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Harvard Forest Symposium Abstract 2019

• Title: Carbon export patterns in relation to diurnal and seasonal carbon and water dynamics in red oak leaves
• Primary Author: Jessica Gersony (Not Specified)
• Additional Authors: Uri Hochberg (Not Specified); Noel Michele Holbrook (Harvard Forest); Fulton Rockwell (Harvard University)
• Abstract:

  Trees typically experience large depressions in water potential throughout the day, which
  may impede carbon export from leaves because of the interactions between xylem and
  phloem water dynamics. Data from the literature suggests that sieve tube sugar
  concentrations are not large enough to draw water from the xylem at midday in red oak
  trees. Therefore, we hypothesized that carbon export from the leaf into the phloem would
  be minimal at midday for red oak trees in Harvard Forest (Petersham, MA). To test this
  hypothesis, we measured diurnal and seasonal patterns of non-structural carbohydrates
  (NSCs), photosynthesis, solute and water potentials, and estimated export in leaves of
  five mature (> 20 m tall) red oak trees between June and September. Surprisingly, we
  found that export occurs throughout the day (at equal or higher rates than at night),
  despite large depressions in water potential. We also found that NSCs accumulate over
  the course of the day (specifically, sucrose), contributing approximately half of the 0.5
  MPa diurnal osmotic adjustment. Higher export patterns during the day were observed in
  saplings of five additional temperate tree species and is consistent with data from the
  literature concerning herbaceous plants. These findings illustrate the robustness of
  phloem functioning despite declines in midday water potentials, the role of NSCs in leaf
  turgor maintenance, and the need to further study phloem dynamics in the context of
  water relations. Currently, we are working on visualizing intracellular sugar
  concentrations to better understand the process by which leaves export carbohydrates to
  the rest of the plant.

• Research Category: Physiological Ecology, Population Dynamics, and Species Interactions