Untitled - 中国植物生理与分子生物学学会

Investigations of photosynthetic electron flow in leaves using
P700 redox kinetics
Wah Soon CHOW a , Husen JIA a , Da-Yong FAN b and Alexander B. HOPE c
a
Photobioenergetics Group, Research School of Biological Sciences, The Australian National
University, Canberra, ACT 0200, Australia
b
Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of
Sciences, Beijing 100093, China
c
School of Biological Sciences, Flinders University, Faculty of Science and Engineering, GPO Box
2100, Adelaide, SA 5001, Australia
When P700, the special chlorophyll (Chl) pair in Photosystem I (PS I), undergoes photo-oxidation,
its absorbance at 810 nm increases. We utilized this near-infra-red signal (reference wavelength
870 nm) to quantify photosynthetic electron flow in leaves in a number of conditions.
(1) The integrated electron flow from PS II to P700 + after a single-turnover flash was linearly
correlated with the content of active PS II, as varied by a photoinhibitory pre-treatment. A single,
empirical linear correlation was applicable to leaves of C 3 and C 4 , woody and herbaceous, monocot
and dicot, wild-type and Chl b-less plants tested.
(2) Allowing for a possible dependence of the rate of P700 photo-oxidation on [P700 + ] due to
excitation quenching by the Chl cation, the ratio of the flash-induced, integrated electron flow
originating in PSII to the total P700 content could be obtained; this ratio was consistent with the
independently-observed ratio of the two photosystems, and exhibited a marked decrease during
ageing of barley plants in darkness. In contrast, the Chl fluorescence ratio, Fv/Fm, did not show any
substantial change over several days of ageing.
(3) The initial rate of re-reduction of P700 + at cessation of continuous illumination gave the
electron flux to P700 + immediately before the light was turned off. This electron flux consisted of
a linear and a cyclic component, but the use of methyl viologen abolished the cyclic component,
such that the rate of cyclic electron flow could be obtained by difference after a slight correction.
(4) The cyclic electron flux so obtained increased from a low value during photosynthetic
induction, reaching a peak at about 30 s, and declined to a low value in steady state during
illukination with low-irradiance far-red light.
(5) The cyclic electron flux was not affected by severe water stress, whereas the linear electron
flux was halved; this differential effect on the two electron fluxes could be explained by the
crowdedness in the stroma.
(6) The cyclic electron flux was confirmed to be negligibly small in the pgr5 mutant of
Arabidopsis, contrary to one report in the literature obtained using a different method. The P700 +
absorbance signal is a versatile and convenient way of monitoring electron transfers in the whole
leaf tissue, in contrast to Chl fluorescence which represents a leaf-surface measurement.
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Keywords:Photosynthetic electron flow; P700 redox kinetics
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