Can CAM cope with copious
carbon dioxide?
One of the main causative agents of global warming,
namely the increasing atmospheric CO2 concentration, will have an effect on plant growth
aside from any other effects of climate change. In general, it is expected that
higher CO2 concentrations will lead to increased photosynthetic carbon fixation
in C3 plants, at least partly because photorespiration will be suppressed.
C4 plants, with their PEP carboxylase-based
CO2 concentrating mechanism, are much less likely to benefit. But what about CAM plants?
Although much of their CO2 fixation occurs at night via PEP carboxylase,
significant C3 photosynthetic activity may occur in the light period and it is
possible that day-time CO2 fixation will be stimulated. To investigate this and
other aspects of CAM, Ceusters et al. (Katholieke
Universitiet, Leuven, Belgium and Newcastle
University, UK; pp. 389–397) have grown a CAM bromeliad (an Aechmea hybrid) in 700 μmol mol-1
CO2 (concentration expected by the middle of the century) for 5
months. This led to a 60 % increase in carbon gain over each 24-h period. The
main effects were in phase II (early in the light period) and especially in phase
IV. Proportionally, the greater increases by far were in C3 fixation,
although, perhaps unexpectedly, day-time C4 fixation was also
stimulated. There was no stimulation of night-time C4 fixation. Further,
because stomatal conductance was lower under elevated
CO2, water use efficiency was two-fold higher over the full 24-h
period. Intriguingly, none of the extra fixed carbon was exported to produce
increased biomass. Indeed, day-time export of sugars was abolished, although
this was balanced by an increase in night-time export. It seems that much of
the extra carbon was kept as hexose in order to
provide substrates for the CAM cycle. The
authors thus suggest that ‘whilst some CAM
species [such as Aechmea] may not show enhanced biomass production in a higher CO2 world,
productivity could be maintained with reduced inputs of water’.
