ECOLOGY OF PLANTS 245
INTERRELATIONSHIPS BETWEEN
ENVIRONMENTAL FACTORSIn addition to having a direct effect on many plant functions,
the water factor affects other factors through the amount
and type of precipitation involved. It affects: (1) amounts of
radiant energy, (2) temperature of air and soil, (3) amount
of nitrogen brought down by rain, (4) amounts of minerals
leached from the soil, (5) availability of various elements to
plants, (6) topography of the region, (7) numbers and types
of organisms in the soil, (8) toxins in the soil, (9) aeration of
soil, and (10) numbers and types of parasites in a region.
All other environmental factors have many indirect
effects also.EFFECTS OF WATER FACTORWarming (1909) stated that “no other influence impresses its
mark to such a degree upon the internal and external struc-
tures of the plant as does the amount of water present in the
air and soil.” This is due primarily, of course, to the following
roles of water: (1) it is important as a solvent system in the
soil and in cells of all living organisms on earth, (2) it serves
as the dispersion medium for the colloidal systems present
in cells, (3) there is a close interaction between the nature
of the hydration shell surrounding protein molecules and the
physicochemical properties of the proteins (Klotz, 1958),
(4) it is important as a raw material in photosynthesis, (5) it
is important as a raw material in all hydrolytic processes,
and (6) it has some influence on leaf temperatures through
transpirational cooling.
Klotz (1958) suggested that protein molecules in cells
have hydration shells of lattice-ordered water which act like
ice-shells around the molecules and that the maintenance
of an active configuration is due to the shells. The effect of
heat is thought to reduce the extent of the ice-shell, whereas
lower temperatures increase the thickness of the shell. Urea
is thought to be an effective denaturizer of proteins because
its strong hydrogen bonding characteristics may break down
the frozen structure of the water envelope. The observed
effects of water stress on the viscosity of protoplasm sug-
gests that reduced hydration has a definite effect on the
ice-shell.
Jacobson (1953) offered considerable evidence for the
presence of an ordered-water lattice around DNA molecules
also. He suggested that such an ice-shell would make sepa-
ration of the two chains of DNA during replication possible
with low energies.
The models of Klotz and Jacobson have apparently been
well received (Stocker, 1960; Slatyer, 1967). If these models
are indeed correct, it is easy to see how water deficiency can
have pronounced effects on all sorts of processes, because
of the role of DNA in protein synthesis and the role of pro-
teins in enzymes. It is probable that the integrity of specific
water-protein structures is necessary for the continuance
of organic processes at optimum rates. It is likely that the
variations in reductions to different species to water stressare due chiefly to different degrees of sensitivity of essential
metabolic systems.
Buckman and Brady (1960) state that a slight lowering
of the available water content in the soil below field capac-
ity stimulates growth of plants generally. Blair et al. (1950)
found that the rate of stem elongation in sunflower (a dwarf
type from an inbred line, apparently of Helianthus annuus ) is
markedly reduced before half the available water is used, and
that zero growth results during the use of the last one-fourth
of the available water. A reduction of 30–40% between field
capacity and the wilting point probably slows growth in most
plants.
Probably the most direct effect of water on plant growth
is its effect on the turgor pressure of the individual cells.
Turgor pressure affects cell enlargement and stomatal clo-
sure. Closing of the stomates due to reduced turgor pressure
in the guard cells reduces both transpiration and photosyn-
thesis and can ultimately reduce growth. Probably, many of
the detrimental effects of moderate water deficits are due
to stomatal closure. One important effect is that the tem-
perature of leaves can rise to detrimental levels under some
conditions when the transpiration rate is lowered markedly
(Slatyer, 1967).
It appears that cell division is influenced less by water
deficits than cell enlargement (Slatyer, 1967). Gates and
Bonner (1959) found that the amount of DNA in water
stressed tomato leaves remained constant during the period
of their experiments whereas the amount increased steadily
in control leaves. This indicated that chromosomal multipli-
cation was continuing to occur in the control leaves but had
ceased in the test leaves. Apparently, cell number is linearly
related to DNA content (Slatyer, 1967) so it appears that there
was a reduction in rate of cell division in the water stressed
leaves. Gardner and Nieman (1964) found that water stress
in cotyledonary leaves of Raphanus sativus (radish) reduced
the rate of increase of DNA.
Gates and Bonner (1959) found that water stress pre-
vented a net increase in RNA in Lycopersicum esculentum
(tomato) leaves also. The water stressed leaves were able to
incorporate P^32 -labeled phosphate into RNA apparently at
the same rate as in control leaves. They concluded, there-
fore, that the rate of destruction of RNA was greater in water
stressed leaves than in controls. Kessler (1961) reported that
drought stress causes a pronounced decrease in the RNA
content of several plants. Despite this, the rate of incorpo-
ration of uracil-C^14 was the same as in control leaves but
had ceased in the test leaves—drought stressed plants. This
was confirmed because the activity of RNase was found to
increase markedly with water stress.
A reduction in the water content of leaves generally
results in a decrease in the rate of photosynthesis (Meyer
and Anderson, 1939). This may seem strange because
less than one per cent of the water absorbed by a plant is
used as a raw material in photosynthesis. According to
Slatyer (1967) however, there are two main modes of action
of water stresses on photosynthesis: (1) stomatal closure and
reduced rates of CO 2 exchange can influence the supply of
CO 2 and (2) there is probably a direct effect of the waterC005_001_r03.indd 245C005_001_r03.indd 245 11/18/2005 10:20:10 AM11/18/2005 10:20:10 AM