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compounds, resulting in the formation of a S-rich macromolecule, (Kok et al.
2000
),
oxidative reticulation (Riboulleau et al.
2001
) where lipids may be crosslinked with
oxygen and selective preservation of highly aliphatic and resistant biopolymers (e.g.
algaenan or cutan; Tegelaar et al.
1989a
; de Leeuw and Largeau
1993
). However,
recent analysis of a wide variety of gymnosperms and angiosperms failed to detect
cutan in plants previously thought to contain it (following chemical degradation tech-
niques different from that used in this investigation, Mösle et al.
1998
; Collinson
et al.
1998
; Gupta et al.
2006a
,
2007
), and it has been suggested that cutan may have
limited occurrence in modern plants (Boom et al.
2005
; Gupta et al.
2006a
). This has
renewed interest in understanding the origin of aliphatic components in fossil organic
matter (Briggs
1999
; Stankiewicz et al.
2000
; Gupta et al.
2006b
) and particularly
fossil leaves (Gupta et al.
2007
). Here we report the results of experimental heating
(350 °C, 700 bars) of various plant tissues attempted for the fi rst time with plants and
examine the role of different biological components in the formation of chemically-
resistant macromolecules.
Materials and Methods
Extant leaves and plant tissues were crushed in liquid N
2
and powdered and matured
in sealed gold cells at 350 °C, under a confi nement pressure of 700 bars (see descrip-
tion below, Monthioux et al.
1985
; Landais et al.
1989
; Michels et al.
1995
) for a
duration of 24 h in the absence of water. This temperature was chosen because dur-
ing a previous investigation of scorpion cuticle and arthropods (Stankiewicz et al.
2000
; Gupta et al.
2006b
) that was the temperature at which the most dramatic
change in chemical composition was observed. Whole leaves of extant
Castanea
,
Acer
,
Pinus
,
Quercus
and
Ginkgo
(see Gupta et al.
2006a
for further information on
plant species investigated) were matured to investigate the products formed from
various constituents: lignin, polysaccharides, cutin, cuticular lipids and internal lip-
ids. All of these plants have a well-documented fossil record and in each case the
composition of the fossil leaves includes a strong aliphatic component (Logan et al.
1993
; Mösle et al.
1998
; Gupta et al.
2007
). The leaf tissues were also matured
after (
1
) lipid extraction in dichloromethane:methanol (2:1 v:v, 3×30 min in an
ultrasonicator) to remove internal + cuticular lipids and (
2
) extraction followed by
saponifi cation (by refl uxing in 1 M methanolic NaOH for 24 h at 70 °C) to remove
cutin (Kolattukudy
1980
) as well as lipids. The following materials were also
matured to evaluate the fate of specifi c structural biopolymers: (
3
) tomato cuticle
representative of cutin (which lacks cutan) prepared by mechanical isolation fol-
lowed by lipid extraction and acid hydrolysis (Mösle et al.
1998
); (
4
) commercially
prepared lignin and cellulose; (
5
) fl eshy tomato mesocarp (i.e., the succulent fl eshy
edible layer in fruits); and (
6
) the mechanically isolated cuticle of
Agave americana
and associated epidermal tissue, which contains cutan (Nip et al.
1986
). Samples
obtained after maturation were either extracted as described above or subjected to
thermodesorption of weakly bound or non-covalently bound components (at 310 °C,
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