Measurement and Measurement Error of Light Used for Photosynthesis & Plant Growth Richard Garcia April 20, 2010

Transcription

1 TRANSCRIPT SLIDE 1 [00:01] Thanks Ashlee, good afternoon from LI-COR Biosciences here in Lincoln, Nebraska. Thanks for joining us. Probably the most important process on our planet, is Photosynthesis and the so called light reactions of Photosynthesis, light energy from the sun, is captured by plant foliage and used to reduce ADP SLIDE 2 [00:21]: and NADP to ATP and NADPH this store of biochemical energy, can be used to fix carbon from atmospheric CO 2 into sugar compounds in The Calvin Cycle or the Dark Reactions. SLIDE 3 [00:40]: Although the sun provides energy across a broad range of wavelengths, plants can only use energy in a much narrower range of wavelengths. Today, we typically think of plants absorbing light in visible wave bands between about nm, represented here by the yellow bar. This wavelength range represents about 50% of the total energy we receive from the Sun. SLIDE 4 [1:09]: Research and technical advances in the 1960 s and early 1970 s, allowed Scientists to refine the definition of light used for plant growth. Keith McCree, published a series of classic articles which he outlined the need for a definition of photosynthetically active radiation which would be accepted across the scientific community. SLIDE 5 [1:35]: McCree studied the leaf spectral properties of 22 different plant species grown under a range of conditions. In his study he specifically evaluated: A, in the top figure, the light Absorptance spectrum determined by measuring the total incident light on the leaf, the reflected light from the leaf surface, Page 1 of 5

2 and the transmitted light through the leaf. And then, by difference, he calculated the light absorbed by the leaf. This procedure requires a Spectroradiometer and an integrating sphere. B, the Action Spectra, was determined through normalizing the photosynthetic rate, by the light energy incident on the leaf. And finally C, the Spectra of Quantum Yield, was evaluated by normalizing the photosynthetic rate to the quanta of light absorbed across the light Absorptance spectra. SLIDE 6 [2:32]: McCree showed that there is some variation in Spectra properties of leaves, due to growth history and species. As you can see in this figure, which shows the variation in the absorption spectra of maze leaves, with varying degrees of chlorophyll density. McCree s conclusion was that, we cannot define a single sensor with a perfect spectral response. He went on to reiterate the importance of establishing a well defined measuring stick that the scientific community would accept. SLIDE 7 [3:11]: After evaluating all his data, McCree concluded that the best definition of photosynthetically ctive radiation or PAR, is a flat response between 400 and 700 nm. In this figure, we have overlaid this proposed definition over a quantum yield spectra of a typical plant. The scientific community has accepted this definition, and it now represents what would be the ideal sensor response. Besides the term PAR Photosynthetically active radiation, other terms that scientist use to describe light used for photosynthesis and plant growth, include PPFD or Photosynthetic Photon Flux Density, and PFD, Photon Flux Density. SLIDE 8 [4:04]: Now that we have a definition of what we want to measure, let s talk about the measurement. Light is typically measure using passive solid state electronic devices, such as photodiodes. There are hundreds of these devises from various manufacturers, all with different spectral properties. None of these devises match the ideal sensor response. But with judicious selection of sensor and the use of optical filters, we can come close to the ideal sensor response. SLIDE 9 [4:38]: Page 2 of 5

3 In this slide, we chosen to compare three commercially available sensors that have been used to measure PAR. The response of the sensor in each case was compared to the ideal response, represented by the rectangle in each figure. The sensors we have chosen to compare are, in figure A; the Gallium Arsenide Phosphide sensor, which is an unfiltered, smaller, more economical sensor. B; a Kipp & Zonen model PAR Lite, which is a filtered photodiode. And C; the LI-COR model LI-190 filtered photodiode. In these figures, any area colored in blue represents overestimation by the sensor within the defined 400 to 700 nm range. Any area colored with yellow, represents underestimation by the sensor within the defined 400 to 700 nm range. And finally, any area colored with red, represents overestimation by the sensor, due to response outside the defined 400 to 700 nm range. SLIDE 10 [5:52]: The standard LI-COR calibration process for each sensor manufactured includes analysis of the specter response of the sensor, to a tungsten halogen reference lamp by output comparison to a reference sensor, with a known spectral response. The procedure by definition results in a total sensor error of zero, where the overestimates and underestimates cancel each other out, as you can see in the table in the bottom left hand corner of this figure. For example in figure A, the Gallium Arsenide Phosphide sensor, has substantial errors, but the underestimation at the low ends and high ends of the PAR region, have been balanced out in the calibration by the overestimates at the mid-range PAR response. Once a sensor s been calibrated with a specific light source, if we change to a different light source with different spectral characteristics, the sensor error will change depending on how the sensitivity of the sensor lines up with the spectral properties of the light source. In order to illustrate these types of errors, we first need to look at typical spectra for light sensors. Moderator: Rick, may I interrupt you with a question? The question I have is, I see that some manufactures offer calibration that are tailored for a specific light source, how does that work? Page 3 of 5

4 Well, in that situation, in the calibration process the manufacturer would use a reference lamp, with spectral properties similar to the properties of the light source the sensor s being tailored for. The problem with this approach is, that the sensor is likely to have greater errors when it s used under other lighting conditions. Let s move on. SLIDE 11 [7:45]: In this slide, we ve plotted the spectral properties of seven different lighting situations. The three most extreme examples in these plots, or plot A, where we have open sky conditions. Plot D, where red LED with a peak output of 680 nm, and plot G, where we have daylight under a fully developed soybean canopy. In the figure at the top of this slide, we ve superimposed the light source spectra for daylight under a soybean canopy, SLIDE 12 [8:20]: over the Kipp & Zonen sensor response, to illustrate how sensor errors change depending on the spectra of the light being measured. In this particular case, where most of the incident light is between 650 and 750 nm this sensor substantially overestimates PAR, because of the sensors high sensitivity outside the PAR waveband, there which is greater than 60% is computed as the integration or summation of the products in the equation, that s above the table. SLIDE 13 [8::59]: In another illustration, we have overlaid the light source spectra for a red LED with peak output at 680 nm over the sensor response spectra of the Gallium Arsenide sensor, although most of the output from the LED falls within the defined PAR region, a significant portion of the LED s output is outside the Gallium Arsenide sensor s range of detection and therefore the sensor underestimates the PAR from this source by more than 84%. SLIDE 14 [9:32]: Page 4 of 5

5 In this table, we want to examine the measurement errors of the LI-COR LI-190 sensor across the full range of lighting conditions. While the response of the LI-COR sensor is not a perfect reflection of the peak PAR definition, it does follow it very closely. And, it is the only sensor which gives reasonable accuracy with errors less than 5%, regardless of the source of light. SLIDE 15 [10:01]: In this presentation, we set out to compare sensors to the ideal measuring stick response. We also have a Technical Note, technical note #126 which you can download from our website. Thank you. SLIDE 16 [10:17]: Moderator: Thanks Rick, I have just one question for you, and that is; I know that LI-COR uses Gallium Arsenide Sensors for some of the LI-6400 s Portable Photosynthesis System Chambers, why do you use them if there measurements are not as actuate? That s an excellent question Ashlee. In Photosynthesis measurements, our objective is to quantify the light as close as possible to the leaf surface; the problem is that the packaging for our standard quantum sensor, which has several filters in it, requires a much larger package. To get the quality of sensor we want, and it s not practical to get that sensor very close to the plane of the leaf in some measurement conditions, so we ve chosen to offer the Gallium Arsenide sensor, which is a much smaller sensor and can be located right at the plane of the leaf, so that it s seeing the same light that the leaf is seeing. Now, in the LI-6400 System we also offer an option to include a standard quantum sensor, so the scientist can get both a very accurate measurement of the light that s available with the standard quantum sensor the LI-190, but at the same time can measure light with the Gallium Arsenide Sensor, right at the plane of the leaf, very closely with the very small sensor that we have in that package. Page 5 of 5