---
title: "Forcing Plants to Participate"
subtitle: "If a leaf isn't photosynthesizing, how can we tell if it's incapable or unwilling?"
author: "Riley Leff"
url: "https://rileyleff.com/blog/forcing-plants-to-participate"
published: "2026-08-21"
series: null
tags: ["Plant Ecophysiology", "Photosynthesis", "Fieldwork", "Open Science", "Methods"]
---

# Forcing Plants to Participate

*If a leaf isn't photosynthesizing, how can we tell if it's incapable or unwilling?*

<!-- Capitalized tags like <Plot /> are interactive figures on the web version: https://rileyleff.com/blog/forcing-plants-to-participate -->

This summer, we set out to measure photosynthesis on the Eastern Shore of Virginia, where rising sea levels are killing forests and converting them into marshes. We measured gas exchange rates near zero everywhere we looked: no meaningful carbon assimilation or transpiration. Plants can close up shop by closing stomata, the pores on their leaves that facilitate gas exchange. By temporarily forcing those stomata open, we revealed that the chemical machinery underneath was still partially intact. In *Phragmites australis*, an invasive reed of broad ecological interest, the ability to convert light into chemical energy was on par with [populations on the Gulf Coast](https://doi.org/10.1093/aobpla/plt016), but their estimated ability to fix carbon into sugar was reduced by a little more than 50%. Without that intervention, we would have mistaken a plant’s unwillingness to photosynthesize for an inability to do so: leaves with roughly half the carbon-fixing capacity of Gulf Coast populations looked like they had almost none.

That distinction has vastly different implications for the future of the planet. If plants can wait out adverse conditions and capitalize on rare favorable opportunities, they may be much more resilient than standard measurements suggest. If they are losing the capacity itself, low-elevation coastal forests may be at risk. Our measurement blindness leads to collisions across scales of understanding: landscape-scale approaches claim that forest mortality rates lag behind rates of sea level rise, while [physiological approaches](https://doi.org/10.1111/gcb.16297) claim coastal plants are experiencing a [“death spiral”](https://doi.org/10.1038/s43017-022-00272-1). We can reconcile these views with better measurement.

We designed an approach that forces plants to participate long enough for us to separate stomatal limitation from biochemical capacity. We built on methods that exploit the difference in response time between slow stomata and fast photochemistry, but which require the stomata to already be open (see [Stinziano et al. 2017](https://doi.org/10.1111/pce.12911) and [Saathoff & Welles 2021](https://doi.org/10.1111/pce.14178)). By temporarily depriving a leaf of CO2, we opened its stomata, then measured gas exchange and chlorophyll fluorescence across a grid of CO2 and light levels before they could close again. We automated the whole protocol on our field’s standard photosynthesis instrument, the LI-COR 6800, and used it to collect a small, highly detailed pilot dataset on *Phragmites* across a salinity gradient at our coastal site.

<LowCo2Experiment
  src="https://assets.rileyleff.com/forcing-plants-to-participate/low-co2-opening.svg"
  mobileSrc="https://assets.rileyleff.com/forcing-plants-to-participate/low-co2-opening-mobile.svg"
  alt="Stomatal conductance and carbon assimilation in a Morella sun leaf as chamber carbon dioxide was lowered from 420 to 125 ppm and returned to ambient"
  captionLead="Demoing our method on a Morella cerifera leaf."
  caption="g_sw measures stomatal openness, while A measures carbon assimilation. Although g_sw is still rising during the initial 420 ppm CO₂ period, in repeated observations, leaves held there generally level off rather than continue to open. Lowering CO₂ keeps g_sw climbing. Returning to 420 ppm then gives us a brief window to measure A and its response to manipulated conditions before stomata become limiting again."
/>

We found wide, overlapping estimates of photosynthetic capacity in the high- and low-salinity populations, with no detectable difference on either the light-converting or carbon-fixing side of photosynthesis. For my PhD work, this is a useful null result: we found no evidence that models of hydraulic and photosynthetic tradeoffs need separate photosynthetic parameterizations across this salinity gradient. This would be awkward to publish in a traditional venue because the dataset is unusually deep within leaves but shallow across them. We measured only 12 leaves across two salinity levels, but collected roughly 60,000 gas-exchange observations and 600 fluorometry measurements.

<PhotosynthesisSurface
	src="/blog-assets/forcing-plants-to-participate/phrag-p10-surface.json"
  captionLead="A full characterization of a Phragmites australis leaf's photosynthetic capacity."
  caption="C_i is intercellular CO₂ concentration, and Q is incident light. As Q and C_i increase, carbon assimilation increases and plateaus. The height and curvature of that response let us calculate the leaf's photochemical limits."
/>

I would love to publish the automated LI-COR program and analysis code on GitHub, the operating protocol on protocols.io, and the raw and processed data with a technical report on Zenodo. I would also write a more narrative account of the work and share it on my blog. In doing this, I would put the method into other researchers’ hands reproducibly, contribute Mid-Atlantic data to the burgeoning effort to understand cross-regional patterns in *Phragmites* invasion, and invite researchers to hunt down interactions in the dataset that we did not set out to test. Plant ecophysiology traditionally compresses entire measurement campaigns into a few summary statistics. There are shockingly few “what did somebody else’s machine actually do?” breadcrumbs out there, even though it’s immensely helpful to have a complete instrument output to reference when standing up a new project.