256
M.e. Ball and J.B. Passioura
have lower quantum yields than shaded leaves (Bjorkman et al. 1988;
Cheeseman et al. 1991). This depression in quantum yield of leaves naturally
receiving high irradiances is correlated with the concentration per unit leaf
area of zeaxanthin pigment (Lovelock and Clough 1992; Lovelock and
De'ath 1993), implying that the loss of photosynthetic activity at limiting
irradiances is due to protective dissipation of excessive excitation energy
through the xanthophyll cycle (Demmig-Adams et al. 1989).
Clearly, there are both coarse and fine adjustments to irradiance regimes.
The coarse adjustments occur primarily through the display and physical
properties of the leaves to avoid prolonged exposure to excessive irradiance,
whereas fine adjustments are made at the biochemical level of chloroplast
functioning. Naturally, there are limits to the extent to which a leaf can
provide protection from excessive irradiance, and when the light absorbed
exceeds the capacities for both useful photochemistry and protective dissipation mechanisms, then the leaf becomes vulnerable to another form of
photo inhibition which results from photodamage to PS II (Kyle 1984). The
relative importance of photoprotection and photo damage to the carbon
balance of mangroves with spatial and temporal variation in environmental
factors affecting photosynthesis is an exciting area for future research.
12.6 Into the Future: Coping with Global Increase
In Atmospheric CO2 Concentration
It is difficult to predict changes in climate and sea level due to the greenhouse
effect, but change in one environmental factor is certain. The concentration
of atmospheric CO2 is increasing and is expected to double current levels
during the latter half of the next century. As previously noted, enhancement
of water use efficiency within the constraints of C3 photosynthetic biochemistry can occur at the expense of the assimilation rate. This is because
stomatal closure reducing the efflux of water vapor also reduces the influx of
CO2, causing the leaf to operate with a low intercellular CO2 concentration
and a correspondingly low assimilation rate (Cowan and Farquhar 1977).
However, this trade-off between water loss and carbon gain can be greatly
ameliorated under elevated atmospheric concentrations of CO2, Higher external levels of CO 2 enable a leaf to maintain higher intercellular concentrations of CO2 at lower stomatal conductances, thereby enabling a leaf to
operate with a higher water use efficiency without a penalty in terms of
carbon gain.
Recent studies by M.C. Ball and H.M. Rawson (unpubl.) indicate that
change in the CO 2 concentration, because of its effects on both carbon gain
and water use characteristics, may be sufficient to change the structure and
function of mangrove vegetation. In tropical Australia, the distribution of
M.e. Ball and J.B. Passioura
have lower quantum yields than shaded leaves (Bjorkman et al. 1988;
Cheeseman et al. 1991). This depression in quantum yield of leaves naturally
receiving high irradiances is correlated with the concentration per unit leaf
area of zeaxanthin pigment (Lovelock and Clough 1992; Lovelock and
De'ath 1993), implying that the loss of photosynthetic activity at limiting
irradiances is due to protective dissipation of excessive excitation energy
through the xanthophyll cycle (Demmig-Adams et al. 1989).
Clearly, there are both coarse and fine adjustments to irradiance regimes.
The coarse adjustments occur primarily through the display and physical
properties of the leaves to avoid prolonged exposure to excessive irradiance,
whereas fine adjustments are made at the biochemical level of chloroplast
functioning. Naturally, there are limits to the extent to which a leaf can
provide protection from excessive irradiance, and when the light absorbed
exceeds the capacities for both useful photochemistry and protective dissipation mechanisms, then the leaf becomes vulnerable to another form of
photo inhibition which results from photodamage to PS II (Kyle 1984). The
relative importance of photoprotection and photo damage to the carbon
balance of mangroves with spatial and temporal variation in environmental
factors affecting photosynthesis is an exciting area for future research.
12.6 Into the Future: Coping with Global Increase
In Atmospheric CO2 Concentration
It is difficult to predict changes in climate and sea level due to the greenhouse
effect, but change in one environmental factor is certain. The concentration
of atmospheric CO2 is increasing and is expected to double current levels
during the latter half of the next century. As previously noted, enhancement
of water use efficiency within the constraints of C3 photosynthetic biochemistry can occur at the expense of the assimilation rate. This is because
stomatal closure reducing the efflux of water vapor also reduces the influx of
CO2, causing the leaf to operate with a low intercellular CO2 concentration
and a correspondingly low assimilation rate (Cowan and Farquhar 1977).
However, this trade-off between water loss and carbon gain can be greatly
ameliorated under elevated atmospheric concentrations of CO2, Higher external levels of CO 2 enable a leaf to maintain higher intercellular concentrations of CO2 at lower stomatal conductances, thereby enabling a leaf to
operate with a higher water use efficiency without a penalty in terms of
carbon gain.
Recent studies by M.C. Ball and H.M. Rawson (unpubl.) indicate that
change in the CO 2 concentration, because of its effects on both carbon gain
and water use characteristics, may be sufficient to change the structure and
function of mangrove vegetation. In tropical Australia, the distribution of