Leaf Carbon Export and Nonstructural Carbohydrates in Relation to Diurnal Water Dynamics in Mature Oak Trees

Abstract

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 nonstructural carbohydrates, photosynthesis, solute, and water potential 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 21.8 MPa at midday. Suc and starch accumulated over the course of the day, with Suc contributing ;50% of the 0.4 MPa diurnal osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always .0.7 MPa. These findings illustrate the robustness of phloem functioning despite diurnal fluctuations in leaf water potential and the role of nonstructural carbohydrates in leaf turgor maintenance.

Canopy Position Influences the Degree of Light Suppression of Leaf Respiration in Abundant Tree Genera in the Amazon Forest

“…This is in apparent contrast to previous observations that have shown that leaves more exposed to sun usually have higher levels of soluble sugars when compared to shade leaves (Weerasinghe et al, 2014;Dayer et al, 2021). One explanation for the difference may be because our samples were collected during the early mornings where leaf NSCs may be depleted due to their use as respiratory substrates at night; leaf NSC concentrations tend to peak at midday when biosynthesis from photosynthesis is high (Dickman et al, 2019;Gersony et al, 2020). The lack of clear variation of leaf starch concentrations with canopy position may also be related to the fact that starch is a reserve carbohydrate with a slower turnover rate relative to sugars (Dickman et al, 2019).…”

Raman spectroscopy reveals high phloem sugar content in leaves of canopy red oak trees

“…Furthermore, because bulk leaf osmolality is c . 935 mM at midday (Gersony et al ., 2020), some cells would need to have a much lower osmolality to balance out the osmolality of the cells with high cytoplasmic sucrose concentrations that are involved in passive loading. Such cellular‐level discrepancies in osmolality have not been observed in plasmolysis studies (Russin & Evert, 1985).…”

“…A neighboring leaf was analyzed for sucrose content using the enzymatic‐colorimetric assay method described by Gersony et al . (2020) to establish the baseline concentration in the extract (average of 34 mM). The extract was then spiked with 0–1250 mM of sucrose.…”

“…1). While one solution would be to perform the majority of phloem loading and transport at night, in red oak, a reported passive loader (Rennie & Turgeon, 2009), the majority of carbon transport out of the leaf takes place during the day (Gersony et al ., 2020). The key question then follows: in red oak, is the source sieve element sucrose concentration high enough to support Münch flow, while being low enough to allow passive loading?…”

Extrapolating Physiological Response to Drought through Step-by-Step Analysis of Water Potential

“…genotype and phenological and physiological stages). In particular, diurnal variation in nonstructural carbohydrates in leaves can significantly affect osmotic pressure and thus leaf turgor (Charrier, 2020;Gersony et al, 2020).…”