Yes my darling daughter ...but don’t
take all the water
Crassulacean acid metabolism (CAM) is an aspect of plant biochemistry that has fascinated me for many years. Of course we
now know that it is not a simple case of CAM versus C3 photosynthesis. There
are constitutive CAM plants and inducible CAM plants, there are C3–CAM
intermediates; there may be changes in C3 versus CAM activity or in CAM
inducibility during development and there are variants of CAM biochemistry
itself. CAM is still a subject for fruitful research, as illustrated by the
work of Pimienta-Barrios et al. (Jalisco, Mexico and UCLA, pp.
363–369). They compared the effects of drought on CO2 uptake in
cladodes of wild and cultivated Opuntia ficus-indica. Under cultivation,
O. ficus-indica forms daughter cladodes very readily, whereas this is
much less common in wild plants, and certainly never occurs in conditions of
drought. These daughter cladodes exhibited C3 photosynthesis with daytime
stomatal opening (again indicating a developmental factor in the ability to
perform CAM) and thus had a higher demand for water than the CAM-performing
mother cladodes. However, the daughter cladodes maintained their relative water
content (RWC), even in periods of drought, and this was achieved at the expense
of the mother cladodes whose RWC decreased, thus exacerbating the effects of
drought on the mothers. In parallel with this decrease in RWC, the mother
cladodes also exhibited a marked decrease, especially during drought, in CO2
uptake within their normal CAM pathway. Further, these effects were cumulative,
as the authors showed by carrying out their measurements on mother cladodes
with between zero and eight daughters, although RWC of the mothers never fell
below about 60 % (the authors suggest that at this value water is held more
tightly by matric potentials). Overall, this paper provides another insight
into the fascinating world of CAM plants (as well as showing the sacrifices
that parents make for their offspring!).
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
