Carbon Gain in Relation to Water Use: Photosynthesis in Mangroves
253
the microclimate surrounding the leaves, which has a direct and immediate
affect on diurnal water use in relation to carbon gain. The transpiration rate
depends on both the leaf conductance to water vapor and the vapor pressure
gradient between the leaf and air (vpd). Diurnal variation in vpd is caused
mainly by variation in leaf temperature because ambient vapor pressure
changes little over the course of a day. The smaller the difference between
leaf temperature and air temperature, the more closely will the evaporative
demand of the leaf reflect the saturation vapor deficit of the air.
Optimal leaf temperatures for photosynthesis in mangroves are very close
to the average air temperatures in the tropical and subtropical environments
in which the plants are grown. Both assimilation rate and stomatal conductance are maximal at leaf temperatures ranging from 25 to 30°C, and
decline precipitously with increase in leaf temperature above 35 °C (Moore
et al. 1972, 1973; Andrews et al. 1984; Andrews and Muller 1985; Ball et al.
1988). The transpiration rates, even at optimal leaf temperatures, are not
sufficient to prevent heating of the leaves above ambient air temperatures
during periods of intense insolation. Leaves operating with high transpiration
rates can take advantage of the high irradiances required for maintenance of
high photosynthetic rates with minimal increase in leaf temperature over air
temperature. In contrast, leaves with more conservative water use must
avoid high irradiances if leaf temperatures are to be kept within physiologically acceptable limits. In such leaves, avoidance of high light intensities
in the middle of the day, when the heat load on the leaf is greatest, would
allow the leaves to maintain fairly constant, but low, assimilation rates
throughout the day, thus achieving a greater net gain of carbon than if the
leaves were horizontal and subject to temperature-dependent inhibition of
photosynthesis for extended periods (Cowan 1982).
It follows that increase in leaf angle is a compromise between requirements for illumination and for maintenance of favorable leaf temperatures
with minimal evaporative cooling. For example, when exposed canopy leaves
of Rhizophora apiculata were held in a horizontal position, leaf temperatures
increased from 4 to 11 °C above ambient air temperatures of approximately
30°C, while incident irradiation increased from 1430 to 2085Iimolm-2s-1. In
contrast, leaves left in their natural, almost vertical, orientation avoided the
maximum heat load during midday when irradiance and air temperatures are
greatest. During midday, these leaves received only 20% of available sunlight
and were approximately 10 °C cooler than they would have been if fully
exposed to the sun. Earlier and later in the day, the leaves received about
500 limol m- 2 S-1 and leaf temperatures were 30°C, conditions nearly optimal
for photosynthesis (Ball et al. 1988). Such maintenance of leaf temperature close to air temperature by avoidance of high irradiance in leaves of
Rhizophora stylosa was critical to maximizing the total integrated gain of
carbon for a minimum expenditure of water during a day (Andrews and Muller
1985). Thus, maximizing carbon gain for a fixed total expenditure of water
involves a complex balance between stomatal behavior in relation to photosyn-
253
the microclimate surrounding the leaves, which has a direct and immediate
affect on diurnal water use in relation to carbon gain. The transpiration rate
depends on both the leaf conductance to water vapor and the vapor pressure
gradient between the leaf and air (vpd). Diurnal variation in vpd is caused
mainly by variation in leaf temperature because ambient vapor pressure
changes little over the course of a day. The smaller the difference between
leaf temperature and air temperature, the more closely will the evaporative
demand of the leaf reflect the saturation vapor deficit of the air.
Optimal leaf temperatures for photosynthesis in mangroves are very close
to the average air temperatures in the tropical and subtropical environments
in which the plants are grown. Both assimilation rate and stomatal conductance are maximal at leaf temperatures ranging from 25 to 30°C, and
decline precipitously with increase in leaf temperature above 35 °C (Moore
et al. 1972, 1973; Andrews et al. 1984; Andrews and Muller 1985; Ball et al.
1988). The transpiration rates, even at optimal leaf temperatures, are not
sufficient to prevent heating of the leaves above ambient air temperatures
during periods of intense insolation. Leaves operating with high transpiration
rates can take advantage of the high irradiances required for maintenance of
high photosynthetic rates with minimal increase in leaf temperature over air
temperature. In contrast, leaves with more conservative water use must
avoid high irradiances if leaf temperatures are to be kept within physiologically acceptable limits. In such leaves, avoidance of high light intensities
in the middle of the day, when the heat load on the leaf is greatest, would
allow the leaves to maintain fairly constant, but low, assimilation rates
throughout the day, thus achieving a greater net gain of carbon than if the
leaves were horizontal and subject to temperature-dependent inhibition of
photosynthesis for extended periods (Cowan 1982).
It follows that increase in leaf angle is a compromise between requirements for illumination and for maintenance of favorable leaf temperatures
with minimal evaporative cooling. For example, when exposed canopy leaves
of Rhizophora apiculata were held in a horizontal position, leaf temperatures
increased from 4 to 11 °C above ambient air temperatures of approximately
30°C, while incident irradiation increased from 1430 to 2085Iimolm-2s-1. In
contrast, leaves left in their natural, almost vertical, orientation avoided the
maximum heat load during midday when irradiance and air temperatures are
greatest. During midday, these leaves received only 20% of available sunlight
and were approximately 10 °C cooler than they would have been if fully
exposed to the sun. Earlier and later in the day, the leaves received about
500 limol m- 2 S-1 and leaf temperatures were 30°C, conditions nearly optimal
for photosynthesis (Ball et al. 1988). Such maintenance of leaf temperature close to air temperature by avoidance of high irradiance in leaves of
Rhizophora stylosa was critical to maximizing the total integrated gain of
carbon for a minimum expenditure of water during a day (Andrews and Muller
1985). Thus, maximizing carbon gain for a fixed total expenditure of water
involves a complex balance between stomatal behavior in relation to photosyn-