Exogenous sugar addition can exacerbate root carbon limitation in trees.

In most tree species, roots serve as major carbon (C) sinks, where C is depleted first when C assimilation is limited. Recent methodological advancements in sugar infusion allow for a better understanding of physiological processes alleviating root C limitation. We conducted a glasshouse experiment with maple (Acer pseudoplatanus L.) and pine (Pinus sylvestris L.) saplings that underwent defoliation followed by either slow, fast, or no C-labeled glucose infusion. We measured photosynthetic parameters, nonstructural carbohydrate (NSC) concentrations, and δC in cellulose of leaves, twigs, and fine roots, as well as the isotopic composition of dark-respired CO. Sugar infusion induced photosynthetic downregulation and leaf senescence in maple but not in pine. Leaf photosynthesis was negatively correlated with leaf NSC concentration in maple. These responses exacerbated root C limitation in maple. Conversely, pine maintained stable photosynthetic rates and needle NSC concentrations across treatments, showing the potential of sugar infusion to mitigate root C limitation. Our study suggests that exogenous sugar supply reduces the root C availability when it impairs a plant's photosynthetic performance. Species-specific differences influence infused sugar transport and overall source-sink responses. Alleviating C limitation in roots via exogenous sugar addition is feasible only if photosynthesis is not impeded.