Extreme undersaturation in the intercellular airspace of leaves: a failure of Gaastra or Ohm?

Abstract

Background Recent reports of extreme levels of undersaturation in the internal leaf air spaces have called into question one of the foundational assumptions of leaf gas exchange analysis, that leaf air spaces are effectively saturated with water vapor at leaf surface temperature. Historically, inferring the biophysical states controlling assimilation and transpiration from the fluxes directly measured by gas exchange systems has presented a number of challenges, including: 1) a mismatch in scales between the area of flux measurement, the biochemical cellular scale, and the meso-scale introduced by the localization of the fluxes to stomatal pores; 2) the inaccessibility of the internal states of CO2 and water vapor required to define conductances; and 3) uncertainties about the pathways these internal fluxes travel. In response, plant physiologists have adopted a set of simplifying assumptions that define phenomenological concepts such as stomatal and mesophyll conductances. Scope Investigators have long been concerned that a failure of basic assumptions could be distorting our understanding of these phenomenological conductances, and the biophysical states inside leaves. Here we review these assumptions and historical efforts to test them. We then explore whether artifacts in analysis arising from the averaging of fluxes over macroscopic leaf areas could provide alternative explanations for some part, if not all, of reported extreme states of undersaturation. Conclusions Spatial heterogeneities can, in some cases, create the appearance of undersaturation in the internal air spaces of leaves. Further refinement of experimental approaches will be required to separate undersaturation from the effects of spatial variations in fluxes or conductances. Novel combinations of current and emerging technologies hold promise for meeting this challenge.

The dynamic multi‐functionality of leaf water transport outside the xylem

“…Conversely, if airspace unsaturation or guard cell water relations is of particular interest, then we would like to know the conductance all the way to airspace and the guard cells. Technological advances enabling quantification of Ψ in vivo at leaf level, using intercellular gel sensors (Jain et al ., 2021; Rockwell et al ., 2022), or at the cellular level, using intracellular protein biosensors (Cuevas‐Velazquez et al ., 2021) may soon enable estimation of K ox dynamics according to both definitions.…”

Stomatal response to VPD is not triggered by changes in soil–leaf hydraulic conductance in Arabidopsis or Callitris

“…As with all methods of measuring K leaf or K s‐l (which encompasses K leaf ), our approach assumes a consistent gradient in Ψ across the mesophyll. Some authors have suggested that under extreme conditions, the Ψ gradient within the leaf may become very steep near the end of the flow path for liquid water (Cernusak et al ., 2018; Rockwell et al ., 2022; Wong et al ., 2022), and this would be partially hidden if only a small proportion of mesophyll cells were affected. The conditions used here did not include the extreme transpiration rates where unsaturated mesophyll humidity has been reported, but this might be an interesting focus for future work.…”

Assessing the CO2 concentration at the surface of photosynthetic mesophyll cells

“…However, it has been shown that under high ASD, patchiness (Mott & Buckley, 2000), or unsaturation may occur (Cernusak et al ., 2018), and the mixture of adaxial and abaxial fluxes might mask the occurrence of both. The failure of one or both of these assumptions compromizes the validity of the results, making their identification crucial (Rockwell et al ., 2022). The measurements presented in Fig.…”