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The protocol introduces a high-throughput method for measuring the relaxation of non-photochemical quenching by pulse amplitude modulated chlorophyll fluorometry. The method is applied to field-grown Glycine max and can be adapted to other species to screen for genetic diversity or breeding populations.
Photosynthesis is not optimized in modern crop varieties, and therefore provides an opportunity for improvement. Speeding up the relaxation of non-photochemical quenching (NPQ) has proven to be an effective strategy to increase photosynthetic performance. However, the potential to breed for improved NPQ and a complete understanding of the genetic basis of NPQ relaxation is lacking due to limitations of oversampling and data collection from field-grown crop plants. Building on previous reports, we present a high-throughput assay for analysis of NPQ relaxation rates in Glycine max (soybean) using pulse amplitude modulated (PAM) chlorophyll fluorometry. Leaf disks are sampled from field-grown soybeans before transportation to a laboratory where NPQ relaxation is measured in a closed PAM-fluorometer. NPQ relaxation parameters are calculated by fitting a bi-exponential function to the measured NPQ values following a transition from high to low light. Using this method, it is possible to test hundreds of genotypes within a day. The procedure has the potential to screen mutant and diversity panels for variation in NPQ relaxation, and can therefore be applied to both fundamental and applied research questions.
Photosynthesis consists of light absorption, primary electron transfer, energy stabilization, and the synthesis and transport of photosynthetic products1. Understanding each step is vital to guide efforts to increase crop photosynthetic efficiency. Light affects the rate of photosynthesis, requiring balancing energy supply, in the form of photons, with demand for reducing equivalents. When supply exceeds demand, for example under high-light or during reduced CO2 fixation caused by stomatal closure, build-up of reducing power increases the probability of reactive oxygen species formation with the potential to damage the photosynthetic apparatus and impair electron transport. Therefore, to prevent damage, plants have developed several photo-protective mechanisms, including detoxification of reactive oxygen species and non-photochemical quenching of the excited chlorophyll states (NPQ)2.
Maintaining high rates of photosynthesis is challenging under a field environment. Seasonal and diurnal changes, along with environmental fluctuations such as wind-induced leaf movements and transient cloud cover, cause shifts in the amount and intensity of light received by plants for photosynthesis3. NPQ dissipates excess light energy and can help prevent photo-damage while allowing for sustained rates of photosynthesis at high-light4. However, prolonged NPQ during high- to low-light transitions continues to dissipate energy that could be used for carbon reduction5. As a result, speeding up the relaxation of NPQ can increase the efficiency of photosynthesis6, making NPQ relaxation an attractive target for crop improvement.
Pulse amplitude modulated chlorophyll fluorescence (PAM) analysis can be used to calculate NPQ from measurable parameters (Supplementary Table 1 and Supplementary Table 2)7,8,9. This article focuses on determining NPQ relaxation rates in field-grown plants for the purpose of screening natural variation in germplasm. However, PAM chlorophyll fluorometry analysis can also be used for a wide variety of purposes, applied to species ranging from algae to higher plants, and is reviewed elsewhere7,8,9.
In a dark-adapted leaf or cell, photosystem II (PSII) reaction centers are open to receive electrons and there is no NPQ. Switching on a low-intensity measuring light elicits chlorophyll fluorescence while avoiding electron transport through PSII. The recorded minimum fluorescence in this dark-adapted state is described by the parameter Fo. Applying a high-intensity light pulse to a dark-adapted leaf can rapidly reduce the first stable electron acceptor pool of quinones bound to the quinone A site. This temporarily blocks electron transfer capacity in PSII reaction centers, which are then said to be closed and unable to receive electrons from water-splitting. By using a short pulse duration, there is insufficient time to stimulate NPQ. The resulting chlorophyll fluorescence is equivalent to the maximum value obtainable in the absence of NPQ, or maximum fluorescence, Fm. The difference between minimal and maximal fluorescence is referred to as variable fluorescence, Fv. The maximum photochemical quantum yield of photosystem II (Fv/Fm) is calculated from these two parameters using the following equation:
Fv/Fm = (Fm-Fo)/Fm
This can provide an important indicator of photosystem function and stress. Turning on an actinic (photosynthetic) light stimulates non-photochemical quenching, and subsequent application of a saturating flash allows for the measurement of light-adapted maximal fluorescence, Fm'. By comparing the difference between dark and light-adapted maximum fluorescence, NPQ can be calculated according to the Stern-Volmer equation10:
NPQ = Fm/Fm' - 1
In higher plants, NPQ has been described as consisting of at least five distinct components, including qE, qT, qZ, qI and qH. The precise mechanisms involved in NPQ are not fully understood; however, qE is considered to be the major component of NPQ in most plants. Crucial factors for full engagement of qE have been found to include the build-up of a proton gradient across the thylakoid membrane, the activity of photosystem II subunit S11,12, and de-epoxidated xanthophylls, antheraxanthin, lutein, and in particular zeaxanthin13. qE relaxes the fastest of any NPQ component (< 2 min)14, and reversible activation of qE is therefore particularly important for adaptation to shifting light intensities. A second slower phase of NPQ relaxation (~2-30 min) encompasses both qT, related to state transitions, and qZ, involving interconversion of zeaxanthin to violaxanthin15. Slow relaxing (> 30 min) of NPQ may include both photoinhibitory quenching (qI)16 and processes independent of photodamage17,18, such as qH, which is sustained quenching in the peripheral antennae of PSII mediated by a plastid lipocalin protein19,20.
NPQ increases during exposure to high light. Subsequent transfer to low light can result in downregulation of NPQ. The decay of fast, intermediate, and slow relaxing phases can be captured in the parameters of a bi-exponential function15,21,22,23
NPQ = Aq1(-t/τ1) + Aq2(-t/τ2) + Aq3
The theoretical basis for the bi-exponential function is based on the assumption of first-order utilization of hypothetical quenchers, including qE (Aq1), the combined relaxation of qZ and qT (Aq2), with the corresponding time-constants τq1 and τq2, and long-term NPQ, which includes qI and photodamage independent processes (Aq3). As such, the bi-exponential function provides a more realistic representation of the multiple connected biological processes involved in quenching chlorophyll fluorescence compared to a simpler Hill equation which lacks a theoretical basis24.
NPQ can be measured using a variety of commercially available PAM fluorometers25,26, from simple hand-held devices27 to more advanced closed systems28. However, a limitation of several of these approaches is a relatively low throughput, which makes screening large collections of plants challenging without multiple devices and a team of researchers. To address this issue, McAusland et al. developed a procedure based on excised leaf tissue and used it to identify differences in chlorophyll fluorescence between two wheat cultivars29. The attraction of this approach is that imaging leaf disks, taken from multiple plants with a single device, can facilitate screening hundreds of genotypes within a day. This makes it possible to assess variation in NPQ relaxation as part of genome wide association studies, or for screening breeding populations with the potential to increase crop photosynthetic efficiency and ultimately yield.
Building on the findings of McAusland et al.29, we use PAM chlorophyll fluorescence analysis of leaf disks for high-throughput screening of NPQ relaxation rates in Glycine max (G. max; soybean). This protocol uses the CF Imager25, which is comparable to other commercially available closed-PAM systems, such as the popular FluorCam26. With a dark room for adaptation of samples, users can image 96-well plates, Petri dishes, and small plants. The key advantage of this approach is the increase in throughput afforded by using leaf disks compared to sequential analysis of individual plants. Herein we present representative results, and a method for sampling, measuring, and analysis of NPQ in field-grown plants.
1. Seed planting
2. Collecting leaf samples from the field
3. Preparing samples for analysis
4. Measuring of non-photochemical quenching using chlorophyll fluorescence imager
5. Processing chlorophyll fluorescence data
Figure 1A depicts a typical measurement of NPQ in field-grown soybean. Plants were grown in Urbana, IL (latitude 40.084604°, longitude -88.227952°) during summer 2021, with seeds planted on June 5th. 2021. The leaf discs were sampled after 30 days of planting seeds, and measurements were made with the protocol provided (Table 1). Fv/Fm and NPQ values were calculated for each leaf disk (Supplementary Table 4) and NPQ relaxation par...
Careful choice and handling of leaf disks are critical to obtain reliable measurements of NPQ. First, damage to the tissue, such as rough handling with tweezers, will introduce stress, resulting in low values for the maximum quantum efficiency of photosynthesis. Non-stressed plants typically have Fv/Fm values of around 0.8318, with significant declines indicating a reduction in photosynthetic performance9. However, plants grown under ...
The authors report no conflicts of interest
This work is supported by the research project Realizing Increased Photosynthetic Efficiency (RIPE) that is funded by the Bill & Melinda Gates Foundation, Foundation for Food and Agriculture Research, and the U.K. Foreign, Commonwealth & Development Office under grant number OPP1172157.
Name | Company | Catalog Number | Comments |
24 well tissue culture plate | Fisher Scientific | FB012929 | Country of Origin: United States of America |
96 well tissue culture plate | Fisher Scientific | FB012931 | Country of Origin: United States of America |
Aluminum foil | Antylia Scientific | 61018-56 | Country of Origin: United States of America |
Black marker pen | Sharpie | SAN30001 | Country of Origin: United States of America |
CF imager | Technologica Ltd. | N/A | chlorophyll fluorescence imager Country of Origin: United Kingdom |
Cork-borer, 7mm | Humboldt Mfg Co | H9665 | Country of Origin: United States of America |
FluorImager V2.305 Software | Technologica Ltd. | N/A | imaging software Country of Origin: United Kingdom |
iHank-Nose 100-Pack of Premium Nasal Aspirator Hygiene Filters | Amazon | B07P6XCTGV | Country of Origin: United States of America |
Marker stakes | John Henry Company | KN0151 | Country of Origin: United States of America |
Paper scissors | VWR | 82027-596 | Country of Origin: United States of America |
Parafilm | Bemis Company Inc. | S3-594-6 | Semi -transparent flexible film Country of Origin: United States of America |
Solid rubber stoppers | Fisher Scientific | 14-130M | Country of Origin: United States of America |
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