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 (CO2)-C-13 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 (V-phloem) decreases under drought. Transport of carbohydrates in the phloem was assessed via C-13 of CO2 efflux measured at two branch positions following (CO2)-C-13 labeling. Pre-dawn water potential ((PD)) 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, V-phloem 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 (PD) (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 V-phloem and (PD) 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 V-phloem. The two mechanisms increasing MRT and reducing V-phloem respond differently to low water availability and impair trees' carbon translocation under drought.