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 ¹³C-labeled glucose infusion. We measured photosynthetic parameters, nonstructural carbohydrate (NSC) concentrations, and δ¹³C in cellulose of leaves, twigs, and fine roots, as well as the isotopic composition of dark-respired CO₂. 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.