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'''Non-photochemical quenching''' (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity.<ref name=Horton>{{cite journal | doi = 10.1093/jxb/eri023 | title = Regulation of Photosynthesis under Stress: Molecular design of the photosystem II light-harvesting antenna: photosynthesis and photoprotection | pmid = 15557295 | journal = J. Exp. Bot. | date = 2005-04 | first = Peter | last = Horton | coauthors = Alexander V. Ruban | volume = 56 | issue = 411 | pages = 365–373 | url = http://jxb.oxfordjournals.org/content/56/411/365.abstract?sid=9489ebee-5b2f-4292-9260-54cea443c6b8 | accessdate = 2011-01-15}}</ref>  It involves the quenching of singlet [[excited state]] [[chlorophyll]]s (Chl) via enhanced [[internal conversion (chemistry)|internal conversion]] to the [[ground state]] (non-radiative decay), thus harmlessly dissipating excess excitation energy as heat through molecular vibrations. NPQ occurs in almost all photosynthetic [[eukaryote]]s (algae and plants), and helps to regulate and protect photosynthesis in environments where [[light energy]] [[Absorption (electromagnetic radiation)|absorption]] exceeds the capacity for light utilization in [[photosynthesis]].<ref name=muller>{{cite journal | doi = 10.1104/pp.125.4.1558 | title = Update on Photosynthesis: Non-Photochemical Quenching. A Response to Excess Light Energy | journal = Plant Physiol | date = 2001-04 | first = Patricia | last = Müller | pmid = 11299337 | coauthors = Xiao-Ping Li, Krishna K. Niyogi | volume = 125 | issue = 4 | pmc = 1539381 | pages = 1558–1566 | id = | url = http://www.plantphysiol.org/cgi/content/full/125/4/1558 | accessdate = 2010-08-04}}</ref>

==Process==
[[File:Photosynthetic parameters of plants.png|thumb|Carbon assimilation (red line) tends to saturate at high light intensities, while light absorption (blue line) increases linearly<ref name="tamoi">{{cite journal|author=Masahiro Tamoi, Miki Nagaoka, Yoshiko Miyagawa and Shigeru Shigeoka|year=2006|title=Contribution of Fructose-1,6-bisphosphatase and Sedoheptulose-1,7-bisphosphatase to the Photosynthetic Rate and Carbon Flow in the Calvin Cycle in Transgenic Plants|journal=Plant & Cell Physiology|volume=29|issue=10|pages=380&ndash;390|publisher=Japanese Society of Plant Physiologists|doi=10.1093/pcp/pcj004|issn=1471-9053|url=http://pcp.oxfordjournals.org/content/47/3/380/F4.expansion.html|accessdate= }}</ref>]]
[[File:Irradiance and carbon assimilation for a monoculture of Woloszynskia halophila at different pH.png|thumb|The relationship between irradiance and carbon assimilation for a [[monoculture]] of the [[plankton]] ''[[Woloszynskia halophila]]'' at different [[pH]]<ref name="spilling">{{cite journal|author=Kristian Spilling|year=2007|title=Dense sub-ice bloom of dinoflagellates in the Baltic Sea, potentially limited by high pH|journal=Journal of Plankton Research|volume=29|issue=10|pages=895&ndash;901|publisher=Oxford University Press|doi=10.1093/plankt/fbm067|issn=1464-3774|url=http://plankt.oxfordjournals.org/content/29/10/895/F4.expansion.html|accessdate= }}</ref>]]
When a molecule of [[chlorophyll]] absorbs light it is promoted from its [[ground state]] to its first singlet excited state. The excited state then has three main fates. Either the energy is; 1. passed to another chlorophyll molecule by [[Förster resonance energy transfer]] (in this way excitation is gradually passed to the photochemical reaction centers ([[photosystem I]] and [[photosystem II]]) where energy is used in photosynthesis (called photochemical quenching); or 2. the excited state can return to the ground state by emitting the energy as heat (called non-photochemical quenching); or 3. the excited state can return to the ground state by emitting a photon ([[fluorescence]]). 

In higher plants, the absorption of light continues to increase as light intensity increases, while the capacity for photosynthesis tends to saturate. Therefore there is the potential for the absorption of excess light energy by photosynthetic [[Photosynthesis#Photosynthetic membranes and organelles|light harvesting]] systems. This excess excitation energy leads to an increase in the lifetime of singlet excited [[chlorophyll]], increasing the chances of the formation of long-lived chlorophyll triplet states by [[inter-system crossing]]. Triplet chlorophyll is a potent [[photosensitiser]] of molecular oxygen forming [[singlet oxygen]] which can cause oxidative damage to the pigments, lipids and proteins of the photosynthetic [[thylakoid membrane]]. To counter this problem, one photoprotective mechanism is so-called non-photochemical quenching (NPQ), which relies upon the conversion and dissipation of the excess excitation energy into heat. NPQ involves conformational changes within the light harvesting proteins of photosystem II that bring about a change in pigment interactions causing the formation of energy traps. The conformational changes are stimulated by a combination of transmembrane proton gradient, the PsbS subunit of [[photosystem II]] and the enzymatic conversion of the carotenoid [[violaxanthin]] to [[zeaxanthin]] (the [[xanthophyll cycle]]).

''For further discussion, see [[Chlorophyll fluorescence]] and [[Plant stress measurement]].''

==Measurement of NPQ==
Non-photochemical quenching is measured by the quenching of chlorophyll fluorescence and is distinguished from photochemical quenching by applying a bright light pulse to transiently saturate photochemical quenching thus removing its contribution from the observed quenching. Non-photochemical quenching is not affected if the pulse of light is short. During the pulse, the fluorescence reaches the level reached in the absence of any photochemical quenching, known as maximum fluorescence, <math>F_m</math>.

''For further discussion, see [[Chlorophyll fluorescence#Measuring fluorescence|Measuring chlorophyll fluorescence]] and [[Plant stress measurement]].''

Chlorophyll fluorescence can easily be measured with a chlorophyll fluorometer. Some fluorometers (such as the [http://www.adc.co.uk/Products/NEW_ADC_OS5p_Advanced_Modulated_Fluorometer/ OS-5p]) can calculate NPQ and photochemical quenching coefficients (including qP, qN, qE and NPQ), as well as light and dark adaptation parameters (including Fo, Fm, and Fv/Fm).

== See also ==
[[Chlorophyll fluorescence]] 

[[Chlorophyll fluorescence#Measuring fluorescence|Measuring chlorophyll fluorescence]]

==References==

{{Reflist}}

          5.   Jörg Standfuss, Anke C Terwisscha van Scheltinga, Matteo Lamborghini and Werner Kühlbrandt, Mechanisms of photoprotection and nonphotochemical quenching in pea lightharvesting complex <br />
                at 2.5A° resolution The EMBO Journal VOL 24 | NO 5 | 200

[[Category:Photosynthesis]]