ECOLOGY OF PLANTS 251
Salvia leucophylla leaves markedly reduce oxygen uptake
by suspensions of mitochondria from Avena fatua (wild
oats) or cucumber. The inhibition appears to be localized in
that part of the Krebs cycle where succinate is converted to
fumarate, or fumarate to malate. They also reported that the
permeability of cell membranes appears to be decreased on
exposure to terpenes.
Most of the inhibitors that my students and I have
identified from our old-field species are phenolics (Abdul-
Wahab and Rice, 1967; Parenti and Rice, 1969; Rice, 1965a,
1965b; Rice and Parenti, 1967; Wilson and Rice, 1968).
Some of the inhibitors identified are: chlorogenic acid,
p-coumaric acid, ferulic acid, gallic acid, p-hydroxybenzal-
dehyde, isochlorogenic acid complex, scopoletin, scopolin,
sulfosaliyclic acid, and several tannins.
Sondheimer (1962) reported that chlorogenic acid is a
strong inhibitor of several enzyme systems and it is pos-
sible that this may be its chief role in growth inhibition.
Schwimmer (1958) found that a 10^3 M concentration
of chlorogenic acid caused a 50% inhibition of potato
phosphorylase activity. Mazelis (1962) reported that a
peroxidase-catalyzed oxidative decarboxylation of methio-
nine is strongly inhibited by a 5 10 ^6 M concentration of
chlorogenic acid. Sondheimer and Griffin (1960) reported
that indoleacetic acid oxidase is inhibited by chlorogenic
acid and its isomers, and by isochlorogenic acid and
dihydrochlorogenic acid.
Zenk and Müller (1963) reported that p-coumaric and
ferulic acids increase decarboxylation of indoleacetic acid
(IAA), resulting in reduced plant growth. Henderson and
Nitsch (1962) found a pronounced inhibiting effect of
p-coumaric acid on IAA induced growth also.
Bendall and Gregory (1963) found that phenol oxidases
which are important in the formation of lignin-like polymers
from coniferyl alcohol and oxidation products of tannins
such as quinones react with each other to produce a complex
that may make phenol oxidase catalytically inactive, or may
produce an active protein of modified properties. Hulme and
Jones (1963) and Jones and Hulme (1961) found that tannins
caused considerable reduction of Krebs cycle succin oxidase
and malic enzyme activity. Gallic and tannic acids were
shown also to inhibit IAA induced growth (Zimsmeister and
Hollmuller, 1964). Hall (1966) and Williams (1963) reported
that relatively low concentrations of tannic acid were very
inhibitory to pectolytic enzymes. Gallic and tannic acids
are very inhibitory to the nodulation of heavily inoculated
legumes and to hemoglobin formation in the nodules (Blum
and Rice, 1969). The hydroxyl groups of tannins are known
to have a strong affinity for the peptide linkages of proteins,
so it is likely that reactions readily occur between the two
compounds when they come in contact at a time when the
ice-shell around the protein is appropriately modified. This
would undoubtedly change the nature of the protein.
Einhellig et al. (1970) found that sunflower ( Helianthus
annuus ) and tobacco plants treated with 10^4 and 5 10 ^4
scopoletin concentrations had significant increases in scopole-
tin and scopolin in their tissues. Net photosynthesis in tobacco
plants treated with a 10^3 M concentration of scopoletin wasdepressed to as low as 34% of that of the controls. Reduced
growth in leaf area over a 12 day experiment correlated well
with the significant reduction in the rate of net photosynthesis.
The surface has just been scratched in identifying plant
produced inhibitors of other plants. Moreover, the field is
wide open for research on the mechanisms of action of the
presently known inhibitors.GENERAL CONCLUSIONSI have discussed briefly the presently known physiological
basis for some of the ecological responses of plants to the
environmental factors of water, temperature, radiant energy,
and toxins. It would obviously require most of the present
volume to discuss the presently known effects of all envi-
ronmental factors. Even so, our knowledge of the effects of
most factors is superficial. Many of the known physiological
effects may be indirect ones also.
In closing, I emphasize that the physiological ecologist is
searching for the physiological basis for ecological behavior.
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