246 ECOLOGY OF PLANTS
deficit on the biochemical processes involved in photosyn-
thesis. Gates (1964) maintained that the rate of photosyn-
thesis is more closely related to the water content of leaves
than to the diffusive capacity of the stomates. Certainly, if
Klotz’s model of protein hydration and behavior is correct,
one would expect Gate’s contention to be logical.
Gates (1964) reported that during water stress, the respira-
tion rate is accelerated but later declines. There is an increase
in starch hydrolysis and an increase in sucrose which later
declines. The respiration rate appears to be correlated with
the sucrose content. Protein synthesis is impaired and active
proteolysis may occur with prolonged wilting, especially in
older leaves. Finally, if the degree of protoplasmic desicca-
tion reaches critical levels, individual cells, tissues and even
the entire organism may die.
Flooding lowers the oxygen supply to the roots or roots
and tops if the tops are covered with water also. Under
these conditions, Crawford and Tyler (1969) found that
plants intolerant of flooding have an increased rate of gly-
colysis including alcohol dehydrogenase induction and the
greatest production of acetaldehyde. On the other hand,
malic acid accumulates in flood tolerant species and falls
in intolerant species. Organic acids can accumulate in cells
in much greater quantities than ethanol without damaging
the plant. In fact, malic acid helps preserve the electrical
neutrality of the cell.EFFECTS OF TEMPERATUREThe cardinal temperatures (i.e., minimum, optimum, maxi-
mum) often differ for the same function in different plants
and different functions in the same plant. They often differ
also for the same function at different stages of development,
and often in different organs of the same plant at a given,
stage. Moreover, they may differ with the general physi-
ological condition of the plant, the duration of temperature
regime, and changes in other environmental factors.
Daubenmire (1959) states that the minimum temperature
for growth of melons ( Citrullus ) and sorghums ( Sorghum ) is
in the range of 15 to 18C; and for peas ( Pisum ), rye ( Secale ),
and wheat ( Triticum ), 2 to 5C. He further states that roots
usually have a lower minimum for growth than shoots of the
same plants. The optimum temperature for photosynthesis is
below that for respiration in all known cases. This is prob-
ably the reason peaches, apples, white potatoes, and many
other plants do not accumulate normal food reserves in low
latitudes.
Seeds of many native plants of temperature or colder
regions have a low temperature requirement for germination.
The value of such a mechanism in seedling survival is obvi-
ous. Work in my laboratory at the University of Oklahoma
has shown that seeds of Ambrosia artemisiifolia (common
ragweed), A. psilostachya (western ragweed) and Sporobolus
pyramidatus (drop seed) require exposure to temperatures in
the range of 4–7C for about 6–10 weeks in moist condi-
tions to break dormancy. Lane (1965) reported that seeds
of Helianthus annuus (annual sunflower) require 90 daysat 4C in moist sand to completely break dormancy. Donoho
and Walker (1957) reported that Elberta peach ( Prunus per-
sica ) seeds usually required stratification for 60–100 days
at 7C or below to break dormancy.
Overwintering buds of many, if not most, perennial plants
of temperature or colder regions also have low temperature
requirements for the breaking of dormancy. Weinberger
(1956) found that Elberta peach buds require 950 hours at
7 C or below to break dormancy. Daubenmire (1959) stated
that peaches in general require 400 or more hours of temper-
atures at or below 7C, blueberries ( Vaccinium spp.) require
800 hours at this level, and apples ( Pyrus malus ) require
even more.
The chemical changes which are necessary for the
breaking of dormancy in seeds and buds, and which are
stimulated by exposure to low temperatures, are not defi-
nitely known. In fact, this is a fertile field for research.
There is growing evidence, however, that the breaking of
dormancy may be related to a change in balance between
growth inhibitors and stimulators. Sondheimer, Tzou, and
Galson (1968) reported that abscisic acid inhibits germi-
nation of excised nondormant embryos and this can be
reversed with a combination of gibberellic acid and kinetin.
Natural amounts of abscisic acid in seeds and pericarps of
several species of Fraximus (ash) were determined, and it
was found that the amount in F. americana decreased 37%
in the pericarp and 68% in the seed with chilling treatment.
They felt that it is unlikely that the amount in the pericarp
influences dormancy but that the inhibitor in the seed does
appear to play a regulatory role. Junttila (1970) found that
dormancy of seeds of Betula nana (birch) was effectively
broken by low temperatures or treatment with gibberellic
acid. Haber, Foard, and Perdue (1969), reported 84% ger-
mination in dormant lettuce ( Lactuca sativa ) seeds treated
with a 5 10 ^4 M gibberellic acid solution, 1.5% germi-
nation in seeds treated with a 5 10 ^4 M gibberellic acid
solution plus 100 mg/l of abscisic acid, and 0% in water
controls.
Donoho and Walker (1957) reported a 98% breaking of
dormancy of Elberta peach buds after only 164 hours at 7C
when sprayed with a 4000 ppm gibberellic acid solution,
despite the usual requirement of 950 hours at 7C or below.
Aung, Hertogh, and Staby (1969) found free gibberellic acid
in Tulipa gesneriana (tulip) increased by 67% over the initial
level in bulbs grown for 4 weeks at 18C. Bound gibberel-
lic acid showed a slight initial increase followed by a rapid
decrease. In bulbs treated at 13C, a marked decline, occurred
in free gibberellic acid and a 2-fold increase occurred in the
bound form. The rates of floral, shoot, and root development
were accelerated at 18C as compared with 13C.
Cold hardening of plants (i.e., improving the ability of
plants to withstand low temperatures without severe damage
or death) is probably just as important to plant survival in
regions with periods of relatively low temperatures as dor-
mancy. Actually, some of the same internal changes may be
responsible for both. Irving (1969) found a buildup during
cold hardening in Acer negundo (box elder) of an inhibitor
similar or identical to abscisic acid which has, of course, beenC005_001_r03.indd 246C005_001_r03.indd 246 11/18/2005 10:20:10 AM11/18/2005 10:20:10 AM