Distribution of Cutan in Modern Leaves - Biopolymers: A Molecular Paleontology Approach - page 27

relative abundances that may be produced, for example, from secondary reactions in
the pyroprobe or non extractable physically entrained waxes (e.g. wax esters; Sylvie
Derenne, personal communication). However, none contained n -alkane/alk-1-ene
homologues after saponifi cation. In addition to Agave , two other examples ( Pinus
and Acer ) of Residue 2 were subjected to acid hydrolysis. A small amount of
Residue 3 was obtained and in both cases this yielded only lignin moieties upon
pyrolysis and no aliphatic polymer, confi rming the absence of cutan. This indicates
that a single acid hydrolysis procedure does not remove lignin from a crushed leaf
preparation but confi rms the absence of any highly aliphatic resistant residue.
The Occurrence of Cutan in Modern Leaves
In this study, following solvent extraction, pyrolysis of all of the modern leaves
(Residue 1) yielded predominantly carbohydrate, lignin, and protein moieties
together with C 16 and C 18 fatty acids, refl ecting the bulk composition of the leaf
(Ralph and Hatfi eld 1991 ; Gupta and Pancost 2004 ). Residue 1 pyrolysates are also
characterised by a series of n -alkane/ n -alk-1-ene homologues. Because, the source
of such an aliphatic signal from biological components other than cutan is unclear,
this aliphatic signature was previously interpreted to result from the pyrolysis of
cutan. The ubiquity of the signal prompted the hypothesis that cutan accounts for
the aliphatic signal typically found in leaf fossils (Table 2.2 ; Tegelaar et al. 1991 ).
However, in our study, pyrolysis of the residue after saponifi cation (Residue 2) of 16
out of 19 leaves released products related solely to lignin and carbohydrates; no
aliphatic components (neither fatty acids nor n -alkane/ n -alk-1-ene homologues)
were detected. Transmission electron microscopy of Residue 2 confi rmed the
absence of cuticle after hydrolysis (e.g., Mösle et al. 1997 , 1998 ). However, cell
walls were retained after the treatment, consistent with the presence of lignin and
polysaccharide moieties in the pyrolysates post saponifi cation. The absence of
n -alkane/alk-1-ene homologues after saponifi cation was also noted in modern
Quercus leaf litter (van Bergen et al. 1998 ).
The absence of the aliphatic signal in the pyrolysates of Residue 2 reveals that the
aliphatic components in the majority of leaves analysed are hydrolysable and thus,
by defi nition, are not cutan. This means that cutan is absent in most of the fl owering
plant leaves previously interpreted as containing it (Tegelaar et al. 1991 ). The excep-
tions are Agave americana , Clivia miniata and Prunus laurocerasus , all of which
yielded a residue diagnostic of cutan after saponifi cation, a residue that was retained
in Agave and Clivia ( Prunus not tested in our study) in Residue 3 after acid hydroly-
sis. The presence of cutan in Agave and Clivia is concordant with many results from
other laboratories (Nip et al. 1986a , b ; Tegelaar et al. 1989c ; McKinney et al. 1996 ;
Mösle et al. 1997 , 1998 ; Schouten et al. 1998 ; Villena et al. 1999 ) and proves that our
protocol, using the whole leaf as a starting material, is able to detect cutan.
Cuticles isolated enzymatically in previous studies Boom et al. ( 2005 ) recorded
cutan in the eudicots Clusia rosea , C. multifl ora and Cereus sp., a cactus—presumably