growth rates of CAM plants

[Joseph.Holtum]

Re Ross Koning's statement on the speed of growth by CAM plants:

The growth of CAM plants that take their CO2 in solely during the night 
(i.e. are operating solely in a CAM mode) is limited by the size of the 
vacuole (into which they stuff the malic acid) and the concentration 
gradient against which they can pump malic acid. 

However, in the presence of water, many CAM plants open their stomata during 
the afternoon and fix CO2 via rubisco (i.e. normal C3 metabolism). Therefore 
they can grow at very good rates. I cant remember the numbers, but there are 
some very high growth rates recorded for CAM species (try Barry Osmonds 1978 
review in Ann reviews of Plant Physiol)

In the late 1920s/early 1930s prickly pear infested around 1,000,000 
acres/annum in south-east Queensland (Oz). The rates of growth of commercial 
CAM species such as pineapple, vanilla orchids or Agave can be substantial.

Many CAM plants are perennial plants that grow in regions of intermittent 
water supply (whether they are cacti in the Sonoran desert or epiphytic 
ferns on the trunks of rainforest trees). Therefore they inhabit niches in 
which fast growth may not be a successful strategy of using the resources 
that are available

There are few CAM annuals....though Mesembryanthemum crystallinum is one. It 
grows as a C3 plant during the wetter winter months and then develops CAM as 
water becomes scarcer approaching summer. CAM allows the plant to last 
longer than the other annuals and the plant is able to invest more C in seed 
production (a paper by Kalus Winter in....damn it, I've forgotten! Sorry).

Most CAM plants that live in semi-arid regions also have other adaptations 
for survival in those habitats. It can be difficult to separate the CAM 
effects from the others. It is the same with C4 plants. Do C4 grasses grow 
well in the tropics because they are C4 or because they evolved in the 
tropics and therefore possess a range of traits that are suited to those 
climes? As far as I understand things, there are advantages to possessing 
the C4 syndrome (at present CO2 concentrations that is!) AND there are also 
advantages in having other adaptations. i.e. the traits are not mutually 
exclusive.....what came first the chicken or the egg?

Joe



 CAM rules!.....but what ARE the rules?

Joe Holtum
Department of Tropical Plant Sciences,
James Cook University of North Queensland,
Townsville, North Queensland 4811

Telephone:-          (077) 81 4391 (lab); 79 5252 (home)
Facsimile:-             (077) 25 1570
electronic mail:-    joseph.holtum at jcu.edu.au