Measurements

Simultaneous fluorescence and gas exchange measurements over the same sample area provide compounding value because the data can reveal more about the sample. The LI-6800 controls environmental conditions for the duration of a measurement, preventing multiple parameters from confounding the plant responses, ensuring more repeatability and confidence in the results.

Gas exchange

Unsurpassed for CO₂ and H₂O gas exchange at the leaf level or CO₂ exchange of aquatic samples, the LI-6800 provides direct measurements of CO₂ uptake to characterize the photosynthetic assimilation rate of samples.

Features and benefits

Long-lasting, durable, Advanced Polymer leaf gaskets ensure a good seal around the irregular leaf surfaces, while rebounding quickly between measurements.
High flow rates enable the system to characterize photosynthesis from large leaves, while precise gas analyzers enable the measurement of very low photosynthesis rates.
IRGAs in the sensor head, excellent mixing of sample air, and high flow rates through the sample volume and IRGA tightly couple the leaf’s response and the LI-6800’s measurement.
IRGA matching to remove offsets, so you can measure the true differential between sample and reference cells. Matching can be done manually, automatically, or across a range of CO₂ and H₂O concentrations (range matching) to save time and suit your experimental protocols.

Dynamic Assimilation™ Technique

The LI-6800, equipped with the 6800-01A Fluorometer, is the only instrument capable of the Dynamic Assimilation Technique. It allows you to collect data for full response curves in a fraction of the time required by steady state measurements. In contrast with the RACiR method, the Dynamic Assimilation Technique does not depend on empirical corrections and is traceable to first principles of gas exchange measurements.

The Dynamic Assimilation Technique is based on a reformulation of the steady-state equation that enables you to measure assimilation curves. Dynamic assimilation data are comparable in uncertainty to traditional survey data. The increase in variability is typically offset by higher data density.

A-Q curve using the Dynamic Assimilation Technigue on the LI-6800 — Comparison of dynamic and steady-state methods for an A-Q curve using soybean (*Glycine max*). For the dynamic method, actinic light was ramped from 2,000 µmol m^-2 s^-1 over 40 minutes. The steady-state method changed the actinic light level every 2 minutes.

A-Ci curve using the Dynamic Assimilation Technigue on the LI-6800 — Soybean (*Glycine max*) A-Ci curve comparing dynamic assimilation and steady-state measurements. The 400 ppm/minute curve was collected in 4 minutes and the 200 ppm/minute curve was collected in 8 minutes. In contrast, data for steady state curves may take up to 35 minutes to collect under ideal circumstances.

The LI-6800, equipped with the 6800-01A Fluorometer, is the only instrument capable of the Dynamic Assimilation Technique.

For details, see
Saathoff, A. J., & Welles, J. (2021). Gas exchange measurements in the unsteady state. Plant, Cell & Environment, 44(11), 3509–3523. https://doi.org/10.1111/pce.14178

Chlorophyll a fluorescence

Chlorophyll a fluorescence measurements are a distinct application of the LI-6800. Chlorophyll a fluorescence measurement provides insights into the energy-producing and energy-consuming reactions of photosynthesis.

6800-081A fluorometer ready to meadure a tree leaf

The 6800-01A Fluorometer chamber is for combined gas exchange and fluorescence measurements of leaves and aquatic samples. Featuring a Pulse-Amplitude Modulation (PAM) fluorometer, the chamber measures chlorophyll a fluorescence and gas exchange simultaneously over the same leaf area. It includes interchangeable apertures for 2 cm² or 6 cm² leaf area.

Why the 6800-01A Fluorometer?

Combined gas exchange and fluorescence over the same sample for simultaneous insights into the energy-producing and energy-consuming reactions of photosynthesis.
Highly uniform optical output over the entire sample area for more controlled measurements.
User configurable modulation rate and intensity for different research questions and sample material.
Capable of both rectangular and multiphase flashes—to achieve maximum fluorescence yield and save time.
High speed detection and logging of fluorescence emission, with sufficiently high frequency for induction kinetics.

chart showing spectra for the 6800-01A fluorometer — The 6800-01A fluorometer peak outputs are at 475 nm for blue, 625 nm for red modulating, and 735 nm for far-red.

LI-6800 OJIP plot — Fluorescence induction transient plotted on a logarithmic time scale, measured on a dark-adapted leaf.

Leaf, needle, aquatic, and soil

LI-6800 chambers and light sources measure small to large leaves, needles, shoots, and sprigs—as well as aquatic samples and soil CO₂ flux—enabling researchers to expand the scope of their research questions.

For a full list of chambers, including those for bryophytes and insects, see Chambers and Light Sources.

Small leaf measurements

6800-12A clear-top chamber measuring a blade of grass

The Clear-top Chamber (6800-12A) measures gas exchange of small to mid-sized leaves under ambient or controlled light. Aperture inserts restrict the sample area to 1x3, 2x3, or 3x3 cm in either side-to-side or front-to-back orientations to accommodate leaves of varying shapes and sizes.

For leaves that do not fill the apertures, the opening features 1-mm graduations around the perimeter to simply leaf area estimation.
Compatible with the Small Light Source (6800-02) for controlled red and blue light.
Independent control of light intensities and spectral blends—both for steady state and programmatically controlled measurements.

spectra for the 6800-02 small light source — The small light source features peak emission at 453 nm for blue and 660 nm for red.

diagram showing different configurations of the 3x3 cm chamber and light source — Light options include ambient light one side, ambient light on both sides, one light source on one side, or light sources on both sides with the Light Source Extension Cable (9968-243).

Large leaf and needle measurements

LI-6800 with Large Leaf and Needle chamber and Large Light Source measuring conifer needles

The Large Leaf and Needle Chamber (6800-13) is used for gas exchange measurements from large leaves, sprigs, and shoots. It offers a 6x6-cm opening with 1-mm scale lines to assist with area estimation of leaves that do not fill the aperture.

The optional Conifer Sprig Kit (9968-271) expands the chamber volume to accommodate shoots and sprigs that do not fit readily into the chamber.
For measurements under controlled lighting, the large leaf and needle chamber is compatible with the Large Light Source (6800-03) in any configuration—with or without the conifer sprig kit.
The large light source offers fully configurable red, green, blue, and while illumination with intensities up to 2400, 1000, 2000, and 1500 μmol m^-1 s^-1 respectively.
The system provides independent control of light intensities and spectral blends—both for steady state and programmatically controlled measurements.

spectra for the 6800-03 large light source — The large light source provides peak emission at 453 nm for blue, 523 nm for green, 660 nm for red. White is nominally at a 4000 K color temperature.

diagram showing different configurations of the 6x6 cm chamber and light source — Light options include ambient light on top, ambient light on both sides, one light source on top, or light sources on both sides with the Light Source Extension Cable (9968-243). Dual light source operation is supported by the chamber alone and with the Conifer Sprig Kit (9968-271).

Aquatic photosynthesis measurements

Aquatic photosynthesis measurements based on direct CO₂ uptake are uncommon due to the complexity of the measurement. The Aquatic Chamber (6800-18) provides measurements of CO₂ exchange and chlorophyll a fluorescence from samples in liquid water, such as algal suspensions, or samples that require high ambient humidity, such as bryophytes.

With the Aquatic Chamber (6800-18), you can measure the photosynthetic response of a sample to controlled parameters such as light, CO₂, pH, or temperature. It includes the Aquatic Sample Adapter Kit (9968-338) that accommodates non-liquid samples that require humid air, such as coral, macro algae, sea grasses, and roots.

Features and Benefits

Achieves rapid equilibration of CO₂ in air and CO₂ in water in the sample chamber.
Humidity equilibrator minimizes the water vapor differential between sample and reference IRGAs to optimize computation of the CO₂ differential.
Condensation protection for the system—by measuring temperatures and humidity and adjusting setpoints to prevent condensation.
Compatible with external recirculating water bath for temperature control over an extended range.
Support for pH probes; pH measurements are recorded with the dataset.
Septum port allows ancillary sensors and probes to access the sample during a measurement.
With samples in water, you can compute optical throughput to estimate photosynthetic absorption.

chart of assimilation measurements with the LI-COR 6800-18 aquatic chamber — Assimilation measurements on Chlorella at ambient oxygen (21%) and low oxygen (0.5%) in response to light (Q). The CO₂ concentration entering the chamber was held constant at 400 μmol mol^-1 by the LI-6800. Chamber temperature was held constant at 25 °C using an external water bath. Cells were measured in a saltwater media at 17 ppm salinity.

For more details, read the recent publication by Hupp, et al.
Hupp, J., McCoy, J., Millgan, A., & Peers, G. (2021). Simultaneously measuring carbon uptake capacity and chlorophyll a fluorescence dynamics in algae. Algal Research, (58). https://doi.org/10.1016/j.algal.2021.102399.

Soil CO₂ flux

For survey measurements with the Soil CO₂ Flux Chamber (6800-09), the LI-6800 operates as a closed system to measure the rate of diffusion of CO₂ from the soil surface.

Why use the LI-6800 for soil measurements?

Patented technologies that are only available in LI-COR soil gas flux systems ensure that soil CO₂ flux measurements represent the true flux.
Patented pressure equilibrator vent to prevent wind from creating pressure excursion that affect measurements.
Excellent mixing inside the chamber to avoid dead volumes, ensuring an accurate measurement.
Bellows mechanism to avoid artifacts that may arise from chamber placement.
Soil moisture/soil temperature measurements recorded with the dataset to assist with computations and interpretation.
Data processing in SoilFluxPro™ Software for evaluation.

Controlled conditions

By controlling environmental conditions in the chamber for the duration of a measurement, the LI-6800 prevents multiple parameters from confounding the plant responses, ensuring repeatability and confidence in results.

CO₂ response curves

By controlling chamber conditions, the photosynthetic response to changes in intercellular CO₂ concentration is not confounded by other independent variables. The LI-6800 controls the leaf chamber environment at the setpoint with low variance (n=3; ± standard error of the means).

The flow rate is maintained precisely, decreasing moment-to-moment variation that impacts calculated carbon assimilation rates.

LI-6800 graph showing light-dependent biochemical reaction rates — Leaves are illuminated with constant irradiance, ensuring consistent light-dependent biochemical reaction rates.

LI-6800 graph showing CO2 assimilation — As the substrate for the dark reactions, photosynthetic responses to CO₂ concentration provide information about the biochemical limitations. In C3 species, these data are used to assess A_max (maximum photosynthesis rate), V_c,max (maximum rate of Rubisco carboxylation), J_max (maximum rate of electron transport for RuBP regeneration), and triose phosphate utilization (TPU) limitations.

Diurnal measurements

LI-6800 graph showing Diurnal carbon assimilation — The LI-6800 controls chamber environmental conditions to maintain ambient or growth conditions around the sample of diurnal or survey measurements. Diurnal carbon assimilation for field-grown bok choy (*Brassica rapa*) with a slight depression at mid-day (n = 24 to 28; ± standard error of the means).

LI-6800 graph showing leaf illumination — The 6800-02 light source consistently illuminates the leaf with intensities that mirror ambient light levels.

LI-6800 graph showing chlorophyll fluorescence — The Φ_PSII, measured from chlorophyll fluorescence, reflects the leaf illumination across the day, driving changes in electron transport.

Light responses in uniform illumination

The chamber light sources illuminate the leaf with uniform light. Variation is typically less than ± 10% over 90% of the aperture, ensuring that photosynthesis is even across the entire chamber aperture. The average photosynthetic carbon assimilation reflects the true spot-to-spot rate across the enclosed leaf area when the leaf is uniformly illuminated (n = 2; ± standard error from the means).

LI-6800 graph showing light intensity distribution

False-color map (left) indicating the light intensity distribution from an LI-6800 fluorometer. A transect (right) of light intensity across the light field at the Multiphase Flash^™ Fluorometer midline shows that illumination is highly uniform across the entire aperture.

LI-6800 histogram showing heat map of light uniformity distribution

Histogram of the heat map, indicating the light uniformity distribution is 10% over 90% of the area (counts).

LI-6800 graph showing Light response of Nicotiana tabacum

Light response of Nicotiana tabacum. Error bars are standard error of the means (n = 3).

Quantum efficiency

Leaf temperature is held steady across a range of light intensities ( Q_abs), from 0 to 100 µmol m^-2 s^-1, ensuring that temperature is not a confounding variable.

LI-6800 graph showing changing carbon assimilation rate

Precisely controlled leaf chamber CO₂ concentrations prevent alterations to the carbon assimilation rate that result from a changing CO₂ concentration.

LI-6800 graph showing constant leaf vapor pressure difference

Constant control of leaf vapor pressure difference (VPD) during light-driven changes in stomatal conductance, which removes the confounding effect of changing chamber H₂O concentration.

The flow rate is maintained precisely at the setpoint, which prevents flow from confounding the calculated carbon assimilation rate.

LI-6800 graph showing photosynthesis quantum efficiency — At low light intensities, the carbon assimilation rate is limited by light capture to drive linear electron transport in the light harvesting complex. Assessing the gross carbon assimilation response to absorbed light ( Q_abs) is a measure of the quantum efficiency of photosynthesis. The quantum efficiency for *Nicotiana tabacum* in non-photorespiratory conditions (0.5% O₂ air) is consistent with theoretical and empirically measured values (n = 2, ± standard error of the means).

Complementary Measurements with the LI-600

The LI-6800 Portable Photosynthesis System characterizes carbon assimilation and numerous other parameters under controlled chamber conditions. In contrast, the LI-600 Porometer/ Fluorometer records high-speed stomatal conductance measurements on many plants in ambient conditions. Combined, the two instruments record larger, more informative data sets in less time.

For example, use the LI-600 to screen a large population, then use the LI-6800 to measure select individuals in greater detail. Characterize stomatal ratio with the LI-600 and use these measurements in the LI-6800 to improve calculation of important parameters related to stomatal ratio.

LI-600	LI-6800
Rapidly screen up to 200 samples per hour to identify candidates for detailed measurements	Measure detailed physiological and photosynthetic parameters, including light response curves and A/Ci curves
Measurements in ambient conditions	Measurements in controlled conditions; capable of multiple independent controls, including light, CO₂, H₂O, and temperature
Easy to use, basic configuration options	Sophisticated configuration options in an intuitive graphical interface
Chlorophyll a fluorescence in ambient light	Chlorophyll a fluorescence with controlled light; capable of induction kinetics measurements

Learn more

To provide more information about complementary LI-6800 and LI-600 measurements, we’ve created an application note with case studies and recorded a webinar.

Theory—combined gas exchange and fluorescence