1021-4437/05/5204- © 2005
Russian Journal of Plant Physiology, Vol. 52, No. 4, 2005, pp. 427–433. From Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 485–491.
Original English Text Copyright © 2005 by Nikolaeva, Bukhov, Egorova.
Variations of light irradiance during plant growth
signiﬁcantly inﬂuence the properties of photosynthetic
apparatus of higher plants. The decrease in total chloro-
phyll content in leaves of sun-type plants grown under
low light is one of the most documented responses of
photosynthetic apparatus to long-term irradiation [1–3].
In addition, the decrease in light irradiance during plant
growth is accompanied by the decline in the chloro-
ratio [3–5]. As chlorophyll
molecules are constituents of only light-harvesting
complexes, especially of photosystem II (PSII) [6, 7],
such a decline evidences a larger antenna size of this
photosystem in chloroplasts of plants grown under low
irradiances [8, 9]. Unlike PSII, the size of PSI photo-
synthetic unit is insensitive to irradiance during plant
cultivation [4, 10, 11].
Long-term irradiance during plant growth was
found to affect not only the amount of constituents of
photosynthetic apparatus but also the rate of electron
transfer in isolated chloroplasts. A common tendency is
the decrease in rates of light-induced electron transfer
(on a per chlorophyll basis) in isolated chloroplasts
from leaves grown under low light as compared to those
isolated from high light-grown leaves [12–14]. Further-
more, the increase in light irradiance during plant
growth accelerated the electron transport driven solely
by PSI  and stimulated the dark activity of ferre-
reductase, the PSI key component .
In experiments with isolated thylakoids, one must
add exogenous soluble cofactors (electron donors and
acceptors) into the assay medium to monitor PSI-
dependent electron transport. Meanwhile, electron
transport reactions driven solely by PSI proceed in
chloroplasts of intact leaves as well . In this case,
the activities of so-called alternative electron transport
pathways can be characterized by the kinetics of dark
reduction of P700, the photoactive pigment of PSI .
Activities of Noncyclic and Alternative Pathways
of Photosynthetic Electron Transport in Leaves of Broad Bean
Plants Grown at Various Light Irradiances
M. K. Nikolaeva, N. G. Bukhov, and E. A. Egorova
Timiryazev Institute of Plant Physiology, Russian Academy of Sciences,
Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (095) 977-8018; e-mail: firstname.lastname@example.org
Received November 2, 2004
—Activities of noncyclic and alternative pathways of photosynthetic electron transport were studied
in intact leaves of broad been (
L.) seedlings grown under white light at irradiances of 176, 36, and
s). Electron ﬂows were followed from light-induced absorbance changes at 830 nm related
to redox transformations of P700, the photoactive PSI pigment. The largest absorbance changes at 830 nm,
induced by either white or far-red light, were observed in leaves of seedlings grown at irradiance of
s), which provides evidence for the highest concentration of PSI reaction centers per unit
leaf area in these seedlings. When actinic white light of 1800
s) was turned on, the P700 oxi-
dation proceeded most rapidly in leaves of seedlings grown at irradiance of 176
s). The rates
of electron transfer from PSII to PSI were measured from the kinetics of dark P700
reduction after turning off
white light. These rates were similar in leaves of all light treatments studied, and their characteristic reaction
times were found to range from 9.2 to 9.5 ms. Four exponentially decaying components were resolved in the
kinetics of dark P700
reduction after leaf exposure to far-red light. A minor but the fastest component of P700
reduction with a halftime of 30–60 ms was determined by electron transfer from PSII, while the three other slow
components were related to the operation of alternative electron transport pathways. Their halftimes and rela-
tive magnitudes were almost independent on irradiance during plant cultivation. It is concluded that irradiance
during plant growth affects the absolute content of PSI reaction centers in leaves but did not inﬂuence the rates
of noncyclic and alternative electron transport.
Key words: Vicia faba - irradiance during plant growth - photosynthetic electron transport
—absorbance changes at 830 nm; PSI and
PSII—photosystems I and II.
The article was translated by the authors.