Repeated summer drought delays sugar export from the leaf and impairs phloem transport in mature beech

Phloem sustains maintenance and growth processes through transport of sugars from source to sink organs. Under low water availability, tree functioning is impaired, i.e., growth/photosynthesis decline and phloem transport may be hindered. In a 3-year throughfall exclusion (TE) experiment on mature European beech (Fagus sylvatica L.) we conducted ¹³CO₂ branch labeling to investigate translocation of recently fixed photoassimilates under experimental drought over 2 years (2015 and 2016). We hypothesized (H1) that mean residence time of photoassimilates in leaves (MRT) increases, whereas (H2) phloem transport velocity (Vₚₕₗₒₑₘ) decreases under drought. Transport of carbohydrates in the phloem was assessed via δ¹³C of CO₂ efflux measured at two branch positions following ¹³CO₂ labeling. Pre-dawn water potential (ΨPD) and time-integrated soil water deficit (iSWD) were used to quantify drought stress. The MRT increased by 46% from 32.1 ± 5.4 h in control (CO) to 46.9 ± 12.3 h in TE trees, supporting H1, and positively correlated (P < 0.001) with iSWD. Confirming H2, Vₚₕₗₒₑₘ in 2016 decreased by 47% from 20.7 ± 5.8 cm h–¹ in CO to 11.0 ± 2.9 cm h–¹ in TE trees and positively correlated with ΨPD (P = 0.001). We suggest that the positive correlation between MRT and iSWD is a result of the accumulation of osmolytes maintaining cell turgor in the leaves under longer drought periods. Furthermore, we propose that the positive correlation between Vₚₕₗₒₑₘ and ΨPD is due to a lower water uptake of phloem conduits from surrounding tissues under increasing drought leading to a higher phloem sap viscosity and lower Vₚₕₗₒₑₘ. The two mechanisms increasing MRT and reducing Vₚₕₗₒₑₘ respond differently to low water availability and impair trees’ carbon translocation under drought.