1021-4437/04/5104- © 2004
Russian Journal of Plant Physiology, Vol. 51, No. 4, 2004, pp. 547–555. Translated from Fiziologiya Rastenii, Vol. 51, No. 4, 2004, pp. 607–616.
Original Russian Text Copyright © 2004 by Zyalalov.
Transpiration is as a unique biological phenomenon
in the aspect of physical meaning of momentum. Annu-
tons of water is transported through the
higher plants of our planet . Within 24 h each plant
renews its water over 20–30 times . Considering the
average velocity of water ﬂow in the xylem, the front
edge of the transpiration ﬂow covers a single cell dis-
tance in the xylem parenchyma within 10–20 ms.
The modern ecological physiology abandoned the
viewpoint on transpiration as a plant defense mecha-
nism against overheating or as inevitable expenses for
providing the plant with carbon dioxide . Transpira-
tion is known to decline typically at noon, i.e., at the
period when the protection against overheating is most
urgent. On the other hand, transpiration remains quite
high even at cool weather, when air temperature does
not reach optimum values. The view on transpiration as
inevitable side effect of photosynthesis was also invali-
dated by studies of photosynthesis and transpiration in
relation to movement of stomatal guard cells .
According to contemporary views, the main role of
transpiration is the delivery of mineral nutrients and
organic substances, produced in roots, to the sink tis-
sues; i.e., the exchange between tissues and organs .
Besides, water ﬂow can also fulﬁll a signaling function,
since it occurs within the uniﬁed hydrodynamic system
of plant, which includes vessels (or tracheids) and free
water spaces between the cell walls (apoplast).
WATER MOVEMENT AS A PRECONDITION
FOR LIFE ACTIVITY OF HIGHER PLANTS
AND THEIR ANCESTORS
An important aspect of movement of huge amounts
of water in higher plants is the position of this phenom-
enon in the system of general biological knowledge.
Solving this problem requires an evolutionary approach
as an important methodological basement of modern
biology . The development of water relations in
higher plants is a striking manifestation of a fundamen-
tal tenet of the evolutionary theory concerning develop-
ment of the organism towards its higher independence
of stochastic environmental factors . The basic path-
way of biological progress is an expansion of capacities
to resist the external factors by attaining new properties
of more general importance. According to contempo-
rary concepts, vascular plants originated as a result of
symbiosis between fungi and green algae.
Intimate relations between fungi and algae led to the
fusion of their heterotypic nuclei , which gave rise to
a nucleus of dual nature with a mixed genome. Subse-
quent proliferative processes led to the development of
a chimeric organism, which consisted of algal and fun-
gal cells, together with intermediate type cells.
An assumption that plants colonized land by emerg-
ing from water habitat seems to be well-grounded. Ter-
restrial plants preserved all physiological processes that
were present in their ancestors: growth, photosynthesis,
respiration, and mineral nutrition. In addition, higher
plants developed the process of transpiration, which
Water Flows in Higher Plants:
Physiology, Evolution, and System Analysis
A. A. Zyalalov
Institute of Ecology of Natural Systems, Tatarstan Academy of Sciences, Daurskaya ul. 28, Kazan, 420087 Russia;
Received February 7, 2003
—Water movement in higher plants is treated as a symplastic ﬂuid ﬂow incorporated into a uniﬁed
hydrodynamic system comprising the apoplast and vessels (or tracheids). Since water ﬂow is of vital necessity
for algae (phylogenetic ancestors of higher plants), it can be stated that higher plants colonized land, still keep-
ing connections with their former water habitat. It is argued that colonization of terrestrial areas by plants
became possible due to the appearance and maintenance of a gradient of water chemical potential between the
rhizosphere and atmosphere, which drives water ﬂows. Autonomization of ﬂows in the symplast is considered
as a vector of evolution, whereas the gradient of water activity is a factor of evolution. The osmotic models of
water uptake by roots are analyzed; the role of potassium circulation in water-transporting system is deter-
mined; and a mechanism of automatic coupling between CO
uptake through stomata and water evaporation
from leaves is presented. An inherent property of the systems to explicitly or implicitly duplicate its structural
or functional elements substantiates possible interactions between the mechanisms underlying opposite water
ﬂows in plants.
Key words: higher plants - water ﬂow - evolution - physiology - system