Variation in Gas Exchange Characteristics Among Desert Plants
377
are primarily C 4 plants (Mulroy and Rundel 1977). Such a distribution
pattern is consistent with the observation that C4 photosynthesis, through its
effects on reduced photorespiration and increased quantum yields at high
temperature, provides a significant potential carbon gain advantage over the
C 3 pathway under hot summer conditions if soil moisture is adequate.
18.5 Longevity and Gas Exchange
While absolute photosynthesis and transpiration rates among species may
exhibit substantial variation, these flux rates decrease as water stress increases (Smith and Nobel 1986; Smith and Nowak 1990). Variation in
maximum flux capacities is loosely associated with life-form, with annuals
typically having higher rates than perennials (Mooney and Gulmon 1982;
Smith and Nobel 1986). However, there are enough counter-examples of
annuals having low photosynthetic capacities (Seeman et al. 1980; Werk et
al. 1983) and perennials having high photosynthetic capacities (Ehleringer
and Bjorkman 1977) that generalities of this type cannot be drawn with a
high degree of certainty. An alternative approach to understanding lifehistory variation in gas exchange parameters is to evaluate ratios of activity
(such as water-use efficiency) or possible set points in photosynthetic gas
exchange.
18.5.1 Water Use in Relation To Carbon Gain
Gas-exchange responses at the leaf level can be viewed from two perspectives: what causes changes in absolute flux rates and what controls changes
in flux rates. Changes in maximum photosynthesis (A) and transpiration (E)
rates of desert plants in response to soil moisture availability (or any other
measures of plant water status for that matter) have been described in
numerous studies (Lange et al. 1976; Ehleringer and Mooney 1983; Smith
and Nobel 1986; Smith and Nowak 1990). Changes in flux rates and canopy
photosynthetic area almost always show a linear response to plant stress
(e.g., water potential), and species vary widely in their capacity to maintain
photosynthetic activity under water stress. As a consequence, instantaneous
measures of gas exchange activity at a single point in time may provide
limited insight into primary productivity and plant fitness, although the
parameters are ultimately linked with each other (Fig. 18.10).
18.5.2 Gas Exchange Flux Versus Set Point
An alternative approach to examining absolute flux rates and their impact
on gas exchange performance is to examine control points or set points in
377
are primarily C 4 plants (Mulroy and Rundel 1977). Such a distribution
pattern is consistent with the observation that C4 photosynthesis, through its
effects on reduced photorespiration and increased quantum yields at high
temperature, provides a significant potential carbon gain advantage over the
C 3 pathway under hot summer conditions if soil moisture is adequate.
18.5 Longevity and Gas Exchange
While absolute photosynthesis and transpiration rates among species may
exhibit substantial variation, these flux rates decrease as water stress increases (Smith and Nobel 1986; Smith and Nowak 1990). Variation in
maximum flux capacities is loosely associated with life-form, with annuals
typically having higher rates than perennials (Mooney and Gulmon 1982;
Smith and Nobel 1986). However, there are enough counter-examples of
annuals having low photosynthetic capacities (Seeman et al. 1980; Werk et
al. 1983) and perennials having high photosynthetic capacities (Ehleringer
and Bjorkman 1977) that generalities of this type cannot be drawn with a
high degree of certainty. An alternative approach to understanding lifehistory variation in gas exchange parameters is to evaluate ratios of activity
(such as water-use efficiency) or possible set points in photosynthetic gas
exchange.
18.5.1 Water Use in Relation To Carbon Gain
Gas-exchange responses at the leaf level can be viewed from two perspectives: what causes changes in absolute flux rates and what controls changes
in flux rates. Changes in maximum photosynthesis (A) and transpiration (E)
rates of desert plants in response to soil moisture availability (or any other
measures of plant water status for that matter) have been described in
numerous studies (Lange et al. 1976; Ehleringer and Mooney 1983; Smith
and Nobel 1986; Smith and Nowak 1990). Changes in flux rates and canopy
photosynthetic area almost always show a linear response to plant stress
(e.g., water potential), and species vary widely in their capacity to maintain
photosynthetic activity under water stress. As a consequence, instantaneous
measures of gas exchange activity at a single point in time may provide
limited insight into primary productivity and plant fitness, although the
parameters are ultimately linked with each other (Fig. 18.10).
18.5.2 Gas Exchange Flux Versus Set Point
An alternative approach to examining absolute flux rates and their impact
on gas exchange performance is to examine control points or set points in