CAM plants seem to have the capacity to photorespire and in many situations
they will need to. The text book version of CAM is that the stomata are
closed during the day and the malate is decarboxylated- hence CO2
concentrations are high and there is no Rubisco oxygenase activity.
However, if you look further into the literature you will find that most
CAM plants- unless they are under very severe conditions of heat and
drought- will open their stomata towards the latter half of the day and go
into a 'phase 4 stage' where both Rubisco and PEP carboxylase are active
and the stomata are allowing gas exchange to occur. Phase 4 is probably
initiated when the malate pools have been depleted- hence internal CO2
concs. fall and the stomata open. It is also more likely to occur when the
plant is well watered- or at least not completely droughted. So, if the
stomata are open in the light then Rubisco is exposed to 'normal air' and
the CAM plants will photorespire. If I can put in an advert for our own
work here we certainly found measurable levels of activity of key
photorespiratory enzymes in both C3 and CAM forms of the facultative CAM
plant Mesembryanthemum crystallinum (WHitehouse et al (1991) J Exp. Bot. 42
485-492)
Dr A.K. Tobin
Plant Science Laboratory
Sir Harold Mitchell Building
University of St Andrews
St Andrews
KY16 9TH
UK
