydrates, phenolic compounds, and amino acids. Results revealed significant spatial variability in exudation along the root axis, with young root tissue exhibiting higher exudation rates than older segments, and double those of WRS. Root hairs and genotypic differences showed less influence than anticipated, with position along the root axis being the dominant factor. Extrapolating exudation rates of individual segments to WRS consistently overestimated whole root system exudation, emphasizing the need for careful interpretation of exudation hotspots and WRS rates. This study highlights the importance of soil-based approaches and ecologically relevant root exudate sampling for spatially resolved insights into carbon input via plant roots into the soil.