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

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 R-2 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 Munch 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 similar to 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.

Automated summary

This study demonstrates the use of Raman spectroscopy to assess sucrose concentration in leaf samples and apply this method to estimate sucrose concentration in sieve elements of canopy red oak trees. The findings support the Munch hypothesis for phloem transport and challenge the passive-loading hypothesis in red oak. The study provides a new avenue for investigating phloem functioning by characterizing sieve element sucrose concentration in plants in a similar to in-situ state.