Leaf relative uptake of carbonyl sulfide to CO₂ seen through the lens of stomatal conductance–photosynthesis coupling

Abstract

Carbonyl sulfide (COS) has emerged as a multi-scale tracer for terrestrial photosynthesis. To infer ecosystem-scale photosynthesis from COS fluxes often requires knowledge of leaf relative uptake (LRU), the concentration-normalized ratio between leaf COS uptake and photosynthesis. However, current mechanistic understanding of LRU variability remains inadequate for deriving robust COS-based estimates of photosynthesis.We derive a set of closed-form equations to describe LRU responses to light, humidity and CO 2 based on the Ball-Berry stomatal conductance model and the biochemical model of photosynthesis. This framework reproduces observed LRU responses: decreasing LRU with increasing light or decreasing humidity; it also predicts that LRU increases with ambient CO 2 .By fitting the LRU equations to flux measurements on a C 3 reed (Typha latifolia), we obtain physiological parameters that control LRU variability, including an estimate of the Ball-Berry slope of 7.1 without using transpiration measurements. Sensitivity tests reveal that LRU is more sensitive to photosynthetic capacity than to the Ball-Berry slope, indicating stomatal response to photosynthesis.This study presents a simple framework for interpreting observed LRU variability and upscaling LRU. The stoma-regulated LRU response to CO 2 suggests that COS may offer a unique window into long-term stomatal acclimation to elevated CO 2 .

Traversing scales: parts‐per‐trillion to gigatons

“…In this issue of New Phytologist , Sun et al . (2022; pp. 1729–1742) clarify the theory connecting atmospheric measurements of OCS and CO 2 to primary production.…”

“…In this issue of New Phytologist , Sun et al . (2022; pp. 1729–1742) connect the mechanistic understanding of OCS and CO 2 uptake at the leaf level with LRU.…”

“…How can this whiff of a gas reveal anything interesting about the carbon cycle? In this issue of New Phytologist, Sun et al (2022;pp. 1729-1742 clarify the theory connecting atmospheric measurements of OCS and CO 2 to primary production.…”

“…Carbon dioxide diffuses into the mesophyll (mesophyll conductance: g m ) where some is ultimately fixed via photosynthesis (reaction rate coefficient with RuBisCO: g c ). In this issue of New Phytologist, Sun et al (2022;pp. 1729-1742 connect the mechanistic understanding of OCS and CO 2 uptake at the leaf level with LRU.…”

Technical note: Novel estimates of the leaf relative uptake rate of carbonyl sulfide from optimality theory

“…% are well known to vary over short timescales in response to diel changes in environmental drivers, as well as along large-scale bioclimatic gradients (Lloyd and Farquhar, 1994). With regard to the former, recent work by Kohonen et al (2022) and Sun et al (2022) demonstrated that contrasting leaf gas exchange theories are able to reproduce and explain the observed short-term response of LRU to key drivers such as incident PAR or the vapor pressure deficit (VPD).…”