Light response curves

Why measure light response? Starting from total darkness, in which there is no photosynthesis, the first few photons to be absorbed by the leaf will be used with the 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 indications of the:

• Dark respiration rate: The photosynthesis rate with no light,
• Light compensation point: Absorbed quantum flux for which photosynthesis and respiration are balanced
• Quantum efficiency: Initial slope of the assimilation rate, and
• Light saturated photosynthetic rate (A_sat).

Craine and Reich (2005)1 Craine, J.M. and Reich, P.B. (2005), Leaf‐level light compensation points in shade‐tolerant woody seedlings. New Phytologist, 166: 710-713. https://doi.org/10.1111/j.1469-8137.2005.01420.x observe that shade-tolerant woody seedlings tend to have lower dark respiration per unit of leaf area (R_d,area) than plants adapted to full sun. They observed lower light-compensation points in shade-tolerant species compared with intermediate and shade-intolerant species were associated with lower Rd and area in 60+ broadleaf and evergreen tree species.

Light curve strategies

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

Rapid

Since the photosynthetic apparatus responds almost immediately to light, especially drops in light intensity, 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 m^‑2 s^‑1 or less. When you do this, you'll find that the stomata have not had time to adjust, and tend to remain more open at low light values than they normally would. This manifests itself as a steadily rising C_i 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.

Slow

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 wait 15 or 20 minutes at each level, you will find that C_i will be fairly constant throughout the measurement, indicating that the stomata are fully adjusted. In fact, you could use C_i as an indicator of when to log the next record at all but the darkest levels.

Survey

A third approach is to generate a light curve using multiple leaves that are equilibrated at a range of light values. 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 the external PAR sensor.

Note: Be careful when using Qabs with a clear top and software v1.4.05 or older. The alpha_ambient coefficient here is used for the energy balance. If you want to use Qabs, make sure you are using a new version of the software or multiply your Qamb_in by ~0.8.

Sunfleck and 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: 100, 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 appropriate for addressing questions of light dynamics in canopies.

Operational considerations

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

Light

The best light sources for light response curves are the 3×3 cm light source or the fluorometer. In the absence of a light source, a light curve cannot be automated, but is still possible to make one. Neutral density filters, for example, can provide means to reduce sunlight or other sources by known amounts. Full sunlight is about 2,000 µmol m^‑2 s^‑1 but there are fast growing plants that might require an even higher light level to saturate. Some plants grown in low light conditions might be fine with a maximum light of 1,000 to 1,500 µmol m^‑2 s^‑1. It may take up to 30 minutes before a plant previously acclimated to 1,500 or 2,000 µmol m^‑2 s^‑1 has adapted. The survey technique discussed earlier also can be done without a light source.

Temperature

Ideally, the response curve should be measured at a constant temperature, so operate the coolers to maintain constant leaf temperature.

CO₂

Control on CO2_s, which is set to ambient CO₂. What ambient is depends on your environment. Perhaps you work in a greenhouse with elevated CO₂. To find ambient CO₂, make sure flow is turned on then turn the CO₂ and H₂O controls off and step away from the console so you aren’t exhaling near the instrument’s air inlet. Look at CO2_r to see approximate ambient CO₂. Enter that value as the setpoint.

H₂O

H2O_r works for the survey style of the light response curve but for rapid or slow curves, control on a fixed RH or VPD.

Matching

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 and set Only match if abs(CO2_r-CO2_s) < 5.

Programs

Programs enable you to record the light curve using predefined settings.

An automatic response curve using the Light_Response program

Here's how to make an automatic light response curve. It uses a program and does a rapid response curve.

1. Set the Environment controls.
2. • Flow, set Flow: On; Pump Speed: Auto; Flow Setpoint: 500 µmol s^-1; and Press. Valve: 0.1 kPa
   • H₂O, set H₂O: On; and RH_air: 50 to 75%
   • CO₂, set CO₂ injector: On; Soda Lime: Scrub Auto; Tap CO2_s and enter a setpoint near ambient. To find ambient CO₂, make sure flow is turned on then turn the CO₂ and H₂O controls off and step away from the console so you aren’t exhaling near the instrument’s air inlet. Look at CO2_r to see approximate ambient CO₂. Enter that value as the setpoint.
   • Fan: Set Mixing fan: On; Fan Speed: 10,000 rpm
   • Temperatures: On; Tleaf: 27.0 °C or something close to ambient
   • HeadLS or Fluor: Use 1,800 µmol m^‑2 s^-1, or a light-saturating level for your leaf
2. Set leaf area and stomatal ratio under Constants > Gas Exchange.
3. Configure other gas exchange settings if needed.
3. Open a log file.
4. Make sure you have the Match Options and Logging Options configured.
4. Clamp onto the leaf.
5. Under Programs, select Light_Response.
6. Configure the settings or just go with the default settings.
6. Tap Start.
7. Watch the measurement as it progresses under the Measurements tab, graph H or G. Data are stored in the log file. You can review logged data under Tools > View Log Files. Select the file, then tap Load.