374
1.R. Ehleringer
plants (Ehleringer and Bjorkman 1977; Ehleringer and Pearcy 1983; Pearcy
and Ehleringer 1984). While such an explanation may be satisfactory for the
northern and central portions of the Sonoran Desert, it falls short on
explaining the lack of C4 photosynthesis in perennials from the southern
desert regions where precipitation comes primarily during the summer
months and where temperatures are high. At present, there is no satisfactory
explanation (other than historical) as to why C4 photosynthesis is not so
taxonomically common in the southern Sonoran Desert.
Two interesting patterns emerge with respect to shifts in photosynthetic
pathway within a plant in response to increased soil moisture deficit. The
first is tissue-dependent differentiation of photosynthetic pathway. Lange
and Zuber (1977) were the first to note this with Frerea indica, a south
African perennial having a succulent stem with CAM photosynthesis and
drought-deciduous C3 leaves. Presumably, by such a mechanism, the plants
is able to gain carbon longer into the drought period than were it to have
onlyC3 metabolism. More common, though, is a second pattern in which
the same leaf tissue switches from C3 to CAM photosynthesis depending on
the external soil moisture stress (Troughton et al. 1977; Winter et al. 1978;
Bloom and Troughton 1979). Perhaps nowhere is this better developed than
in the south African genus Aloe, whose members span the entire range from
100% C3 to 100% CAM photosynthesis (Fig. 18.8).
18.4.2 Environment and Life-Form Distribution
It is difficult to evaluate or predict leaf-level gas exchange characteristics
of different life-forms in the absence of information about other possible
constraints within the plant that influence overall carbon gain. For photosynthetic gas exchange, two of the most critical aspects will be factors
related to mineral nutrition and to water acquisition and transport. Mineral
nutrition (particularly nitrogen) limits those aspects of gas exchange most
closely associated with capacity (Field and Mooney 1986; Evans 1989).
Models exist to predict the optimal allocation of nitrogen to maximize
carbon gain (Field 1983). Photosynthetic capacity in desert species is linearly
related to leaf nitrogen contents (Mooney et al. 1981; Field and Mooney
1986). A priori, there is no reason to expect that life form should impose
constraints on the maximum photosynthetic capacity of leaves in desert
plants, although field observations indicate distinct trends (Mooney and
Gulmon 1982; Smith and Nobel 1986). Stomatal and biochemical aspects of
gas exchange are known to be closely integrated (Wong et al. 1979; Woodrow
and Berry 1988), suggesting that water-related aspects of gas exchange may
account for the life-form-dependent patterns recognized by Mooney and
Gulmon (1982).
There are reasons to expect that life-form characters should impose
constraints on actual photosynthetic rates, particularly in perennial plants
1.R. Ehleringer
plants (Ehleringer and Bjorkman 1977; Ehleringer and Pearcy 1983; Pearcy
and Ehleringer 1984). While such an explanation may be satisfactory for the
northern and central portions of the Sonoran Desert, it falls short on
explaining the lack of C4 photosynthesis in perennials from the southern
desert regions where precipitation comes primarily during the summer
months and where temperatures are high. At present, there is no satisfactory
explanation (other than historical) as to why C4 photosynthesis is not so
taxonomically common in the southern Sonoran Desert.
Two interesting patterns emerge with respect to shifts in photosynthetic
pathway within a plant in response to increased soil moisture deficit. The
first is tissue-dependent differentiation of photosynthetic pathway. Lange
and Zuber (1977) were the first to note this with Frerea indica, a south
African perennial having a succulent stem with CAM photosynthesis and
drought-deciduous C3 leaves. Presumably, by such a mechanism, the plants
is able to gain carbon longer into the drought period than were it to have
onlyC3 metabolism. More common, though, is a second pattern in which
the same leaf tissue switches from C3 to CAM photosynthesis depending on
the external soil moisture stress (Troughton et al. 1977; Winter et al. 1978;
Bloom and Troughton 1979). Perhaps nowhere is this better developed than
in the south African genus Aloe, whose members span the entire range from
100% C3 to 100% CAM photosynthesis (Fig. 18.8).
18.4.2 Environment and Life-Form Distribution
It is difficult to evaluate or predict leaf-level gas exchange characteristics
of different life-forms in the absence of information about other possible
constraints within the plant that influence overall carbon gain. For photosynthetic gas exchange, two of the most critical aspects will be factors
related to mineral nutrition and to water acquisition and transport. Mineral
nutrition (particularly nitrogen) limits those aspects of gas exchange most
closely associated with capacity (Field and Mooney 1986; Evans 1989).
Models exist to predict the optimal allocation of nitrogen to maximize
carbon gain (Field 1983). Photosynthetic capacity in desert species is linearly
related to leaf nitrogen contents (Mooney et al. 1981; Field and Mooney
1986). A priori, there is no reason to expect that life form should impose
constraints on the maximum photosynthetic capacity of leaves in desert
plants, although field observations indicate distinct trends (Mooney and
Gulmon 1982; Smith and Nobel 1986). Stomatal and biochemical aspects of
gas exchange are known to be closely integrated (Wong et al. 1979; Woodrow
and Berry 1988), suggesting that water-related aspects of gas exchange may
account for the life-form-dependent patterns recognized by Mooney and
Gulmon (1982).
There are reasons to expect that life-form characters should impose
constraints on actual photosynthetic rates, particularly in perennial plants