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Defining the Value: Photochemistry Parameters Measured by the LI-6800

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Defining the Value: Photochemistry Parameters Measured by the LI-6800 1 Defining the Value: Photochemistry Parameters Measured by the LI-6800 We often think of photosynthesis as representing a single phenomenon; the conversion of light energy to biochemical energy. The underlying processes that represent photosynthesis, however — collectively termed photochemistry — are considerably more complex. No single metric exists that directly represents photosynthesis; rather, the LI-6800 Portable Photosynthesis System measures specific processes and states of the components of photochemistry. These individual parameters are indicators of plant health and/or stress, and are important for biomass, yield, water use efficiency, or other studies where it is important to estimate photosynthetic performance as a measure of the response of plants to their environment. Here we provide a brief, introductory overview of some of the more commonly measured and/or physiologically relevant parameters that are provided either directly by LI-6800, or that can be derived from its outputs. References Net assimilation rate: Anet Farquhar GD, von Caemmerer S, Berry JA (1980) A The balance between carbon uptake by biochemical model of photosynthetic CO₂ assimilation in carboxylation and carbon loss due to leaves of C3 species. Planta 149: 78-90. photorespiration and other respiratory processes. Carbon assimilation to support von Caemmerer S, Farquhar GD (1981) Some relationships plant growth and metabolism. between the biochemistry of photosynthesis and the gas exchange of leaves. Planta 153: 376-387. Reported as the rate of CO₂ uptake per unit time von Caemmerer S, ed (2000) Biochemical Models of Leaf per unit leaf area (µmol of CO₂ m-2 s-1) and is a Photosynthesis. CSIRO Publishing, Collingwood, VIC, Australia. real-time output of the LI-6800. Anet is commonly measured with conditions inside the measurement chamber set to mimic the growth environment of the plant. 2 Reference Light saturated assimilation rate: Asat Atkin OK, Westbeek MHM, Cambridge ML, Lambers H, Net assimilation when light is not Pons TL (1997) Leaf respiration in the light and darkness: limiting photochemistry; net assimilation A comparison of slow- and fast-growing Poa species. Plant at a saturating light intensity. The Physiology 113: 961-965. light intensity at which A is reached sat See also citations under A represents the maximum usable light net intensity by the leaf. Photorespiration: R Reported as the rate of CO₂ uptake per unit time PR per unit leaf area (µmol of CO₂ m-2 s-1) and can The result of the oxygenase activity of either come from a point measurement of A net Rubisco. Where Rubisco oxygenates made at saturating light intensity or be derived rather than carboxylates. Photorespiration from a light response (AQ) curve. is a competing process to photosynthetic See citations under Anet carbon assimilation and ultimately leads to the release of previously fixed carbon. Maximum assimilation rate: A Reported as a rate of CO₂ release per unit time max per unit leaf area (µmol of CO₂ m-2 s-1). Multiple methods exist to estimate R , including CO Net assimilation when neither light nor PR 2 response (A-C ) curves coupled with chlorophyll CO₂ are limiting photochemistry; net i fluorescence and assimilation measured under a assimilation at a saturating light intensity low O₂ concentration. and a saturating CO₂ concentration. Amax represents the maximum capacity of the References leaf for carbon assimilation. Björkman O (1966) The effect of oxygen concentration on photosynthesis in higher plants. Physiologia Plantarum 19: Reported as the rate of CO₂ uptake per unit time 618-633. per unit leaf area (µmol of CO₂ m-2 s-1) and can either come from a point measurement Anet made Busch FA (2012) Current methods for estimating the rate of at saturating conditions or be derived from a CO₂ photorespiration in leaves. Plant Biology 15 (4): 648-655. response (A-Ci) curve. See citations under Anet Stomatal conductance: gs The capacity for gas transport through Respiration rate: RX the stomata. The inverse of the diffusive resistance imposed by the stomata to The rate of carbon loss from the leaf due water leaving or CO₂ entering the leaf. to mitochondrial respiration. Physiologically it provides a relative measure of how open or closed the Reported as the rate of CO₂ release per unit stomata are. time per unit leaf area (µmol of CO₂ m-2 s-1). In C3 plants, several methods exist to estimate Reported as a transport rate per unit time per unit mitochondrial respiration: Respiration in the light, -2 -1 -2 leaf area (moles of H₂O m s or moles of CO2 m R , be estimated from nesting CO₂ response -1 d s ). Stomatal conductance to water vapor (gsw) measurements within light response (A-Ci nested is the more widely cited conductance parameter within AQ) curves. Respiration in the dark, Rn, can reported by the LI-6800 and can be a sensitive be estimated from a measure of Anet in the dark or indicator of stress. It is a real-time output of from the y-intercept of a light response (AQ) curve. the LI-6800. 3 References Long SP, Bernacchi CJ (2003) Gas exchange measurements, what can they tell us about the underlying limitations to Farquhar GD, Sharkey TD (1982) Stomatal conductance and photosynthesis? Procedures and sources of error. Journal of photosynthesis. Annual Review of Plant Physiology 33: 317- Experimental Botany 54 (392): 2393-2401. 345. Flexas J, Medrano H (2002) Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal Maximum velocity of carboxylation: Vc max limitations revisited. Annals of Botany 89: 183-189. The first of the three rate-limiting processes used to describe the response Intrinsic water use efficiency: WUE g of assimilation to CO₂ concentration (ACi curve). Vc max represents the maximum The ratio of net carbon assimilation (Anet ) velocity of carbon fixation by Rubisco. to stomatal conductance to water vapor (gsw ); a metric that describes the potential Derived from fitting a function to the initial portion water cost of carbon assimilation. of the A-Ci curve where the availability of CO₂ limits the rate of carboxylation. Intrinsic water use efficiency measured by gas exchange follows from the same physical process References as water use efficiency derived from the stable Dubois JJB, Fiscus EL, Booker FL, Flowers MD, Reid carbon isotope ratio of plant tissue, only integrated CD (2007) Optimizing the statistical estimation of the over a much shorter time scale. It is related to parameters of the Farquhar-von Caemmerer-Berry model of instantaneous water use efficiency (WUEi) by photosynthesis. New Phytologist 176: 402-414. evaporative demand. WUEg can be calculated Duursma RA (2015) Plantecophys - An R package for from Anet and gsw, but is not calculated by default by the LI-6800. analyzing and modelling leaf gas exchange data. PLOS One 10. Reference Sharkey TD, Bernacchi CJ, Farquhar GD, Singsaas EL (2007) Seibt U, Rajabi A, Griffiths H, Berry JA (2008) Carbon Fitting photosynthetic carbon dioxide response curves for C3 isotopes and water use efficiency: sense and sensitivity. leaves. Plant, Cell and Environment 30: 1035-1040. Oecologia 155 (3): 441-54. Stinziano JR, Morgan PB, Lynch DJ, Saathoff AJ, McDermitt DK, Hanson DT (2017) The rapid A-Ci response: Stomatal limitation: l photosynthesis in the phenomic era. Plant, Cell and Environment, doi: 10.1111/pce.12911. The inherent limitation to photosynthesis Walker AP, Beckerman AP, Gu L, Kattge J, Cernusak LA, imposed by the diffusive resistance of the Domingues TF, Scales JC, Wohlfahrt G, Wullschleger SD, stomata. The proportion of assimilation at Woodward FI (2014) The relationship of leaf photosynthetic growth conditions relative to that when traits – Vcmax and Jmax – to leaf nitrogen, leaf phosphorus, and the leaf’s internal CO₂ concentration is specific leaf area: a meta-analysis and modeling study. Ecology equal to that of growth conditions. and Evolution 4: 3218-3235. See also citations under A Stomatal limitation is typically derived from a CO₂ net response (A-Ci) curve. References Jones HG (1985) Partitioning stomatal and non-stomatal limitations to photosynthesis. Plant, Cell and Environment 8: 95-104. 4 Maximum electron transport rate: Jmax Non-photochemical quenching: NPQ The second of the three rate-limiting The efficiency of energy dissipation processes used to describe the response from Photosystem II (PSII) through of assimilation to CO₂ concentration non-photochemical means. The current (A-Ci curve). Jmax represents the maximum efficiency of heat dissipation relative to the rate of electron transport through the electron leaf in a dark-adapted state. transport chain (µmol of electrons m-2 s-1). NPQ can be thought of as representing excess light energy beyond what the leaf can use for Derived from fitting the portion of the A-Ci curve where the production of ATP and NADPH by the photochemistry. An increase in NPQ in the light reactions limits the rate of carboxylation. absence of an increase in light intensity therefore represents a decrease in photochemistry and/or its See citations under V and A c max net efficiency. It is reported as a dimensionless ratio and is derived from measurements of chlorophyll fluorescence on leaves in a dark adapted and light Velocity of triose phosphate utilization: VTPU adapted state. Where maximum fluorescence in the dark is known (via a prior measurement) NPQ The last of the three rate-limiting is a direct output from the LI-6800. processes used to describe the response See citations under F /F of assimilation to CO₂ concentration v m (A-Ci curve). Triose phosphate utilization is an assessment of the rate of export of photosynthetic carbon assimilation Maximum quantum efficiency:v F /Fm products (triose phosphates), their utilization in sucrose synthesis, and the The maximum, or intrinsic, efficiency return of the phosphate to the chloroplast. for the capture of light energy by chlorophyll at Photosystem II (PSII) Reported as a rate (µmol of triose phosphate m-2 and its use in photochemistry.
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