Variation in Gas Exchange Characteristics Among Desert Plants
383
environments that characterize aridlands, the value of (0 converges on the
actual mean growing-season value of v (Comstock and Ehleringer 1992). (0
is but one way of characterizing the evaporative gradient in leaves, but
provides a means for predicting plant gas exchange parameters for plants
that would be active at different seasons through the year. Habitats in which
the evaporative gradients are higher during the growing season are characterized by higher (0 values. As such, (0 appears to be a useful means for
differentiating habitats and seasonal variation in the evaporative gradient
across sites in a way that encompassed both the driving potential for transpiration as well as moisture input into the soil.
At the ecotypic level, Comstock and Ehleringer (1992) have shown that
variation in A values reflected shifts in habitat quality in Hymenoclea salsola,
a common shrub in the Mojave and Sonoran Deserts. The carbon isotope
discrimination values in H. salsola can vary by >2%0 in the field, suggesting
that Cj values among populations differ by more than 301111-1. Under common garden conditions, the isotopic variation was greater than 2%0 and was
negatively related to the (0 (the average leaf-to-air water vapor gradient
weighted for periods when soil moisture was available) values for the habitats
from which the plants originated (Fig. 18.12). Hymenoclea salsola has both
photosynthetic twigs and leaves, with twigs always having lower Cj and A
values. Since leaves and twigs both have small diameters, resulting in strong
convective exchange and equivalent tissue temperatures, twigs also always
have a greater water-use efficiency (Comstock and Ehleringer 1988). The
fraction of leaf to twig photosynthetic areas is also negatively related to (0,
resulting in plants from drier habitats (atmospheric drought) having both
lower A values (higher water-use efficiencies) at the leaf level as well as a
greater allocation to the more water-use efficient twig tissues in these environments. Overall, this results in a combined morphological-physiological progression towards canopies of greater water-use efficiency in climates with
drier atmospheric conditions. This pattern of decreasing A values in plants
from drier environments and an increased allocation to photosynthetic twigs
is consistent with possible tradeoffs between A, as a set point for gas
exchange, and drought stress.
The implication of the Comstock and Ehleringer (1992) study is that the
seasonality of soil moisture inputs is important in affecting absolute A
values; in desert habitats where precipitation occurred during the hotter
summer months, plants had lower A values than from sites receiving equivalent amounts of precipitation during cooler winter-spring periods of the year.
Implicit in this interpretation is that those ecotypes growing in summer-wet
habitats have the capacity to utilize summer precipitation. For H. salsola,
this is the case (Ehleringer and Cook 1991). In broader terms, it is expected
that along a north-to-south transect through the Sonoran Desert, which is a
gradient of increasing summer precipitation, intraspecific Cj values of leaves
of plants should decrease even though there might be more summer moisture
inputs at those locations.
383
environments that characterize aridlands, the value of (0 converges on the
actual mean growing-season value of v (Comstock and Ehleringer 1992). (0
is but one way of characterizing the evaporative gradient in leaves, but
provides a means for predicting plant gas exchange parameters for plants
that would be active at different seasons through the year. Habitats in which
the evaporative gradients are higher during the growing season are characterized by higher (0 values. As such, (0 appears to be a useful means for
differentiating habitats and seasonal variation in the evaporative gradient
across sites in a way that encompassed both the driving potential for transpiration as well as moisture input into the soil.
At the ecotypic level, Comstock and Ehleringer (1992) have shown that
variation in A values reflected shifts in habitat quality in Hymenoclea salsola,
a common shrub in the Mojave and Sonoran Deserts. The carbon isotope
discrimination values in H. salsola can vary by >2%0 in the field, suggesting
that Cj values among populations differ by more than 301111-1. Under common garden conditions, the isotopic variation was greater than 2%0 and was
negatively related to the (0 (the average leaf-to-air water vapor gradient
weighted for periods when soil moisture was available) values for the habitats
from which the plants originated (Fig. 18.12). Hymenoclea salsola has both
photosynthetic twigs and leaves, with twigs always having lower Cj and A
values. Since leaves and twigs both have small diameters, resulting in strong
convective exchange and equivalent tissue temperatures, twigs also always
have a greater water-use efficiency (Comstock and Ehleringer 1988). The
fraction of leaf to twig photosynthetic areas is also negatively related to (0,
resulting in plants from drier habitats (atmospheric drought) having both
lower A values (higher water-use efficiencies) at the leaf level as well as a
greater allocation to the more water-use efficient twig tissues in these environments. Overall, this results in a combined morphological-physiological progression towards canopies of greater water-use efficiency in climates with
drier atmospheric conditions. This pattern of decreasing A values in plants
from drier environments and an increased allocation to photosynthetic twigs
is consistent with possible tradeoffs between A, as a set point for gas
exchange, and drought stress.
The implication of the Comstock and Ehleringer (1992) study is that the
seasonality of soil moisture inputs is important in affecting absolute A
values; in desert habitats where precipitation occurred during the hotter
summer months, plants had lower A values than from sites receiving equivalent amounts of precipitation during cooler winter-spring periods of the year.
Implicit in this interpretation is that those ecotypes growing in summer-wet
habitats have the capacity to utilize summer precipitation. For H. salsola,
this is the case (Ehleringer and Cook 1991). In broader terms, it is expected
that along a north-to-south transect through the Sonoran Desert, which is a
gradient of increasing summer precipitation, intraspecific Cj values of leaves
of plants should decrease even though there might be more summer moisture
inputs at those locations.