Variation in Gas Exchange Characteristics Among Desert Plants
371
(Shreve 1942; Werger 1986; Shmida and Burgess 1988). Shreve and Wiggins
(1964) described 25 categories of life forms within the Sonoran Desert.
While many of the different life-forms relate to overall physiognomy (tree,
subtree, shrub, annual, etc.), 19 relate to succulence and to the high diversity
associated with shrubs as a life-form. For photosynthetic purposes, we will
examine the diversity of life-forms related to leaf types and succulence
differences, because these are of particular importance to understanding the
distribution patterns of photosynthetic pathways.
Within the C3 photosynthetic pathway, there is wide variation in photosynthetic tissue types in that leaves, twigs, stems, or any combinations of these
serve as the photosynthetic surface. Green-stem or green-photosynthetic
nonleaf tissues are perhaps the most frequent variant (Ehleringer et al. 1987;
Osmond et al. 1987; Smith and Osmond 1987; Comstock and Ehleringer
1988, 1990). Species exhibiting photosynthesis by nonleaf tissues increase
both as precipitation decreases and as the fraction of summer precipitation
increases (Shmida and Burgess 1988; Cody 1989). Twig and stem photosynthesis contribute significantly to the overall carbon balance of these
species, and in some cases, leaves no longer serve a significant role in
the plant's carbon balance (Comstock et al. 1988). In all instances, the
intercellular CO2 concentrations of twigs are lower than those of leaves and
the twig tissues are more drought-tolerant than leaf tissues. As elaborated in
Section 18.5.4, leaves of C3 species in the desert exhibit over a 1001111-1
variation in intercellular CO2 concentrations, which will have significant
bearing on both the extent to which stomata limit photosynthesis and on
water-use efficiency.
In terms of carbon gain, advantages and disadvantages can be associated
with each of the three photosynthetic pathways depending on environmental
conditions. Under today's CO2 environment, the C4 photosynthetic pathway
offers little or no intrinsic advantage over C3 under cool temperatures
( ~25 0C). In fact, the increases in quantum yield and reductions in photorespiration associated with C3 photosynthesis under cooler temperatures
suggest an advantage to plants possessing that pathway in cooler environments (Ehleringer 1978; Osmond et al. 1982). While C4 photosynthesis may
not have originally evolved under hot, arid conditions (Ehleringer et al.
1991b), C4 photosynthesis does result in a reduced photo respiration rate and
increased quantum efficiency at high temperatures (~35 QC). Thus, we should
expect C4 photosynthesis to predominate in hot temperature environments
and C3 photosynthesis in cool temperature environments, provided that
adequate moisture for growth is available. There is no evidence that intrinsic
aspects of each photosynthetic pathway should confer any competitive advantage with respect to drought tolerance. However, because of the higher
water-use efficiency of C4 photosynthesis, these plants are expected to be at
a competitive advantage in saline environments (Pearcy and Ehleringer
1984).
CAM photosynthesis provides a mechanism for a potential advantage
over C3 and C4 plants under limited moisture conditions (Lange et al. 1974,
371
(Shreve 1942; Werger 1986; Shmida and Burgess 1988). Shreve and Wiggins
(1964) described 25 categories of life forms within the Sonoran Desert.
While many of the different life-forms relate to overall physiognomy (tree,
subtree, shrub, annual, etc.), 19 relate to succulence and to the high diversity
associated with shrubs as a life-form. For photosynthetic purposes, we will
examine the diversity of life-forms related to leaf types and succulence
differences, because these are of particular importance to understanding the
distribution patterns of photosynthetic pathways.
Within the C3 photosynthetic pathway, there is wide variation in photosynthetic tissue types in that leaves, twigs, stems, or any combinations of these
serve as the photosynthetic surface. Green-stem or green-photosynthetic
nonleaf tissues are perhaps the most frequent variant (Ehleringer et al. 1987;
Osmond et al. 1987; Smith and Osmond 1987; Comstock and Ehleringer
1988, 1990). Species exhibiting photosynthesis by nonleaf tissues increase
both as precipitation decreases and as the fraction of summer precipitation
increases (Shmida and Burgess 1988; Cody 1989). Twig and stem photosynthesis contribute significantly to the overall carbon balance of these
species, and in some cases, leaves no longer serve a significant role in
the plant's carbon balance (Comstock et al. 1988). In all instances, the
intercellular CO2 concentrations of twigs are lower than those of leaves and
the twig tissues are more drought-tolerant than leaf tissues. As elaborated in
Section 18.5.4, leaves of C3 species in the desert exhibit over a 1001111-1
variation in intercellular CO2 concentrations, which will have significant
bearing on both the extent to which stomata limit photosynthesis and on
water-use efficiency.
In terms of carbon gain, advantages and disadvantages can be associated
with each of the three photosynthetic pathways depending on environmental
conditions. Under today's CO2 environment, the C4 photosynthetic pathway
offers little or no intrinsic advantage over C3 under cool temperatures
( ~25 0C). In fact, the increases in quantum yield and reductions in photorespiration associated with C3 photosynthesis under cooler temperatures
suggest an advantage to plants possessing that pathway in cooler environments (Ehleringer 1978; Osmond et al. 1982). While C4 photosynthesis may
not have originally evolved under hot, arid conditions (Ehleringer et al.
1991b), C4 photosynthesis does result in a reduced photo respiration rate and
increased quantum efficiency at high temperatures (~35 QC). Thus, we should
expect C4 photosynthesis to predominate in hot temperature environments
and C3 photosynthesis in cool temperature environments, provided that
adequate moisture for growth is available. There is no evidence that intrinsic
aspects of each photosynthetic pathway should confer any competitive advantage with respect to drought tolerance. However, because of the higher
water-use efficiency of C4 photosynthesis, these plants are expected to be at
a competitive advantage in saline environments (Pearcy and Ehleringer
1984).
CAM photosynthesis provides a mechanism for a potential advantage
over C3 and C4 plants under limited moisture conditions (Lange et al. 1974,