Jessica Toby Gersony  - Harvard University CITY: Cambridge STATE: Massachusetts POSTAL_CODE: 02138 United States Of America
Fulton E Rockwell  - Harvard University CITY: Cambridge STATE: MA POSTAL_CODE: 02138 United States Of America. 
Maria Park  - Department of Organismic and Evolutionary Biology, Harvard University CITY: Cambridge STATE: Massachusetts 02138 United States Of America.
Paul Pg Gauthier  - Princeton University CITY: Princeton STATE: New Jersey POSTAL_CODE: 08540 United States Of America.  
N Michele Holbrook  - Harvard University CITY: Cambridge STATE: Massachusetts POSTAL_CODE: 2138 United States Of America.  

Trees typically experience large diurnal depressions in water potential, which may impede carbon export from leaves during the day because the xylem is the source of water for the phloem. As water potential becomes more negative, higher phloem osmotic concentrations are needed to draw water in from the xylem. Generating this high concentration of sugar in the phloem is particularly an issue for the ~50% of trees that exhibit passive loading. These ideas motivate the hypothesis that carbon export in woody plants occurs predominantly at night, with sugars that accumulate during the day assisting in mesophyll turgor maintenance or being converted to starch. To test this, diurnal and seasonal patterns of leaf non-structural carbohydrates, photosynthesis, solute, and water potentials were measured, and carbon export was estimated in leaves of five mature (> 20 m tall) red oak (Quercus rubra) trees, a species characterized as a passive loader. Export occurred throughout the day at equal or higher rates than at night despite a decrease in water potential to -1.8 MPa at midday. Sucrose and starch accumulated over the course of the day, with sucrose contributing approximately 50% of the 0.4 MPa diurnal osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always greater than 0.7 MPa. These findings illustrate the robustness of phloem functioning despite diurnal fluctuations in leaf water potential and the role of NSCs in leaf turgor maintenance.