12 Carbon Gain in Relation to Water Use:
Photosynthesis in Mangroves
M.e. Ball and J.B. Passioura
12.1 Introduction
"Mangrove" is an ecological term referring to a taxonomically diverse association of woody trees and shrubs that form the dominant vegetation in
tidal, saline wetlands along tropical and subtropical coasts (Tomlinson 1986).
The photosynthetic characteristics of mangroves are clearly those of plants
utilizing C 3 photosynthetic biochemistry (Ball 1986). There is, however, a
remarkable feature of the gas exchange characteristics of mangroves. Despite
growing in environments with an abundant water supply, they transpire
slowly and maintain high water use efficiencies for C3 plants (Ball and
Farquhar 1984a,b). These water use characteristics become increasingly
conservative with increase in the salinities in which the plants are grown and
with increase in the salt tolerance of the species (Ball 1988a). Conservative
water use may have adaptive significance for survival in saline environments,
but such behavior has far-reaching consequences for plant functioning.
Maximizing carbon gain relative to water use is a whole-plant phenomenon
involving a complex balance between several levels of plant function:
stomatal behavior in relation to photosynthesis, variation in leaf properties
in relation to light interception and evaporative demand, and the partitioning
of carbon between structures supplying and those consuming carbon-based
assimilates (Cowan and Farquhar 1977; Cowan 1986). Differences in water
use characteristics thus find expression at all levels of plant form and function, and indeed are major determinants of mangrove forest structure along
natural salinity gradients (Ball 1988a).
12.2 Water Relations: Why Be Conservative?
Several possible explanations come to mind for the mangroves' conservatism
in water use, which are best discussed against a background of the essentials
of plant water relations. The roots of mangroves in coastal environments are
bathed in salt water whose osmotic potential is of the order of seawater,
namely, - 2.5 MPa, but which may at times be much saltier. At low ele-
Photosynthesis in Mangroves
M.e. Ball and J.B. Passioura
12.1 Introduction
"Mangrove" is an ecological term referring to a taxonomically diverse association of woody trees and shrubs that form the dominant vegetation in
tidal, saline wetlands along tropical and subtropical coasts (Tomlinson 1986).
The photosynthetic characteristics of mangroves are clearly those of plants
utilizing C 3 photosynthetic biochemistry (Ball 1986). There is, however, a
remarkable feature of the gas exchange characteristics of mangroves. Despite
growing in environments with an abundant water supply, they transpire
slowly and maintain high water use efficiencies for C3 plants (Ball and
Farquhar 1984a,b). These water use characteristics become increasingly
conservative with increase in the salinities in which the plants are grown and
with increase in the salt tolerance of the species (Ball 1988a). Conservative
water use may have adaptive significance for survival in saline environments,
but such behavior has far-reaching consequences for plant functioning.
Maximizing carbon gain relative to water use is a whole-plant phenomenon
involving a complex balance between several levels of plant function:
stomatal behavior in relation to photosynthesis, variation in leaf properties
in relation to light interception and evaporative demand, and the partitioning
of carbon between structures supplying and those consuming carbon-based
assimilates (Cowan and Farquhar 1977; Cowan 1986). Differences in water
use characteristics thus find expression at all levels of plant form and function, and indeed are major determinants of mangrove forest structure along
natural salinity gradients (Ball 1988a).
12.2 Water Relations: Why Be Conservative?
Several possible explanations come to mind for the mangroves' conservatism
in water use, which are best discussed against a background of the essentials
of plant water relations. The roots of mangroves in coastal environments are
bathed in salt water whose osmotic potential is of the order of seawater,
namely, - 2.5 MPa, but which may at times be much saltier. At low ele-