Light Response Curves

Starting from total darkness, in which there can be no photosynthesis, the first few photons to be absorbed by the leaf will be used with greatest efficiency. As light increases, the efficiency drops, and eventually subsequent increases in light yield little or no increase in photosynthesis. Thus, a light response curve can provide measures of dark respiration rate, the light compensation point (absorbed quantum flux for which photosynthesis and respiration are balanced), the quantum efficiency (initial slope), and the maximum photosynthetic rate. Shade adapted species tend to have lower dark respiration rates, lower compensation points, and lower maximum photosynthetic rates than sun adapted leaves. Quantum efficiency tends to be conservative, however.

Light Curve Strategies

Depending upon what you are trying to measure, there are a couple of approaches to light curves.


Since the photosynthetic apparatus responds almost immediately to light, especially drops in light, the quickest method is to start with a leaf equilibrated to high light, and decrease the light, spending perhaps 1 or 2 minutes at each light value, and dropping in steps of 200 μmol mol-1 or less. When you do this, you’ll find that the stomata have not had time to adjust, and tend to be more open at the low light values than they normally would. This manifests itself as a steadily rising Ci throughout the measurement. There’s nothing wrong with this, but be careful how you use the conductance measurements from a rapid light curve, because they are not equilibrated values.


Another approach is to do a slow curve, giving the stomata time to equilibrate at each light level. Going slowly, you can work from dark to light, or light to dark. (If you are using a red only light source, however, beware; the stomatal behavior will be artificial. Our comparisons of the red+blue LED source and sunlight show them to have the same influence for opening stomata, however.) If you wait 15 or 20 minutes at each light level, you will find that Ci will be fairly constant throughout the measurement, indicating that the stomata are fully adjusted. In fact, you could use Ci as an indicator of when to log the next record at all but the darkest light levels.


A third approach is to generate a light curve using multiple leaves that are equilibrated at a range of light values. Experiment #6 Sun and Shade Leaf Survey uses this approach. This has the advantage of being fairly quick, yet has equilibrated values. The potential for difficulty comes from using multiple leaves, thus bringing age differences and other factors into the response curve. The survey approach is better suited for some species than others. In deciduous trees, for example, leaf age is not particularly related to position in the canopy. With this approach you can achieve a range of light levels by selecting leaves that are tilted with respect to the sun, and in varying degrees of shade. The orientation of the sunlit leaves is a problem, however, unless you are using a light source when you clamp onto them. With a clear chamber top, leaves that are tilted with respect to the sun will be shaded by the chamber wall, and this is to be avoided at all costs. If, however, you use a light source, you can set the appropriate value first, or have it automatically track the ambient light as measured by an external PAR sensor.

Sunfleck / Shade Method

The fourth approach offered here is to separate each new light level with the starting light value, with time to equilibrate. That is, use a sequence such as: 1800, 1000, 1800, 500, 1800, 300, 1800 μmol m-2 s-1. (The starting point needn’t be high; you could work the other direction with shade leaves.) Data collected in this manner might be most appropriate for addressing questions of light dynamics in canopies.

Operational Considerations

Once you decide on the strategy you wish to take, you then need to decide on how the chamber controls should be set, and on how data is to be collected.


The best light source for light response curves is the red+blue 6400-02B or 6400-40 LCF. The red only 6400-02 source has the potential problem of allowing excessive (that is, more than normal) stomatal closure as light decreases, or delaying stomatal opening as light increases.

Without the LED light source, a light curve cannot be automated, but is still possible. Neutral density filters, for example, can provide means to reduce sunlight or other sources by known amounts. The survey technique discussed above could be done without a light source.


It is important to maintain the chamber CO2 concentrations as constant as possible while measuring a light response curve. Otherwise, the effects of CO2 on photosynthesis will be confounded with the effects of light. If you have a CO2 mixer, this is simple to do: set it to control on sample CO2 concentration.


Ideally, the response curve should be measured at a constant leaf temperature.

Humidity Control

Operate the flow control for constant water mole fraction. If you go from light to dark, expect conductances and transpiration rates to fall, so leave room for the flow to fall as well (or rise, if you are going from dark to light).


Since the concentrations in the IRGAs aren’t going to be changing much during a light curve, there’s no real reason to match after every measurement. Match once before starting. If you are doing a slow curve, however, matching won’t hurt anything, since you’ll have time to burn.

With OPEN version 3.2 and above, you are asked for a matching threshold (the absolute value of the ΔCO2 value). Thus, you don’t have to decide whether to match before each observation or not; it will match on the ones with ΔCO2 smaller than your threshold, and skip the rest.


There are at least two possibilities here: “LightCurve” (described on page 9-35 in the instruction manual) and “TimedLamp” (described on page 9-35). “LightCurve” lets you specify the sequence of light values you want. A minimum and maximum wait time is specified. (Logging can’t occur before the minimum time expires; after that, a record is logged when stability is achieved. “TimedLamp” also lets you specify a sequence of light values, but at each one, you specify a) how long to maintain that light level, and b) how often to log data within that period. This program is good for recording events throughout the experiment, letting you record how the leaf responded with time, as well as with light.

Rapid Light Curve, Step-By-Step

Here’s how to make an automatic light response curve. It uses “LightCurve”, and does a rapid response curve.

  1. Prepare the chamber
  2. Light: Typically 1500 μmol s-1 for C3 plants, or 2000 for C4 plants.
  3. CO2: Constant reference CO2, about 400 μmol mol-1, or your choice. (This is temporary - we’ll switch to constant sample in a few minutes.)
  4. Flow: Constant flow, 500 μmol s-1.
  5. Clamp onto the leaf
  6. Set the temperature
  7. Set the temperature control for constant leaf temperature.
  8. Set the chamber humidity
  9. After the chamber has been clamped onto the leaf for a few minutes, note the H2OS_mml value, then change the flow control to constant mole fraction control, and target that value.
  10. Set the chamber CO2
  11. Control constant sample CO2, targeting the desired value.
  12. Set log options, and open a log file
  13. Make sure you’ve got the computations, prompts, log list, etc. that you need.
  14. Area and Stomatal Ratio
  15. Are they correct?
  16. Match the IRGAs
  17. Be sure CO2S_μml is stable before you do this.
  18. Launch the “LightCurve” Autoprogram
  19. Press 5 then f1. Pick “Light Curve” from the list.
  20. When asked “Append to current data file?” Press Y.
  21. When asked “Desired lamp settings (µmol/m2/s)”, edit the list as needed, and press enter.
  22. When asked “Minimum wait time”, enter the desired value. 120 seconds is usually adequate. This is the time after each light level change that the system will wait before checking stability to see if it can log.
  23. When asked “Maximum wait time”, enter the desired time, in seconds. After the minimum time, it will check stability up to this time to see if it can log. Enter 200. That gives it 80 seconds after the initial 120 second delay for photosynthesis to stabilize.
  24. When asked “Match if |ΔCO2| less than ppm”, enter 15.
  25. When asked “Stability Definition OK (Y/N)”, press Y to keep it, or N to change it.
  26. Trigger the first point
  27. If the first point is the current value, there’s not much point in waiting. Press escape, then T to log it, and start the next one.
  28. Watch the curve develop
  29. Press 4 then f3, and watch the curve develop.
  30. When it’s done...
  31. Once the curve is done, you may want to set the light high again by hand, to let the leaf recover. Or just take the leaf out of the chamber if you are done with it.
  32. After the fact analysis
  33. Before you close the data file, you may want to access GraphIt (press View File (f2 level 1) in New Measurements mode). If the axes are not defined for a light curve, press QuikPik Config (f1) and select “Light Curve”. Press REPLOT GRAPH (f2) and draw it.
  34. Answer these questions by plotting the appropriate data. Did sample cell CO2 stay constant? Did the sample cell humidity stay constant? How did stomatal conductance behave as a function of light? What does a graph of photosynthesis vs. conductance look like?
  35. Exit GraphIt, and close the file
  36. Press escape until you get back to New Measurements mode, then press CLOSE_FILE (f3 level 1) to close the file.