Defoliation alters the relative contributions of recent and non‐recent assimilate to root exudation from Festuca rubra

Abstract

ABSTRACTThe deposition of organic compounds from plant roots is a key determinant of rhizosphere microbial activity and community structure. Consequently, C‐flow from roots to soil is an important process in coupling plant and microbial productivity, via impacts on microbial nutrient cycling in soil. Experimentally, isotopic tracers (13C or 14C) are used to track C inputs to soil and microbial communities. However, in many such studies the relationship between labelled C‐flows and total C‐flows are not established, limiting the interpretative value of the results. In this study, we applied steady‐state near natural abundance 13CO2 labelling to determine the impact of partial defoliation of Festuca rubra on root exudation. This approach in axenic culture facilitated determination of the contribution of pre‐ and post‐defoliation assimilates both to root C‐flow and plant tissues. The results demonstrated that total root exudation was increased in the two days following defoliation. This was concurrent with reduced net CO2 assimilation and reduced allocation of post‐defoliation assimilates below‐ground and to active root meristems. Through determination of the δ13C of root exudates, it was established that the source of the increased root exudation was pre‐defoliation assimilate. However, this response was transient, with reduced deposition of pre‐ and post‐defoliation assimilates from roots during the period 2–4 d following defoliation. The results highlight the limitations of pulse‐labelling approaches as a means of quantifying impacts of treatments on root exudation, particularly where the treatment is likely to affect plant C‐partitioning or the balance between deposition to, and re‐mobilization from, C‐storage pools.