254
M.e. Ball and J.B. Passioura
thesis and variation in leaf properties in relation to light interception and
evaporative demand (Cowan and Farquhar 1977; Cowan 1986).
12.4 Implications of Conservative Water Use for Display
and Properties of Leaves
Interspecific differences in the display and properties of foliage reflect the
increasingly conservative water-use characteristics associated with increasing
salinity tolerance. This is shown by variation in three major characteristics of
leaves that contribute to maintenance of favorable leaf temperatures with
minimal evaporative cooling. As discussed above, leaf angle affects the radiant
heat loading on the leaf. The greater the leaf angle, the lower the proportion of
leaf area projected on a horizontal surface. In the Rhizophoraceae, leaf angle
in foliage fully exposed to the sun was greater, and hence the proportion of
projected leaf area was smaller, the greater the salinity tolerance of the species
(Table 12.1). Thus, the species which are more conservative in water use are
those that tend most to avoid intense radiation (Ball et al. 1988).
A second leaf property influencing leaf temperature is that of leaf size.
Heat convection between a leaf and its environment depends on resistance
to transfer imposed by a boundary layer, the characteristics of which are a
function of leaf geometry and wind speed. Decrease in leaf size enhances
boundary layer conductance and results in the temperature of the leaf being
closer to ambient air temperature without putting the leaf at a disadvantage
in terms of light interception. Leaf size in the Rhizophoraceae is smallest in
the most salt-tolerant (and most water-conservative) species (Table 12.1),
and decreases with increasing exposure (Ball et al. 1988). Similarly, leaves
of mangroves species that dominate humid low salinity wetlands (e.g.,
Heritiera littoralis, Rhizophora mucronata, and Xylocarpus granatum) are
much larger than those of species that dominate hypersaline environments
along the arid coasts of North Australia (e.g., Avicennia marina, Ceriops
australis, Excoecaria ovalis, Lumnitzera racemosa and Osbornia octodonta).
Apparently, mangrove leaves are smallest under conditions in which, due to
intense radiation and/or limitations to evaporative cooling, they experience
the greatest heat load (Ball et al. 1988).
Heat capacity per unit area, which increases with dry weight and water
content per unit area, is a third leaf property influencing leaf temperature.
Among the Rhizophoraceae, specific leaf weight and succulence, and thus
also heat capacity, increase with salinity (Camilleri and Ribi 1983), exposure
(Ball et al. 1988), and with increase in the salinity tolerance of the species
(Table 12.1). The heat capacities of the leaves in Table 12.1 range from 1.1
to 2.2 x 10 3 Jm- 2 C- 1 in Bruguiera gymnorrhiza and Ceriops australis,
respectively. Leaf temperatures of both species would increase during a
M.e. Ball and J.B. Passioura
thesis and variation in leaf properties in relation to light interception and
evaporative demand (Cowan and Farquhar 1977; Cowan 1986).
12.4 Implications of Conservative Water Use for Display
and Properties of Leaves
Interspecific differences in the display and properties of foliage reflect the
increasingly conservative water-use characteristics associated with increasing
salinity tolerance. This is shown by variation in three major characteristics of
leaves that contribute to maintenance of favorable leaf temperatures with
minimal evaporative cooling. As discussed above, leaf angle affects the radiant
heat loading on the leaf. The greater the leaf angle, the lower the proportion of
leaf area projected on a horizontal surface. In the Rhizophoraceae, leaf angle
in foliage fully exposed to the sun was greater, and hence the proportion of
projected leaf area was smaller, the greater the salinity tolerance of the species
(Table 12.1). Thus, the species which are more conservative in water use are
those that tend most to avoid intense radiation (Ball et al. 1988).
A second leaf property influencing leaf temperature is that of leaf size.
Heat convection between a leaf and its environment depends on resistance
to transfer imposed by a boundary layer, the characteristics of which are a
function of leaf geometry and wind speed. Decrease in leaf size enhances
boundary layer conductance and results in the temperature of the leaf being
closer to ambient air temperature without putting the leaf at a disadvantage
in terms of light interception. Leaf size in the Rhizophoraceae is smallest in
the most salt-tolerant (and most water-conservative) species (Table 12.1),
and decreases with increasing exposure (Ball et al. 1988). Similarly, leaves
of mangroves species that dominate humid low salinity wetlands (e.g.,
Heritiera littoralis, Rhizophora mucronata, and Xylocarpus granatum) are
much larger than those of species that dominate hypersaline environments
along the arid coasts of North Australia (e.g., Avicennia marina, Ceriops
australis, Excoecaria ovalis, Lumnitzera racemosa and Osbornia octodonta).
Apparently, mangrove leaves are smallest under conditions in which, due to
intense radiation and/or limitations to evaporative cooling, they experience
the greatest heat load (Ball et al. 1988).
Heat capacity per unit area, which increases with dry weight and water
content per unit area, is a third leaf property influencing leaf temperature.
Among the Rhizophoraceae, specific leaf weight and succulence, and thus
also heat capacity, increase with salinity (Camilleri and Ribi 1983), exposure
(Ball et al. 1988), and with increase in the salinity tolerance of the species
(Table 12.1). The heat capacities of the leaves in Table 12.1 range from 1.1
to 2.2 x 10 3 Jm- 2 C- 1 in Bruguiera gymnorrhiza and Ceriops australis,
respectively. Leaf temperatures of both species would increase during a