Carbon Gain in Relation to Water Use: Photosynthesis in Mangroves
255
lull in air movement because of reduction in boundary layer conductance.
However, the rate of temperature increase would be slower in the leaf with
a greater heat capacity. Thus, there is a tendency for mangrove leaves to
have a greater mass per unit area under conditions in which, due to intense
irradiation and/or limitations to evaporative cooling, they would be most
vulnerable to rapid fluctuations in leaf temperature (Ball et al. 1988).
The display and properties of foliage contribute to conservative water
use, but not without costs to the plant. Increasing the angle of inclination
reduces heat loading on a leaf, but is at the expense of light harvesting in
that a larger leaf area index is required to intercept a given amount of light.
Decrease in leaf size enhances heat transfer rates, but this requires greater
investment in supportive and conductive tissue per unit of exposed leaf area
than in large leaves. Increase in heat capacity of leaves buffers against rapid
changes in temperatures, but at the expense of leaf carbon which might
otherwise be invested in expansion of leaf area. Thus, maintenance of
favorable leaf temperatures with minimal evaporative cooling is at the expense of the assimilative capacity of the plant, with the expense increasing
as water use becomes more conservative (Ball et al. 1988).
12.5 Coping with Excessive Light: Another By-Product
of Conservative Water Use
Photoinhibition is light-dependent loss in photosynthetic functioning of
photosystem II, which is manifest in whole leaves as a decline in the
quantum efficiency of photosynthesis (i.e., mol CO2 fixed or mol O 2 evolved
per mol photons absorbed) under limiting light intensities (see Chap. 10,
this Vol.). Photo inhibition occurs when more light is absorbed than can
be used in photosynthetic photochemistry (Osmond 1981). One form of
photoinhibition is actually photo-protection in that excessive excitation
energy is deflected away from PS II and dissipated harmlessly, primarily as
heat. Protective dissipation occurs mainly by means of the transthylakoid
pH gradient (Krause and Behrend 1986) and xanthophyll cycle pigments
(Demmig-Adams 1990), enabling a so-called "down-regulation" to balance
the light energy received by PS II with its capacity to use it (Chow 1993).
It follows from the low photosynthetic rates of mangroves that light
requirements for maximal photosynthesis are considerably less than the
amounts of light available on bright, sunny days. Rates of photosynthesis in
field-grown mangrove leaves generally become light-saturated at incident
quantum flux densities ranging from 25 to 50% full sunlight (Ball and
Critchley 1982; Bjorkman et al. 1988; Cheeseman et al. 1991), consistent
with the light levels received in their normal orientations under field conditions (Ball et al. 1988). Nevertheless, naturally displayed sun leaves can
255
lull in air movement because of reduction in boundary layer conductance.
However, the rate of temperature increase would be slower in the leaf with
a greater heat capacity. Thus, there is a tendency for mangrove leaves to
have a greater mass per unit area under conditions in which, due to intense
irradiation and/or limitations to evaporative cooling, they would be most
vulnerable to rapid fluctuations in leaf temperature (Ball et al. 1988).
The display and properties of foliage contribute to conservative water
use, but not without costs to the plant. Increasing the angle of inclination
reduces heat loading on a leaf, but is at the expense of light harvesting in
that a larger leaf area index is required to intercept a given amount of light.
Decrease in leaf size enhances heat transfer rates, but this requires greater
investment in supportive and conductive tissue per unit of exposed leaf area
than in large leaves. Increase in heat capacity of leaves buffers against rapid
changes in temperatures, but at the expense of leaf carbon which might
otherwise be invested in expansion of leaf area. Thus, maintenance of
favorable leaf temperatures with minimal evaporative cooling is at the expense of the assimilative capacity of the plant, with the expense increasing
as water use becomes more conservative (Ball et al. 1988).
12.5 Coping with Excessive Light: Another By-Product
of Conservative Water Use
Photoinhibition is light-dependent loss in photosynthetic functioning of
photosystem II, which is manifest in whole leaves as a decline in the
quantum efficiency of photosynthesis (i.e., mol CO2 fixed or mol O 2 evolved
per mol photons absorbed) under limiting light intensities (see Chap. 10,
this Vol.). Photo inhibition occurs when more light is absorbed than can
be used in photosynthetic photochemistry (Osmond 1981). One form of
photoinhibition is actually photo-protection in that excessive excitation
energy is deflected away from PS II and dissipated harmlessly, primarily as
heat. Protective dissipation occurs mainly by means of the transthylakoid
pH gradient (Krause and Behrend 1986) and xanthophyll cycle pigments
(Demmig-Adams 1990), enabling a so-called "down-regulation" to balance
the light energy received by PS II with its capacity to use it (Chow 1993).
It follows from the low photosynthetic rates of mangroves that light
requirements for maximal photosynthesis are considerably less than the
amounts of light available on bright, sunny days. Rates of photosynthesis in
field-grown mangrove leaves generally become light-saturated at incident
quantum flux densities ranging from 25 to 50% full sunlight (Ball and
Critchley 1982; Bjorkman et al. 1988; Cheeseman et al. 1991), consistent
with the light levels received in their normal orientations under field conditions (Ball et al. 1988). Nevertheless, naturally displayed sun leaves can