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

Abstract

Summary A robust understanding of phloem functioning in tall trees evades us because current methods for collecting phloem sap do not lend themselves to measuring actively photosynthesizing canopy leaves. We show that Raman spectroscopy can be used as a quantitative tool to assess sucrose concentration in leaf samples. Specifically, we found that Raman spectroscopy can predict physiologically relevant sucrose concentrations (adjusted R2 of 0.9) in frozen leaf extract spiked with sucrose. We then apply this method to estimate sieve element sucrose concentration in rapidly frozen petioles of canopy red oak (Quercus rubra) trees and found that sucrose concentrations are > 1100 mM at midday and midnight. This concentration is predicted to generate a sieve element turgor pressure high enough to generate bulk flow through the phloem, but is potentially too high to allow for sucrose diffusion from photosynthetic cells. Our findings support the Münch hypothesis for phloem transport once the carbon is in the phloem and challenge the passive‐loading hypothesis for carbon movement into the phloem for red oak. This study provides the first ˜in‐situ (frozen in the functioning state) source sieve element sucrose concentration characterization in any plant, opening a new avenue for investigation of phloem functioning.

Phloem turgor is maintained during severe drought in Ricinus communis

“…Despite this important role, how and when the phloem loses function are both unknown due to methodological limitations. Because the phloem is microscopic (micron‐scale) and living (unlike the xylem), it is difficulty to study directly (but see recent advances in phloem imaging and phloem sucrose detection in Gersony et al, 2021; Guendel et al, 2018; Ray & Savage, 2020; Torode et al, 2018). An example of an existing knowledge gap is the role solutes play in phloem loss of function: the phloem may lose function because it can no longer sustain turgor due to the solute concentration of the sap not being high enough to draw water into the phloem, or it may lose function because it has too many solutes that viscosity becomes a limiting factor in transport (Sevanto, 2018).…”

“…Because the phloem is microscopic (micron-scale) and living (unlike the xylem), it is difficulty to study directly (but see recent advances in phloem imaging and phloem sucrose detection in Gersony et al, 2021;Guendel et al, 2018;Ray & Savage, 2020;Torode et al, 2018).…”

Insights into source/sink controls on wood formation and photosynthesis from a stem chilling experiment in mature red maple

“…Given that we observed contrasting reactions of NSCs in adjacent tissues, such as phloem and stem soluble sugar, it begs the question whether these gradients remain stable under varying conditions or whether only particular aspects of the gradient, such as phloem concentrations, are maintained. Repeated and spatially highly resolved measurements of NSC concentrations, using techniques such as Raman spectroscopy (Gersony et al ., 2021), promise to advance our understanding of these gradients and their dynamics in response to internal and external factors. These gradients are likely to be crucial to understanding the impact of carbon dynamics on specific processes of wood formation (e.g.…”

“…3). Although trees appear to be able to load the phloem against strong concentrations gradients (Gersony et al ., 2021), the gradual NSC accumulation in leaves over the season presumably resulted from the backup of carbon transport in the phloem in chilled red maple, which is best characterized as a passive phloem loader (Eschrich & Fromm, 1994) like about half of all characterized tree species (Liesche, 2017). Higher NSC concentrations in turn have been theorized to inhibit photosynthesis (Salmon et al ., 2020), which has long been confirmed in herbaceous plants and tree leaves (Iglesias et al ., 2002; Vaughn et al ., 2002), but lacked evidence along the long transport pathways between the trunk at breast height and the canopy of mature trees.…”