Stress differentially causes roots of tree seedlings to exude carbon.

How carbon (C) flows through plants into soils is poorly understood. Carbon exuded comes from a pool of non-structural carbohydrates (NSC) in roots. Simple models of diffusion across concentration gradients indicate that the more C in roots, the more C should be exuded from roots. However, the mechanisms underlying the accumulation and loss of C from roots may differ depending on the stress experienced by plants. Thus, stress type may influence exudation independent of NSC. We tested this hypothesis by examining the relationship between NSC in fine roots and exudation of organic C in aspen (Populus tremuloides Michx.) seedlings after exposure to shade, cold soils and drought in a controlled environment. Fine root concentrations of NSC varied by treatment. Mass-specific C exudation increased with increasing fine root sugar concentration in all treatments, but stress type affected exudation independently of sugar concentration. Seedlings exposed to cold soils exuded the most C on a per mass basis. Through 13C labeling, we also found that stressed seedlings allocated relatively more new C to exudates than roots compared with unstressed seedlings. Stress affects exudation of C via mechanisms other than changes in root carbohydrate availability.