V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
293
cell sap may be surpassed, even in the most extreme halophytes, and the
plants die. Even below this tolerance limit the plants grow poorly and exhibit a heavy red coloring, which is quite common in Chenopodiaceae.
Plants in salty soil often exhibit a yellowing of normally green parts
(chlorosis) or just a lower chlorophyll content (see Shetty, 1971, p. 106 for
Acrostichum aureum). Clerodendrum inerme (Verbenaceae, a shrub growing on salt soil on the edge of the mangrove vegetation at the coast of Bombay, India, and also grown as hedge plant in salt-free soils) growing in
a natural habitat has succulent and chlorotic leaves which absorbed more
14
C0 2 during the daytime than did the green leaves of the cultivated
Clerodendrum plants, although the chlorophyll content of the former plants
was lower. Chlorosis seems not to change the light compensation point,
an index for the ability for CO. assimilation. The leaves of the wild
Clerodendrum plants had a lower concentration of sugar and organic acids,
but a higher concentration of amino acids than the leaves from cultivated
plants from salt-free soil (Mishra, 1967, p. 143). Amino acid synthesis increases in the dark at the expense of the organic acids. For different
species of marine plants the amino acid synthesis is directly related to the
chloride content of the plant tissue (Joshi, 1965, p. 260). Such enhanced
amino acid synthesis was also observed in the mesophytic Bryophyllum
pinnaîum grown in nutrient media containing 0.04 M NaCl (Karmarkar
and Joshi, 1968, p. 43). These observations may indicate the involvement
of an alternate pathway of carbohydrate metabolism in halophytes (C 4 -
dicarboxylic acid pathway; see Hatch, 1969).
Opposite to the hygro-halophytes are the xerohalophytes, growing on
elevated locations in soils which are not continuously wet. The salt content
of these soils, relative to the dry weight, may be very low, often only 0.1%
or less. This low salt content causes doubt as to whether plants growing on
these soils are truly halophytic plants. However, the water content in the
soil often is very low also (a few percent), and this results in a rather
high salt concentration of the soil solution. For instance, the Atriplex species in the Australian "salt bush" region have low osmotic potentials and
a high chloride fraction of the leaf press sap (Walter, 1964, p. 462). Like
all true halophytes, Atriplex nummularia and A. vesicaria grow better in
nutrient solutions with NaCl added than in solutions without NaCl. It was
also proven by Brownell and Wood (1957) that Na is an essential element
for A. vesicaria.
Other xerohalophytes are the Atriplex species of the western arid regions of North America, the various Chenopodiacean shrubs and semishrubs of the Middle Asian and Central Asian deserts, the Zygophyllum
species in these latter deserts, and in the North African and South African
deserts, most of the Mesembryanthemum species, and others.
293
cell sap may be surpassed, even in the most extreme halophytes, and the
plants die. Even below this tolerance limit the plants grow poorly and exhibit a heavy red coloring, which is quite common in Chenopodiaceae.
Plants in salty soil often exhibit a yellowing of normally green parts
(chlorosis) or just a lower chlorophyll content (see Shetty, 1971, p. 106 for
Acrostichum aureum). Clerodendrum inerme (Verbenaceae, a shrub growing on salt soil on the edge of the mangrove vegetation at the coast of Bombay, India, and also grown as hedge plant in salt-free soils) growing in
a natural habitat has succulent and chlorotic leaves which absorbed more
14
C0 2 during the daytime than did the green leaves of the cultivated
Clerodendrum plants, although the chlorophyll content of the former plants
was lower. Chlorosis seems not to change the light compensation point,
an index for the ability for CO. assimilation. The leaves of the wild
Clerodendrum plants had a lower concentration of sugar and organic acids,
but a higher concentration of amino acids than the leaves from cultivated
plants from salt-free soil (Mishra, 1967, p. 143). Amino acid synthesis increases in the dark at the expense of the organic acids. For different
species of marine plants the amino acid synthesis is directly related to the
chloride content of the plant tissue (Joshi, 1965, p. 260). Such enhanced
amino acid synthesis was also observed in the mesophytic Bryophyllum
pinnaîum grown in nutrient media containing 0.04 M NaCl (Karmarkar
and Joshi, 1968, p. 43). These observations may indicate the involvement
of an alternate pathway of carbohydrate metabolism in halophytes (C 4 -
dicarboxylic acid pathway; see Hatch, 1969).
Opposite to the hygro-halophytes are the xerohalophytes, growing on
elevated locations in soils which are not continuously wet. The salt content
of these soils, relative to the dry weight, may be very low, often only 0.1%
or less. This low salt content causes doubt as to whether plants growing on
these soils are truly halophytic plants. However, the water content in the
soil often is very low also (a few percent), and this results in a rather
high salt concentration of the soil solution. For instance, the Atriplex species in the Australian "salt bush" region have low osmotic potentials and
a high chloride fraction of the leaf press sap (Walter, 1964, p. 462). Like
all true halophytes, Atriplex nummularia and A. vesicaria grow better in
nutrient solutions with NaCl added than in solutions without NaCl. It was
also proven by Brownell and Wood (1957) that Na is an essential element
for A. vesicaria.
Other xerohalophytes are the Atriplex species of the western arid regions of North America, the various Chenopodiacean shrubs and semishrubs of the Middle Asian and Central Asian deserts, the Zygophyllum
species in these latter deserts, and in the North African and South African
deserts, most of the Mesembryanthemum species, and others.