ISSN 1021-4437, Russian Journal of Plant Physiology, 2007, Vol. 54, No. 5, pp. 633–638. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © L.N. Timergalina, L.B. Vysotskaya, S.Yu. Veselov, G.R. Kudoyarova, 2007, published in Fiziologiya Rastenii, 2007, Vol. 54, No. 5, pp. 715–721.
Under natural conditions, illumination changes fre-
quently because of cloudiness, which affects transpira-
tion. Enhanced transpiration creates a disbalance
between water uptake and loss and increases water def-
icit in the shoot. As an expected consequence is slowing
leaf expansion. However, this growth cessation caused
by water deﬁcit in the shoot is transient: leaf growth
resumes partially or completely [1–3]. However, the
mechanism of this growth resumption is poorly studied.
Turgor maintenance due to the accumulation of
osmotically active compounds in the cells at water def-
icit  favors growth resumption during osmotic stress
. Nevertheless, under drought, leaf growth was
retarded in spite of complete turgor restoration .
Osmotic adjustment is not the only mechanism for
growth regulation under limited water availability .
The cell-wall properties are known to be rapidly
changed in response to environmental changes, and the
rapid resumption of leaf growth could follow from the
increased cell-wall extensibility .
Leaf growth depends on water availability, which is
determined by hydraulic conductivity [5, 7, 9]. In
plants, hydraulic conductivity varies and can be
increased with transpiration enhancement [10, 11].
However, the contribution of elevated hydraulic con-
ductivity to leaf expansion is not properly studied.
It seemed important to evaluate the contribution of
the above-listed factors (osmoregulation, cell-wall
extensibility, and hydraulic conductivity) in the mainte-
nance of leaf expansion after changes in the light
regime. It was also of interest to understand what fac-
tors were involved in plant adaptation to new illumina-
tion conditions. It is believed that plant response to light
depends on endogenous factors, the hormone level in
particular . Until now, the principal attention of
researchers was paid to plant transfer from complete
darkness to light at deﬁnite wave length or optimum
intensity [13, 14]. Hormonal responses to varying illu-
mination are much less studied . In this connection,
the objective of this work was to evaluate the contribu-
tion of osmotic adjustment, cell-wall extensibility, and
hydraulic conductivity to the maintenance of tissue
water relations and leaf expansion and also to examine
the role of hormones in plant response to the increase in
MATERIALS AND METHODS
Desf., cv. Bezenchukskaya
139) plants were ﬁrst grown in darkness on tap water at
and then in water culture on 0.1 Hoagland–
Arnon nutrient medium at illumination of 400
PAR and a 14-h photoperiod. Seven-day-old seedlings
were used in experiments. A day before, the seeds were
removed and seedlings were placed for adaptation in
glasses containing 100 ml of nutrient medium. After
Effect of Increased Irradiance on the Hormone Content,
Water Relations, and Leaf Elongation in Wheat Seedlings
L. N. Timergalina
, L. B. Vysotskaya
, S. Yu. Veselov
, and G. R. Kudoyarova
Institute of Biology, Ufa Research Center, Russian Academy of Sciences, pr. Oktyabrya 69, Ufa, 450054 Russia;
Bashkir State University, Ufa, Russia
Received July 13, 2006
—In wheat (
Desf., cv. Bezenchukskaya 139) seedlings, an increase in irradiance from
20 to 400
s) PAR enhanced transpiration and increased stomatal conductance by three times on the
background of reduced relative water content (RWC). After this treatment, leaves quickly ceased to grow and
became even shrunk later. In 40 or 50 min, leaf growth was resumed. At this period, we observed an increase
in hydraulic conductivity and RWC and also in leaf extensibility. As soon as 10 min after treatment, some
changes in hormone content were noted. In the zones of leaf growth and its mature part, zeatin and zeatin ribo-
side were accumulated, whereas ABA accumulation was observed in the zone of leaf growth and in the roots.
The results obtained indicate that leaf expansion at increased irradiance was related to changes in cell-wall
extensibility and hydraulic conductivity. The ﬁrst effect could be due to cytokinin accumulation, whereas the
second one, to ABA accumulation.
Key words: Triticum durum - growth - water relations - phytohormones
: DTT—dithiothreitol; PAR—photosynthetically
active radiation; RWC—relative water content.