Journal of Experimental Botany, Vol 49, 1563-1573, Copyright © 1998 by Oxford University Press
A Roberts, A Borland, K Maxwell and H Griffiths
The seasonal changes in crassulacean acid metabolism (CAM) activity in
response to daily integrated photon flux density (PFD)
and precipitation were compared in sun and shade leaves of the C3-CAM
intermediate tree Clusia minor L. Measurements of CAM
activity (
ARTICLES
Ecophysiology of the C3-CAM intermediate Clusia minor L. in Trinidad: seasonal and short-term photosynthetic characteristics of sun and shade leaves
Department of Agricultural and Environmental Science, Ridley Building, The University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK; Corresponding author; Fax: +44 191 222 5229; E-mail: Howard.Griffiths@newcastle.ac.uk
H+) showed that maximum leaf
acidity consistently occurred 4 h after dawn, suggesting that new sampling
procedures need to be adopted in order to quantify CAM in
Clusia species. Whilst exposed leaves responded to
intermittent dry conditions, shaded leaves showed a clear induction of CAM
activity as conditions became drier. The magnitude of CAM activity
correlated well with daily integrated PFD, such that
the extent of decarboxylation of organic acids was consistently associated
with increased acidification during the subsequent dark period. Over two
sampling days, both sun and shade leaves exhibited the four phases of CAM,
although PEPc remained active throughout phase II with the result that 50%
of the maximum leaf acidity in shade leaves was accumulated during this
time. During phase III, internal CO2 supply was augmented by substantial
citrate decarboxylation, in addition to malic acid. Chlorophyll
fluorescence characteristics were dominated by high rates of PSII electron
transport, together with an extremely high potential for thermal
dissipation, such that excess light was maintained within safe limits at
times of maximum PFD. Photochemical stability was
maintained by matching supply and demand for internal CO2: in the morning,
C3 and C4 carboxylation processes were regulated by extended PEPc activity,
so that decarboxylation was delayed until temperature and light stress were
highest at midday.Keywords: Clusia
minor, Crassulacean acid metabolism, citrate, chlorophyll
fluorescence, instantaneous carbon isotope discrimination.
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