Estrés Luminico

BULG. J. PLANT PHYSIOL., 1999, 25(3–4), 3–16

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PHOTOSYNTHESIS AND HIGH LIGHT STRESS*
Hartmut K. Lichtenthaler, Stefan Burkart
Botanisches Institut, Universität Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe,
Germany
Received 30 July 1999
Summary. Exposure of plants to irradiances far above the light saturation
point of photosynthesis, known as high-light stress, induces variousresponses including light adaptation of the photosynthetic apparatus and chloroplast
ultrastructure by formation of sun-type chloroplasts. The latter possess a lower cross section for light absorption (less light-harvesting chlorophyll proteins) and higher rates of photosynthetic quantum conversion than shade-type
chloroplasts. De-epoxidation of violaxanthin to zeaxanthin, increase in heat
emission,rise of non-photochemical de-excitation of absorbed light quanta
(qN) and photoinhibition of the photosynthetic pigment apparatus are further high-light stress responses.
The degree of photoinhibition can clearly be determined via measurements
of the chlorophyll fluorescence relaxation kinetics and depends on the photon flux density of the high-light stress as is shown here with soybean leaves.A decrease in photochemical quenching qP and variable chlorophyll fluorescence ratios Fv/Fm or ∆F/Fm, and an increase of non-photochemical quench′′
′
ing qN are not yet an indication of a photoinhibition. One always has to determine the photoinhibitory quenching coefficient qI which is one of three qN
components. However, in soybean leaves a photoinhibition (increase in qI)
of thechloroplasts, as measured at the adaxial light exposed upper leaf side,
did not affect the photosynthetic CO2 assimilation rates (PN) of the whole
leaf. Thus, chlorophyll fluorescence measurements, usually carried out with
low irradiance light, taken at the upper leaf-side alone are not representative
for the physiological situation of the whole leaf. Chlorophyll fluorescence
signatures have to bedetermined as well from the abaxial lower leaf side in
order to correctly judge the physiological state of the leaf.
* This paper is dedicated to the 70-th anniversary of Prof. Dr. Sci. Ivan Yordanov.

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H. K. Lichtenthaler and S. Burkart

Key words: Chlorophyll fluorescence, light adaptation, non-photochemical
quenching, photoinhibition, sun-type chloroplast
Abbreviations: Chl –chlorophyll; Fv and Fm – variable and maximum Chl
fluorescence of PSII in the dark adapted state; Fv and Fm – variable and max′
′
imum Chl fluorescence in the light adapted state; Fd – Chl fluorescence decrease from Fm to Fs at continuous illumination, Fo – ground value of Chl
fluorescence; Fs – steady state Chl fluorescence 5 min after onset of illumination, PN – photosynthetic net CO2-assimilation;PPFD – photosynthetic photon flux density; PSII – photosystem II; qN – non-photochemical quenching coefficient of chlorophyll fluorescence; qP – photochemical quenching
coefficient of Chl fluorescence; qI – non-photochemical quenching due to
photoinhibition of photosystem 2; qT – non photochemical quenching due
to state transitions; qE – non-photochemical quenching due to the build-up
of apH-gradient.

Introduction
The stress concept
Terrestrial plants are exposed to many kinds of natural stressors, such as water stress,
heat and high-light stress or anthropogenic stresses, such as air pollution (exposure
to SO2, NOX, O3), acid rain, and acid morning dew (as reviewed by Lichtenthaler,
1996). Plants respond to these day-to-day or long-term stress exposures by particularstress-induced responses and stress coping mechanisms as well as non-specific
stress responses which efficiently help plants to survive and which have been joined
in a unifying stress concept for plants (Lichtenthaler, 1996, 1998). Within this concept four stages of plants in response to stress can be differentiated. During stress
exposure they are: 1) the response phase (alarm reaction), 2) the stage...